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Butterflies and exurban development in southeastern Arizona
Landscape and Urban Planning 80 (2007) 34–44
Butterflies and exurban development in southeastern Arizona Carl E. Bock a,∗ , Richard A. Bailowitz b , Douglas W. Danforth c , Zach F. Jones d , Jane H. Bock a a
Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA b 1331 West Emerine Drive, Tucson, AZ 85704, USA c P.O. Box 232, Bisbee, AZ 85603, USA d Department of Biology, Colorado College, Colorado Springs, CO 80903, USA Received 2 March 2006; received in revised form 17 April 2006; accepted 29 May 2006 Available online 3 July 2006
Abstract Ranches are being converted to exurban housing developments in the southwestern United States, with potentially significant impacts on biological diversity. We surveyed butterflies on 48 plots in grasslands, mesquite savannas, and oak savannas in southeastern Arizona that were grazed by livestock, embedded in low-density housing developments, or both, or neither. Results suggest that livestock grazing had little impact on butterfly species richness or abundance, while exurban development had minor impacts compared to negative effects that have been documented elsewhere in more fully developed urban and suburban landscapes. However, our data indicate that conversion of ranchland to exurban development has not been without consequence to butterflies. First, relatively immobile species with multiple broods and/or generalized diets were positively associated with development in grasslands, unaffected in mesquite savannas, and often negatively associated with development in oak savannas. Second, while abundance and variety of butterflies were positively correlated with plant species richness and cover in undeveloped landscapes, such correlations were not present in exurban areas. These results suggest that increased resources associated with housing development, including water, shade, and nectar, and possible negative impacts of increased avian predation and pesticide use, caused relationships between butterflies and native vegetation to be less tightly coupled in exurban than in undeveloped landscapes. © 2006 Elsevier B.V. All rights reserved. Keywords: Grassland; Lepidoptera; Livestock grazing; Mesquite; Oak; Rural
1. Introduction Throughout much of the American West, ranchlands are being converted into low-density housing developments that are not part of urban centers (Knight et al., 1995). This widespread land use change, termed exurbanization, has profound implications for regional biological diversity (Theobald, 2004; Hansen and Brown, 2005). However, the ecological consequences of exurbanization have been much less studied than the effects of human population growth in urban and suburban areas (Hansen et al., 2005; but see Maestas et al., 2003). A confounding fac-
∗ Corresponding author at: 5224 Lighthouse Point Court, Loveland, CO 80537, USA. Tel.: +1 970 593 0343; fax: +1 970 593 0347. E-mail addresses: [email protected] (C.E. Bock), [email protected] (R.A. Bailowitz), [email protected] (D.W. Danforth), [email protected] (Z.F. Jones), [email protected] (J.H. Bock).
0169-2046/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.landurbplan.2006.05.003
tor in evaluating the effects of exurban development is that it does not always exempt landscapes from the effects of grazing, because many exurbanites keep livestock (especially horses) on their “ranchettes” (Maestas et al., 2002; Sengupta and Osgood, 2003). Butterflies (Lepidoptera: Papilionoidea and Hesperioidea) are charismatic and familiar insects of longstanding interest to conservation biologists (New et al., 1995). Patterns of butterfly abundance and diversity have been described for urban and suburban environments (Blair and Launer, 1997; Shapiro, 2002; Collinge et al., 2003; Hogsden and Hutchinson, 2004), but we are aware of no studies that have measured butterfly responses to exurban development. The Sonoita Valley of southeastern Arizona is a mixture of grassland and oak/mesquite savanna where some cattle ranches have been converted into low-density exurban housing developments (Bock and Bock, 2000). We surveyed butterflies during two summers (2003–2004) on 48 plots in the valley that were grazed by livestock, or embedded in exurban housing developments, or both, or neither. The objective of our
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
study was to examine the independent and interactive effects of livestock grazing and exurban development on butterfly species richness and abundance. 2. Methods 2.1. Study area and sampling design The Sonoita Valley lies at 1300–1600 m elevation, between the Santa Rita and Huachuca Mountains, in Santa Cruz County, Arizona (31◦ 33–44 N, 110◦ 29–42 W). Predominant ground cover includes a variety of perennial bunchgrasses in the genera Bouteloua, Aristida, Eragrostis, and Hilaria interspersed with scattered low shrubs and succulents and a large variety of forbs (McLaughlin et al., 2001). Certain parts of the valley are treeless, whereas others are savannas with scattered mesquite (Prosopis velutina), or savannas with Emory oak (Quercus emoryi) and Arizona white oak (Quercus arizonica). Vegetation of the study area is dominated by native species, although two species of African lovegrasses (Eragrostis spp.) have become locally common in some parts of the valley. Temperatures vary from a mean January daily minimum of −3.0 ◦ C to a mean June daily maximum of 32.6 ◦ C; annual precipitation is 43.6 cm, about 60% of which falls during the summer monsoon period, from early July to early September (McLaughlin et al., 2001). An analysis conducted in 2000 showed that the Sonoita Valley consisted of about 50% private and 50% public land, the latter including areas managed by the U.S. Department of Agriculture Forest Service, the U.S. Department of Interior Bureau of Land Management, and the state of Arizona (Sonoita Crossroads Community Forum, 2002). Of the private lands, about 77% were cattle ranches, and the remainder consisted mostly of exurban single family housing developments and small commercial centers in the unincorporated towns of Elgin and Sonoita. Housing density in the exurban developments where we worked averaged about 1 home/5.2 h. In the fall of 2002 we established 48 200 m diameter plots in the valley, distributed over an area of about 480 km2 . The average distance from the center of each plot to that of its nearest neighbor was 750 m (range: 325–2000 m). We established the plots in a balanced experimental design, with 12 replicates in each of four landscape and land-use categories: (1) on undeveloped ranches grazed by cattle and a few horses (the Babacomari and Empire ranches), (2) on the undeveloped Appleton-Whittell Research Ranch, a sanctuary of the National Audubon Society that has been ungrazed since 1968 (Bock and Bock, 2000), (3) on exurban properties whose owners did not keep livestock, and (4) on exurban properties whose owners grazed small numbers of either horses (six sites), cattle (four sites), both cattle and horses (one site), or sheep (one site). We further balanced our sampling design such that each group of 12 plots included four in open grassland, four in mesquite savanna, and four in oak savanna. Finally, 12 of the 24 exurban plots included at least one house, while the remaining 12 were embedded in developments but did not include any houses. Landscaping was restricted to the immediate vicinity of home sites, so most of the land in the exurban
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neighborhoods supported natural grassland and savanna vegetation. A large wildfire (the Ryan Fire) burned about 85% of the Research Ranch and parts of the adjacent Babacomari Ranch in April of 2002, 1 year prior to the start of our butterfly surveys. Because of the extent and location of this burn, we had to place 8 of the 12 plots on the Research Ranch, including all four grassland plots and three of four mesquite plots, in areas that had burned in this fire. While vegetation had substantially recovered prior to the start of our surveys in May 2003, lack of pre-burn data made it impossible for us to quantify possible residual fire effects on butterflies. 2.2. Butterfly surveys We surveyed butterflies four times on each plot, during the last week of May and last week of July 2003, and during the last week of July and second week of September 2004. The two co-authors with extensive field experience with butterflies in southeastern Arizona (RAB and DWD) worked together on each survey, walking systematically through all parts of each 200 m diameter circle, counting and identifying all individuals to species. Each survey occurred between mid-morning and early afternoon of a relatively calm, clear, and warm day, and took approximately 30 min. We make no assumption that results represented actual densities, or that all species present were detected. However, because effort was equivalent for all plots over the two summers, we have interpreted count results as accurately reflecting differences in relative abundance and relative species richness among landscapes and habitats. Categorization of the different butterfly species in terms of number of broods per year (voltinism) and degree of larval dietary specialization were based on information provided by Tilden and Smith (1986) and Stewart et al. (2001), confirmed and in some cases modified by personal experience in the study area and elsewhere in southeastern Arizona. Mobility is another attribute important to consider in any study of butterfly landscape ecology. We found no published indices for mobility of the species in our study area, and possible surrogates such as wing or body size have proven unsatisfactory elsewhere (Cowley et al., 2001). Therefore, we adopted the approach used in previous studies (Cowley et al., 2001; Komonen et al., 2004), and ranked each of our study species in terms of relative mobility based on personal experience in the study area. 2.3. Vegetation measurements Between January and March of 2003, we visually estimated canopy cover of mesquite and/or oak trees to the nearest 10% on forty 10 m diameter circles spaced at 20 m intervals along eight 100 m transects radiating at 45◦ intervals from each plot center point. We averaged results from the 40 circles to generate a single tree canopy coverage estimate for each of the 48 plots. In July–August of 2003 and 2004, we sampled ground vegetation at 400 points spaced at 1 m intervals along four 100 m transects radiating in cardinal compass directions from each plot center point. We recorded the presence and identity of any grass,
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forb, succulent, or shrub whose canopy covered each point, and averaged these frequency data to generate percent canopy cover estimates for grasses, forbs, shrubs and succulents, and bare ground on each plot. We collected data only in areas of natural vegetation by making adjustments in the locations of both tree canopy and ground vegetation sampling transects in those few cases where they would have crossed structures or landscaping on some of the exurban plots. Cover data for planted non-native vegetation would have been valuable as well, but landowners were understandably reluctant to permit us to obtain such information by trampling on their landscaping. 2.4. Data analyses Butterfly species richness was calculated as the number of species detected cumulatively during the four surveys on each plot. We estimated relative abundance of species in two ways, first as the average number counted during the four sampling events, and second as the maximum number on any one of the four surveys. We then summed these values for each family
Fig. 1. Mean (+S.E.) species richness and abundance of butterflies on 48 plots in southeastern Arizona, that were in exurban vs. undeveloped landscapes, grazed vs. ungrazed by livestock, and in three different habitats. There was a significant interaction between landscape and habitat for species richness, it being positively related to development in grasslands, and negatively related to development in oak savannas (see Table 1).
and for butterflies as a whole. Results of the two abundance estimates were highly positively correlated for all species combined (r = 0.963, n = 48, P < 0.0001), and for each family individually (r > 0.90 in each case). We chose to present and analyze average rather than maximum count data, because they more accurately reflected species population sizes over the 2-year duration of the study. However, given the high correlations between the two abundance indices, results would have been the same using either approach. We square-root transformed count data prior to all analyses, which is an appropriate way of reducing heterogeneity of variances (Zar, 1999). Resulting abundance and species richness data were sufficiently homogeneous to permit analyses of variance, given our balanced experimental design (Zar, 1999). We compared butterfly abundance and species richness among plots that were exurbanized versus undeveloped (n = 24 per treatment), grazed versus ungrazed (n = 24 per treatment), and for which habitats were either grass or mesquite savanna or oak savanna (n = 16 per habitat), using three-way analyses of variance. We also used three-way analysis of variance to compare butterfly abundance and richness between exurban plots that included at least one house (n = 12) with those exurban plots that did not include houses (n = 12), with grazing and habitat as the second and third independent variables.
Fig. 2. Same as Fig. 1, but for exurban plots only, with landscape difference being presence vs. absence of homes on the plots. There were no significant differences related to landscape type, presence vs. absence of grazing, or habitat type (see Table 1).
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
Presence versus absence of livestock grazing had no statistically significant relationships to butterfly abundance or species richness, either independently or interactively with habitat or exurban development. Therefore, for clarity of data presentation and analysis, we limited comparisons of the different butterfly families and ecological groupings, and of plot vegetation, to the landscape and habitat variables alone, using two-way analyses of variance. We computed product moment correlations (r) to quantify the strengths of relationships between butterfly abundance and species richness and various measures of plant species richness and vegetative canopy cover. We computed chi-square contingency (χ2 ) statistics to test for independence of attributes among the different butterfly species, including dietary specialization, mobility, voltinism, and family membership. All statistical tests were performed in Statview 5.0.1 (SAS Publishing, 1999), with P < 0.05 considered significant, and 0.05 < P < 0.10 as marginally significant. 3. Results 3.1. Butterfly species richness and abundance We recorded 70 butterfly species in the Sonoita Valley (Appendix A). Species richness was unrelated to presence ver-
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sus absence of livestock grazing, to exurban versus undeveloped landscapes, or to habitat type taken as individual variables (Fig. 1, Table 1). However, there was a significant interaction between landscape and habitat, attributable to the fact that species richness was positively associated with development in grasslands, but negatively associated with development in oak savannas, with mesquite savanna intermediate (Fig. 1). Butterfly abundance was unrelated to livestock grazing or landscape context, but it differed among habitats, being higher in mesquite than in grassland or oak (Fig. 1, Table 1). The interaction between landscape and habitat was not significant (Table 1), but the trend was similar to that for species richness, with abundance being higher in exurban than in undeveloped grasslands, but higher in undeveloped than in exurban oak savannas (Fig. 1). Both average species richness and abundance were higher in ungrazed than in grazed grasslands in undeveloped landscapes (Fig. 1). However, these apparent differences did not approach statistical significance, even in one-way analyses of variance with grazing as the lone independent variable (species richness: F1,6 = 0.95, P = 0.37; abundance: F1,6 = 2.41, P = 0.17). There were no differences in butterfly species richness or abundance between exurban plots that included at least one home site versus those embedded in exurban developments that did not include houses, regardless of habitat or livestock grazing (Fig. 2, Table 1). This was despite the fact that the butterfly
Table 1 Results of three-way analysis of variance of data on butterfly species richness and abundance: (1) on 48 plots in southeastern Arizona that differed in terms of presence vs. absence of exurban development, presence vs. absence of livestock, and habitat type (Fig. 1), and (2) on the 24 exurban plots that differed in terms of presence vs. absence of homes on the plots (Fig. 2) Comparison
Dependent variable
Independent variable
d.f.
F (P)
Species richness
A. Exurbanization B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.40 (0.532) 0.42 (0.535) 1.23 (0.304) 0.33 (0.570) 4.67 (0.016) 0.18 (0.833) 1.04 (0.363)
Abundancea
A. Exurbanization B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.16 (0.690) 1.39 (0.246) 3.83 (0.031) 0.01 (0.959) 2.32 (0.113) 1.17 (0.322) 0.72 (0.492)
Species richness
A. Homes on plots B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.10 (0.753) 0.84 (0.378) 0.72 (0.506) 0.75 (0.405) 0.01 (0.998) 0.32 (0.731) 0.83 (0.459)
Abundancea
A. Homes on plots B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
1.57 (0.335) 1.69 (0.218) 2.70 (0.108) 0.02 (0.897) 0.18 (0.841) 0.57 (0.580) 1.28 (0.314)
Exurban vs. undeveloped areas
Exurban plots with vs. without homes
a
Abundance data square-root transformed prior to analysis.
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surveys included landscaped areas when there were homes on the plots. 3.2. Differences among butterfly groups The distribution and abundance of butterflies in the Sonoita Valley appeared to be positively affected by development in grasslands, but negatively affected by development in oak savannas, with mesquite intermediate (Fig. 1). Forty-one of the 70 species contributed to this general pattern by being at least 1.5 times more abundant in exurban than in undeveloped grasslands, or 1.5 times more abundant in undeveloped than in exurban oak savannas, or both (Appendix A). These 41 species were nonrandomly distributed among butterfly families and among other groupings based on natural history. First, the pattern was significant for both species richness and abundance of the families Hesperiidae and Lycaenidae, but not for the families Papilionidae, Pieridae, or Nymphalidae (Table 2). Second, the pattern was strongly developed among 28 less mobile butterflies as a group (Table 3), but it was only marginally present among 42 relatively mobile species. Third, the affinity for exurban grass-
lands, coupled with avoidance of development in oak savannas, was stronger among 52 species with three or more broods per year than among 18 species with only one or two broods per year (Table 3). Finally, species richness and abundance of 37 dietary generalists fit the pattern, but there were no significant interactions between landscape and habitat for 33 dietary specialists whose larvae are known to feed on only one or a group of closely related plant species (Table 3). Butterfly mobility was statistically independent of either voltinism (Appendix A; χ2 = 2.44, d.f. = 1, P = 0.12) or dietary breadth (χ2 = 1.87, d.f. = 1, P = 0.17). However, there was a marginally significant positive association between voltinism and diet breadth, with 31 of 37 diet generalists having three or more broods, but only 21 of 33 diet specialists having three or more broods (Appendix A; χ2 = 3.71, d.f. = 1, P = 0.053). Family membership was independent of voltinism (χ2 = 5.91, d.f. = 4, P = 0.20) or diet breadth (χ2 = 1.22, d.f. = 4, P = 0.87), but species in the families Pieridae, Papilionidae, and Nymphalidae were more likely to be highly mobile than species of Hesperiidae or Lycaenidae (see Appendix A; χ2 = 15.77, d.f. = 4, P = 0.003).
Table 2 Mean (S.E.) and results of two-way analysis of variance comparing species richness and abundance per survey for four butterfly families on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for different effectsa
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Hesperiidae Richness
Undeveloped Exurban
2.6 (0.7) 5.1 (0.7)
4.9 (0.8) 5.1 (0.8)
7.4 (1.1) 5.0 (0.9)
0.03 (ns)b
3.65 (0.035)
4.06 (0.024)
Abundance
Undeveloped Exurban
1.4 (0.6) 3.6 (0.7)
2.8 (0.9) 3.2 (0.9)
4.9 (1.0) 3.1 (0.8)
0.52 (ns)
2.17 (ns)
3.73 (0.032)
Richness
Undeveloped Exurban
2.4 (0.7) 6.0 (0.2)
4.6 (0.7) 3.6 (0.4)
4.3 (0.5) 2.5 (0.5)
0.47 (ns)
1.51 (ns)
15.62 (0.001)
Abundance
Undeveloped Exurban
3.8 (2.2) 7.2 (1.1)
10.8 (1.7) 9.4 (2.1)
7.3 (1.5) 2.5 (0.5)
0.06 (ns)
7.36 (0.002)
6.82 (0.003)
Richness
Undeveloped Exurban
0.4 (0.2) 0.5 (0.3)
0.6 (0.3) 1.3 (0.2)
0.8 (0.3) 0.8 (0.3)
1.42 (ns)
1.93 (ns)
0.83 (ns)
Abundance
Undeveloped Exurban
0.2 (0.1) 0.3 (0.1)
0.7 (0.3) 1.7 (0.6)
0.3 (0.1) 0.3 (0.2)
2.70 (ns)
7.52 (0.002)
2.04 (ns)
Richness
Undeveloped Exurban
6.5 (0.6) 6.6 (0.2)
5.9 (0.5) 6.1 (0.5)
7.1 (0.7) 5.6 (0.6)
0.77 (ns)
0.60 (ns)
1.73 (ns)
Abundance
Undeveloped Exurban
8.4 (1.4) 13.3 (1.5)
13.5 (3.1) 13.6 (1.4)
11.6 (2.4) 11.5 (3.0)
1.28 (ns)
0.90 (ns)
1.14 (ns)
Richness
Undeveloped Exurban
3.6 (0.4) 4.3 (0.6)
4.3 (0.6) 4.9 (0.4)
5.1 (0.7) 5.1 (0.4)
0.92 (ns)
2.48 (0.10)
0.23 (ns)
Abundance
Undeveloped Exurban
7.8 (2.2) 5.9 (1.0)
8.7 (1.3) 8.7 (1.5)
6.8 (1.2) 6.3 (1.1)
0.41 (ns)
1.60 (ns)
0.13 (ns)
Lycaenidae
Papilionidae
Pieridae
Nymphalidae
a b
d.f.: 1, 2, 2, 42 for each ANOVA; abundance data were square-root transformed prior to analysis. ns: not statistically significant (P > 0.10).
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
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Table 3 Mean (S.E.) and results of two-way analysis of variance comparing species richness and abundance per survey for butterfly groups with different attributes, on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for different effectsa
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Low mobility Richness
Undeveloped Exurban
2.1 (0.7) 5.6 (0.7)
5.3 (1.1) 4.5 (0.7)
6.1 (1.1) 3.4 (0.5)
0.60 (ns)b
0.91 (ns)
7.78 (0.001)
Abundance
Undeveloped Exurban
2.1 (1.3) 4.4 (0.6)
4.4 (1.3) 2.9 (0.7)
4.1 (1.1) 2.8 (0.6)
0.48 (ns)
0.35 (ns)
3.82 (0.030)
Richness
Undeveloped Exurban
13.4 (1.2) 16.6 (1.1)
15.3 (1.5) 16.5 (1.3)
18.5 (1.6) 15.5 (1.5)
0.19 (ns)
1.11 (ns)
2.80 (0.072)
Abundance
Undeveloped Exurban
18.7 (4.2) 26.2 (2.4)
31.2 (4.7) 33.7 (3.1)
26.7 (4.1) 20.2 (4.3)
0.23 (ns)
4.49 (0.017)
2.53 (0.092)
Richness
Undeveloped Exurban
2.3 (0.4) 3.2 (0.5)
2.6 (0.6) 2.6 (0.3)
5.3 (0.7) 4.4 (0.7)
0.01 (ns)
10.13 (0.001)
Abundance
Undeveloped Exurban
4.3 (1.3) 3.3 (0.7)
4.1 (1.1) 5.8 (1.1)
6.5 (1.0) 5.9 (2.2)
0.00 (ns)
1.39 (ns)
0.78 (ns)
Richness
Undeveloped Exurban
9.3 (1.5) 14.6 (1.1)
13.6 (1.8) 14.5 (1.4)
14.4 (1.4) 10.8 (1.3)
0.57 (ns)
1.18 (ns)
5.01 (0.011)
Abundance
Undeveloped Exurban
17.3 (5.1) 26.8 (3.0)
32.2 (4.4) 30.6 (3.2)
24.1 (4.5) 17.7 (2.7)
0.19 (ns)
4.95 (0.012)
2.89 (0.067)
Richness
Undeveloped Exurban
4.6 (0.6) 6.3 (0.7)
6.1 (1.0) 6.6 (0.8)
8.1 (1.4) 6.4 (1.0)
0.03 (ns)
1.74 (ns)
1.46 (ns)
Abundance
Undeveloped Exurban
3.9 (0.9) 4.7 (1.2)
5.9 (1.1) 7.5 (1.8)
6.1 (1.6) 6.0 (1.6)
0.20 (ns)
1.65 (ns)
0.14 (ns)
Richness
Undeveloped Exurban
10.9 (1.3) 16.0 (0.7)
14.1 (1.4) 14.4 (0.8)
16.5 (1.2) 12.4 (1.2)
0.22 (ns)
0.46 (ns)
8.16 (0.001)
Abundance
Undeveloped Exurban
17.7 (5.3) 25.6 (2.6)
30.6 (4.2) 29.1 (3.0)
24.7 (3.6) 18.3 (3.8)
0.05 (ns)
3.93 (0.027)
2.86 (0.069)
High mobility
Broods 1–2 1.48 (ns)
Broods > 2
Diet specialists
Diet generalists
a b
d.f.: 1, 2, 2, 42 for each ANOVA; abundance data were square-root transformed prior to analysis. ns: not statistically significant (P > 0.10).
3.3. Plot vegetation
3.4. Correlations between butterflies and vegetation
Plant species richness was highest in oak savanna and lowest in grassland, with mesquite savanna intermediate (Table 4). Plant species richness was higher in exurban mesquite and grassland plots compared to their undeveloped counterparts, a marginally significant effect attributable to differences in species richness of forbs rather than of grasses or woody plants (Table 4). Percent canopy cover of grasses, shrubs and succulents, and trees did not differ between exurban and undeveloped landscapes, whereas forb cover was marginally significantly higher in exurban landscapes (Table 4). Percent canopy of exotic lovegrasses (Eragrostis spp.) averaged <5% across all 48 plots, while no other exotic species comprised >1% canopy. There were no differences among landscape, grazing, or habitat categories in canopy cover of any exotic plant species.
Butterfly species richness was positively correlated with four measures of plant species richness, and with percent canopy cover of trees and forbs, among plots in undeveloped landscapes (Table 5). However, there were no correlations between butterfly species richness and any of the same vegetation measurements on exurban plots. Correlations between butterfly abundance and vegetation generally were weaker than those involving butterfly species richness, but those that were significant (P < 0.05) or marginally significant (P < 0.10) all occurred among undeveloped plots (Table 5). The positive correlation between butterfly species richness and total plant species richness in undeveloped landscapes was driven by low richness values in grasslands and relatively high values in oak woodlands, with mesquite intermediate (Fig. 3). By
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C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
Table 4 Mean (S.E.) and results of two-way analysis of variance comparing various measures of plant species richness and vegetative canopy cover, on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for independent variablea
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Plant species richness All species
Undeveloped Exurban
46.5 (3.3) 55.8 (3.9)
56.3 (2.8) 66.1 (4.3)
76.9 (2.9) 73.4 (1.8)
3.81 (0.058)
27.15 (0.001)
2.66 (0.082)
Grasses
Undeveloped Exurban
16.6 (1.2) 15.3 (1.0)
18.5 (1.1) 19.3 (1.2)
23.1 (1.3) 22.1 (1.8)
0.26 (ns)b
13.47 (0.001)
0.39 (ns)
Forbs
Undeveloped Exurban
25.4 (2.5) 37.1 (2.9)
31.4 (2.6) 38.4 (4.1)
41.1 (2.6) 41.3 (2.9)
6.74 (0.013)
5.74 (0.006)
1.94 (ns)
Woody or succulent
Undeveloped Exurban
4.5 (0.7) 3.4 (1.1)
6.4 (0.8) 8.5 (1.4)
12.6 (1.4) 10.0 (1.8)
0.28 (ns)
17.33 (0.001)
1.88 (ns)
Grasses
Undeveloped Exurban
76.5 (2.6) 83.5 (7.7)
69.6 (2.7) 76.2 (7.0)
76.0 (4.9) 80.3 (1.9)
2.14 (ns)
1.09 (ns)
0.05 (ns)
Forbs
Undeveloped Exurban
16.9 (2.5) 26.5 (3.4)
20.4 (4.3) 27.6 (3.4)
20.0 (2.4) 22.5 (4.3)
2.95 (0.093)
0.20 (ns)
0.31 (ns)
Shrubs and succulents
Undeveloped Exurban
2.8 (0.8) 4.0 (1.3)
3.4 (1.0) 5.8 (2.1)
6.0 (1.3) 8.7 (2.5)
2.53 (ns)
3.10 (0.054)
0.12 (ns)
Trees
Undeveloped Exurban
0.0 (0.0) 0.3 (0.3)
5.1 (1.3) 5.5 (0.9)
18.1 (2.4) 13.5 (2.8)
0.94 (ns)
47.46 (0.001)
1.57 (ns)
Vegetative canopy (%)
a b
d.f.: 1, 2, 2, 42 for each ANOVA. ns: not statistically significant (P > 0.10).
contrast, most grassland plots in exurban landscapes supported relatively high numbers of both butterfly and plant species, such that there was no correlation between these variables across the exurban plots as a whole (Fig. 3).
4. Discussion Results of our study suggest that low-density exurban housing developments in the Sonoita Valley had relatively minor
Table 5 Product-moment correlations (and associated probabilities) between butterfly species richness or butterfly abundance, and various measures of plant species richness and vegetative canopy cover, on 24 grassland and mesquite/oak savanna plots in the exurbanized portions of Sonoita Valley of southeastern Arizona, and on 24 plots in undeveloped portions of the valley, in 2003–2004 Butterfly variable
Environmental variable
Exurban
Undeveloped
Species richness
Plant species richness Grass species richness Forb species richness Woody plant species richness Percent bare ground Percent grass canopy Percent forb canopy Percent shrub/succulent canopy Percent tree canopy
0.04 (ns)a 0.05 (ns) 0.04 (ns) 0.02 (ns) 0.02 (ns) 0.01 (ns) −0.10 (ns) 0.17 (ns) 0.09 (ns)
0.57 (0.004) 0.57 (0.004) 0.37 (0.072) 0.59 (0.002) 0.10 (ns) −0.12 (ns) 0.42 (0.041) 0.18 (ns) 0.59 (0.002)
Total countb
Plant species richness Grass species richness Forb species richness Woody plant species richness Percent bare ground Percent grass canopy Percent forb canopy Percent shrub/succulent canopy Percent tree canopy
0.01 (ns) −0.21 (ns) 0.19 (ns) −0.14 (ns) −0.19 (ns) 0.07 (ns) 0.14 (ns) −0.06 (ns) −0.11(ns)
0.38 (0.065) 0.39 (0.063) 0.30 (ns) 0.29 (ns) 0.13 (ns) −0.35 (0.093) 0.56 (0.004) −0.05 (ns) 0.22 (ns)
a b
ns: not statistically significant (P > 0.10). Count data square-root transformed prior to calculation of correlation coefficients.
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
41
ness and cover in undeveloped landscapes, but these correlations were not present in exurban areas. We consider possible explanations for these two patterns in the following sections. 4.1. Positive and negative impacts of exurban development
Fig. 3. Scatter plots of butterfly species richness vs. plant species richness in three different habitats, on 24 plots in southeastern Arizona that were in undeveloped landscapes, and on 24 plots in exurban landscapes.
impacts on the abundance and variety of butterflies, compared to the negative effects of habitat loss and landscape alterations that have been documented for butterflies in highly developed urban and suburban areas (Blair and Launer, 1997; Hogsden and Hutchinson, 2004; Koh and Sodhi, 2004). Nevertheless, two general patterns in our data suggest that conversion of ranchland to exurban development in the Sonoita Valley has not been without consequence to butterflies. First, there were significant exurban effects involving interactions between habitat and type of butterfly, independent of livestock grazing. Relatively immobile species with multiple broods and/or generalized diets usually were strongly positively associated with development in grasslands, unaffected in mesquite savannas, and often negatively associated with development in oak savannas. Richness and abundance of other sorts of butterflies differed little between exurban and undeveloped landscapes. The second general pattern is that both abundance and variety of butterflies were positively correlated with various measures of plant species rich-
Butterfly habitat requirements include food plants for larvae that often are quite specific (Scott, 1986), and a different suite of more general but no less important resources for adults, including shade, water, and especially nectar (Weiss et al., 1988; Scott, 1986; Wood and Samways, 1992; Schultz and Dlugosch, 1999; Fleishman et al., 2005). Our study involved surveys of adult butterflies. However, their abundances on our plots likely reflected larval as well as adult habitat requirements, to the degree that adult presence was linked to the larval host plants on which they emerged and to which females returned to lay their eggs. This may explain in part why numbers and variety of adult butterflies were positively associated with various measures of native plant species richness and abundance on plots in undeveloped landscapes (Table 5). Adding houses and landscaping to a treeless southwestern grassland brings increased resources for adult butterflies (water, shade, and nectar) to an otherwise arid and essentially two-dimensional landscape. Therefore, exurban development of grasslands in our study area probably increased habitat quality primarily for adult butterflies. However, exurban parts of our study area also supported an increased variety and abundance of native forbs, perhaps the result of disturbances associated with roads and paths, and these could have been of direct benefit to certain larvae. Therefore, the positive effects of exurban development probably involved both adults and larvae. Increased resources linked to exurbanization apparently more than compensated for the small amounts of native grassland lost to development in the Sonoita Valley, particularly for butterflies that were dietary generalists, multivoltine, and/or relatively immobile. These results are consistent with those from urban and suburban areas, where dietary generalists with multiple broods have fared better than other sorts of butterflies, probably because of their abilities to take advantage of increased floral diversity and longer growing seasons associated with landscaping (Blair and Launer, 1997; Kitahara et al., 2000; Hogsden and Hutchinson, 2004). We found no previous studies that have examined the mobility of butterflies or differences among families in relation to anthropogenic landscapes. In our study area, butterflies with relatively low mobility, mostly in the families Hesperiidae and Lycaenidae, particularly benefited from the addition of houses in grasslands—perhaps because it provided them access to water and nectar resources in closer proximity than was the case in undeveloped landscapes. However, butterfly species richness and abundance did not differ between exurban plots that included a home site and those embedded in exurban developments that did not include a home. This suggests that the benefits of exurban development in grasslands operated at scales beyond the immediate vicinity of the houses. Mesquite savannas in the Sonoita Valley included more water, shade, and structural diversity than grasslands, which may
42
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explain why housing development had little or no impact on butterflies in this habitat. It is more difficult to explain why the same groups of species that apparently benefited from development in grasslands often suffered from it in oak savannas. Housing densities did not differ among the habitats, and there were no obvious features of the built environment in exurban oak savannas that did not occur in grasslands or mesquite. However, insectivorous birds were more than two times more abundant in exurban than in undeveloped parts of the study area, regardless of habitat (unpublished data). There also may have been increased use of pesticides in exurban areas, although we have no evidence this was the case. One untested possibility is that avian predation and/or pesticides negatively impacted butterflies in all exurban areas, but that the additional resources associated with development overcompensated for resulting losses in grasslands but not in oak savannas. Livestock grazing can have major effects on the structure, function, and biological diversity of southwestern grassland and savanna ecosystems (Jemison and Raish, 2000). However, it appears that differences in the height and percentage of vegetative ground cover associated with grazing in our study area had little or no impact on butterflies. Fire is another significant environmental variable in many southwestern ecosystems, and one effect of fire in the grasslands of southeastern Arizona can be a temporary increase in the abundance and variety of forbs (McPherson, 1995). It is possible that the relatively high species richness and abundance of butterflies on our ungrazed and undeveloped grassland plots (Fig. 1) was a consequence of increased abundance of forbs resulting from the April 2002 Ryan wildfire that burned all of these plots 1 year prior to our study. Elsewhere, fires have had positive, negative, or little effects on butterflies, depending upon the nature of the fire and its effects on vegetation, and the sorts of butterflies involved (Swengel, 1996, 2001; Fleishman, 2000). Whatever the effects of the Ryan fire in our study area, it cannot explain the apparent benefits of exurbanization to butterflies in grasslands, because the effect was even stronger in grazed areas (Fig. 1), none of which burned. 4.2. Relationships between butterflies and vegetation Various studies have documented positive correlations between butterfly species richness and/or abundance and the abundance and variety of plants (Thomas and Mallorie, 1985; Ries et al., 2001; Simonson et al., 2001). Implicit in these correlations is the possibility of a cause-and-effect relationship, given butterflies’ well-documented associations with particular larval host plants (Scott, 1986). Our results for undeveloped parts of the Sonoita Valley are consistent with this general pattern, but we found no such correlations between butterflies and vegetation in exurban areas. Much of the positive correlation in undeveloped areas was driven by low species richness of both butterflies and plants in grasslands relative to savannas, whereas species richness of both groups were relatively high in exurban grasslands (Fig. 3). However, plant species richness on our exurban plots still differed by a factor of more than two-fold, and yet there was no sign of any correlation between plants and butterflies among these plots.
The lack of correlation between butterfly and plant species richness and abundance in exurban areas suggests that butterflies in arid environments may be more limited by resources such as nectar and water than by the availability of larval host plants. The uncoupling of associations between butterflies and vegetation in exurban areas indicates that positive correlations elsewhere may not be entirely the result of cause-and-effect relationships. Instead, it could be that butterflies and plants on our undeveloped plots responded positively to a set of shared resources, which in arid environments would include factors such as water and shade associated with topographic heterogeneity. The association between butterfly and plant species richness might then have been weaker in exurban environments, first because mobility would permit butterflies to take advantage of widely scattered resources associated with home sites, and second because of a possible (but as yet undocumented) increased predation/pesticide risk near houses. 4.3. Implications for land use planning Perennial surface waters were much more abundant in the southwestern United States prior to watershed alterations caused by heavy livestock grazing in the 1890s (Bahre, 1991). Exurban development returns surface water and associated resources to otherwise arid places, perhaps creating landscapes similar to those of the early 19th Century. Results of our study show that butterflies can benefit from this circumstance, at least in grasslands. This finding is consistent with other studies documenting the conservation value of low-density rural “countrysides” (Daily et al., 2001; Horner-Devine et al., 2003; Rosenzweig, 2003; Mayfield and Daily, 2005). Housing density in most of the Sonoita Valley is zoned at about 2 ha per home, while actual density in the developed area we studied was about 5.2 ha per home (Sonoita Crossroads Community Forum, 2002). Our results suggest that such a landscape can sustain a rich abundance and variety of butterflies, if lands beyond the immediate vicinity of the home sites are left in natural vegetation. However, losses of native habitat resulting from further development eventually would overwhelm the benefits of increased resources such as water and shade, as demonstrated by various studies of butterflies in urban and suburban areas (Blair and Launer, 1997; Hogsden and Hutchinson, 2004). It remains to be determined just where such a transition might occur in southwestern landscapes, along a gradient from rural to urban housing densities. Acknowledgments We thank M. Donaldson and J. Donaldson, K. Simms, S. McFarlin, and the U.S. Bureau of Land Management for hosting our work on the Empire Ranch, and D. Ruppel, B. Brophy, and A. Gibson for allowing us to work on the Babacomari Ranch. L. Kennedy and B. Branan generously made available the lands and facilities of the Appleton-Whittell Research Ranch. We are grateful to the following individuals for granting access to their individual home sites in exurban parts of the Sonoita Valley: M. Bartol, D. Beal and H. Beal, J. Church, S. Clark and J. Clark, S. Dinham, B. Cook and N. Cook, J. Donaldson and B. Donald-
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
son, M. Donaldson and B. Donaldson, M. Douglas, J. Dowling and E. Dowling, B. Eifrig and G. Eifrig, S. Franklin, T. Hanson and C. Hanson, P. Hoffman and F. Hoffman, M. Johnson, J. Kolbe, V. Michael and J. Michael, D. Reilly, S. Shields, G. Sill, S. Strom and K. Strom, D. Sturges, L. Wilkening, J. Woods, and P. Workizer and K. Workizer. We thank the following individuals for field assistance: B. Audsley, K. Bishop, T. Crook, D. Goodheim, B. Loomis, L. Jones, L. Kennedy, J. MacAdam, A. Marshall, L. Schevets, C. Venot, and C. Wonkka. Y. Linhart and two anonymous reviewers provided helpful comments on the manuscript. This study was supported by the Ecology Program and the Research Experience for Teachers Program of the National Science Foundation. Appendix A Species of butterflies recorded on 48 plots in grasslands and mesquite/oak savannas in the Sonoita Valley of southeastern Arizona, 2003–2004.
Family and species
Mobilitya
Dietary specialist?b
Multiple broods?c
Hesperiidae Amblyscirtes aenus Amblyscirtes eosd Amblyscirtes exoteriad Amblyscirtes nereusd,e Amblyscirtes nysad Amblyscirtes oslarid,e Amblyscirtes texanaef Atrytonopsis edwardsi Cogia hippalusd Copaeodes aurantiacus Erynnis funeralisd Erynnis tristis Hesperia pahaska Hylephila phyleusd,f Lerodea eufalad,f Pholisora catullusd Piruna cingod,e Polites carusf Pyrgus albescensd Pyrgus philetasd Staphylus ceosd Systasea zampad Thorybes drusiusd Thorybes pyladesd
Low Low Low Low Low Low Low Low High High High High High High Low Low Low Low High Low High Low High High
No Yes Yes Yes No Yes Yes Yes Yes No No No No Yes No No Yes Yes No No No No Yes No
No Yes No No Yes No No No Yes Yes Yes Yes No Yes Yes Yes No Yes Yes Yes Yes Yes No No
Papilionidae Battus philenor Papilio polyxenes Papilio multicaudatusd
High High High
Yes No No
Yes Yes No
Pieridae Colias cesoniad Colias eurythemed Eurema mexicanumd Eurema nicippe Eurema proterpia Kricogonia lysided Nathalis ioled Phoebis agarithe Phoebis sennae
High High High High High High High High High
No No No Yes Yes Yes No Yes No
Yes Yes Yes Yes Yes Yes Yes Yes No
43
Appendix A (Continued) Mobilitya
Dietary specialist?b
Multiple broods?c
High High
No No
Yes Yes
Lycaenidae Apodemia mormo Apodemia palmeriie Atlides halesus Brephidium exiled Calephelis arizonensisd,e Euphilotes ritad,f Hemiargus ceraunusd Hemiargus isolad Leptotes marinad Ministrymon ledad Mitoura sivad,f Plebejus acmond Strymon melinusd
Low Low High Low Low Low Low Low High High Low High High
Yes Yes Yes No Yes Yes No No No Yes Yes No No
No Yes Yes Yes Yes No Yes Yes Yes Yes No Yes Yes
Nymphalidae Adelpha bredowiid Agraulis vanillaef Asterocampa celtisd Chlosyne laciniad Cyllopsis pyracmond,e Danaus gilippus Danaus plexippus Dymasia dymase Euptoieta claudia Junonia coenia Junonia nigrosuffusaf Libytheana carinenta Megisto rubricata Poladryas arachne Texola eladae Thessalia theonad,e Vanessa atalantad,f Vanessa cardui Vanessa virginiensisd,f
High High Low High Low High High Low High High High High Low High Low Low High High High
Yes Yes Yes No No Yes Yes Yes No No No Yes No Yes Yes No Yes No No
No Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes No Yes Yes Yes Yes Yes
Family and species Pieris protodiced Pieris rapae
a
Species were divided into two groups, based on extensive personal observations of their relative mobility in the study area and elsewhere in southeastern Arizona. b Dietary specialists were those species whose larval foods were restricted to one or a few closely related plant species, based on information in Stewart et al. (2001), modified in some cases by personal experience in the study area. c Multiple brooded species were those with three or more annual broods, according to Tilden and Smith (1986) and/or personal experience in the study area. d These species were >1.5 times more abundant in exurban than in undeveloped grassland, and/or >1.5 times more abundant in undeveloped than in exurban oak savanna. e These species were found only on undeveloped plots. f These species were found only on exurban plots.
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McPherson, G.R., 1995. The role of fire in desert grasslands. In: McClaran, M.P., Van Devender, T.R. (Eds.), The Desert Grassland. University of Arizona Press, Tucson, pp. 130–151. New, T.R., Pyle, R.M., Thomas, J.A., Thomas, C.D., Hammond, P.C., 1995. Butterfly conservation management. Ann. Rev. Entomol. 40, 57–83. Ries, L., Debinski, D.M., Wieland, M.L., 2001. Conservation value of roadside prairie restoration to butterfly communities. Conserv. Biol. 15, 401– 411. Rosenzweig, M.L., 2003. Win–Win Ecology: How the Earth’s Species can Survive in the Midst of Human Enterprise. Oxford University Press, New York. SAS Publishing, 1999. Statview 5.01. SAS Institute, Cary, NC. Schultz, C.B., Dlugosch, K.M., 1999. Nectar and hostplant scarcity limit populations of an endangered Oregon butterfly. Oecologia 119, 231–238. Scott, J.A., 1986. The Butterflies of North America: A Natural History and Field Guide. Stanford University Press, Stanford, CA. Sengupta, S., Osgood, D.E., 2003. The value of remoteness: a hedonic estimation of ranchette prices. Ecol. Econ. 44, 91–103. Shapiro, A.M., 2002. The Californian urban butterfly fauna is dependent on alien plants. Divers. Distrib. 8, 31–40. Simonson, S.E., Opler, P.A., Stohlgren, T.J., Chong, G.W., 2001. Rapid assessment of butterfly diversity in a montane landscape. Biodiversity and Conserv. 10, 1369–1386. Sonoita Crossroads Community Forum, 2002. A comprehensive plan for northeast Santa Cruz County, Arizona. Sonoita Crossroads Community Forum, Sonoita, AZ. Stewart, B., Brodkin, P., Brodkin, H., 2001. Butterflies of Arizona: A Photographic Guide. West Coast Lady Press, Arcata, CA. Swengel, A.B., 1996. Effects of fire and hay management on abundance of prairie butterflies. Biol. Conserv. 76, 73–85. Swengel, A.B., 2001. A literature review of insect responses to fire, compared to other conservation managements of open habitat. Biodivers. Conserv. 10, 1141–1169. Theobald, D.M., 2004. Placing exurban land-use change in a human modification framework. Frontiers Ecol. Environ. 2, 139–144. Thomas, C.D., Mallorie, H.C., 1985. Rarity, species richness and conservation: butterflies of the Atlas Mountains in Morocco. Biol. Conserv. 33, 95–117. Tilden, J.W., Smith, A.C., 1986. A Field Guide to Western Butterflies. Houghton Mifflin, Boston. Weiss, S.B., Murphy, D.D., White, R.R., 1988. Sun, slope, and butterflies: topographic determinants of habitat quality for Euphydryas editha. Ecology 69, 1486–1496. Wood, P.A., Samways, M.J., 1992. Landscape element pattern and continuity of butterfly flight paths in an ecologically landscaped botanic garden. Biol. Conserv. 58, 149–166. Zar, J.H., 1999. Biostatistical Analysis, 4th ed. Prentice-Hall, Upper Saddle River, NJ.
Butterflies and exurban development in southeastern Arizona Carl E. Bock a,∗ , Richard A. Bailowitz b , Douglas W. Danforth c , Zach F. Jones d , Jane H. Bock a a
Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA b 1331 West Emerine Drive, Tucson, AZ 85704, USA c P.O. Box 232, Bisbee, AZ 85603, USA d Department of Biology, Colorado College, Colorado Springs, CO 80903, USA Received 2 March 2006; received in revised form 17 April 2006; accepted 29 May 2006 Available online 3 July 2006
Abstract Ranches are being converted to exurban housing developments in the southwestern United States, with potentially significant impacts on biological diversity. We surveyed butterflies on 48 plots in grasslands, mesquite savannas, and oak savannas in southeastern Arizona that were grazed by livestock, embedded in low-density housing developments, or both, or neither. Results suggest that livestock grazing had little impact on butterfly species richness or abundance, while exurban development had minor impacts compared to negative effects that have been documented elsewhere in more fully developed urban and suburban landscapes. However, our data indicate that conversion of ranchland to exurban development has not been without consequence to butterflies. First, relatively immobile species with multiple broods and/or generalized diets were positively associated with development in grasslands, unaffected in mesquite savannas, and often negatively associated with development in oak savannas. Second, while abundance and variety of butterflies were positively correlated with plant species richness and cover in undeveloped landscapes, such correlations were not present in exurban areas. These results suggest that increased resources associated with housing development, including water, shade, and nectar, and possible negative impacts of increased avian predation and pesticide use, caused relationships between butterflies and native vegetation to be less tightly coupled in exurban than in undeveloped landscapes. © 2006 Elsevier B.V. All rights reserved. Keywords: Grassland; Lepidoptera; Livestock grazing; Mesquite; Oak; Rural
1. Introduction Throughout much of the American West, ranchlands are being converted into low-density housing developments that are not part of urban centers (Knight et al., 1995). This widespread land use change, termed exurbanization, has profound implications for regional biological diversity (Theobald, 2004; Hansen and Brown, 2005). However, the ecological consequences of exurbanization have been much less studied than the effects of human population growth in urban and suburban areas (Hansen et al., 2005; but see Maestas et al., 2003). A confounding fac-
∗ Corresponding author at: 5224 Lighthouse Point Court, Loveland, CO 80537, USA. Tel.: +1 970 593 0343; fax: +1 970 593 0347. E-mail addresses: [email protected] (C.E. Bock), [email protected] (R.A. Bailowitz), [email protected] (D.W. Danforth), [email protected] (Z.F. Jones), [email protected] (J.H. Bock).
0169-2046/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.landurbplan.2006.05.003
tor in evaluating the effects of exurban development is that it does not always exempt landscapes from the effects of grazing, because many exurbanites keep livestock (especially horses) on their “ranchettes” (Maestas et al., 2002; Sengupta and Osgood, 2003). Butterflies (Lepidoptera: Papilionoidea and Hesperioidea) are charismatic and familiar insects of longstanding interest to conservation biologists (New et al., 1995). Patterns of butterfly abundance and diversity have been described for urban and suburban environments (Blair and Launer, 1997; Shapiro, 2002; Collinge et al., 2003; Hogsden and Hutchinson, 2004), but we are aware of no studies that have measured butterfly responses to exurban development. The Sonoita Valley of southeastern Arizona is a mixture of grassland and oak/mesquite savanna where some cattle ranches have been converted into low-density exurban housing developments (Bock and Bock, 2000). We surveyed butterflies during two summers (2003–2004) on 48 plots in the valley that were grazed by livestock, or embedded in exurban housing developments, or both, or neither. The objective of our
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
study was to examine the independent and interactive effects of livestock grazing and exurban development on butterfly species richness and abundance. 2. Methods 2.1. Study area and sampling design The Sonoita Valley lies at 1300–1600 m elevation, between the Santa Rita and Huachuca Mountains, in Santa Cruz County, Arizona (31◦ 33–44 N, 110◦ 29–42 W). Predominant ground cover includes a variety of perennial bunchgrasses in the genera Bouteloua, Aristida, Eragrostis, and Hilaria interspersed with scattered low shrubs and succulents and a large variety of forbs (McLaughlin et al., 2001). Certain parts of the valley are treeless, whereas others are savannas with scattered mesquite (Prosopis velutina), or savannas with Emory oak (Quercus emoryi) and Arizona white oak (Quercus arizonica). Vegetation of the study area is dominated by native species, although two species of African lovegrasses (Eragrostis spp.) have become locally common in some parts of the valley. Temperatures vary from a mean January daily minimum of −3.0 ◦ C to a mean June daily maximum of 32.6 ◦ C; annual precipitation is 43.6 cm, about 60% of which falls during the summer monsoon period, from early July to early September (McLaughlin et al., 2001). An analysis conducted in 2000 showed that the Sonoita Valley consisted of about 50% private and 50% public land, the latter including areas managed by the U.S. Department of Agriculture Forest Service, the U.S. Department of Interior Bureau of Land Management, and the state of Arizona (Sonoita Crossroads Community Forum, 2002). Of the private lands, about 77% were cattle ranches, and the remainder consisted mostly of exurban single family housing developments and small commercial centers in the unincorporated towns of Elgin and Sonoita. Housing density in the exurban developments where we worked averaged about 1 home/5.2 h. In the fall of 2002 we established 48 200 m diameter plots in the valley, distributed over an area of about 480 km2 . The average distance from the center of each plot to that of its nearest neighbor was 750 m (range: 325–2000 m). We established the plots in a balanced experimental design, with 12 replicates in each of four landscape and land-use categories: (1) on undeveloped ranches grazed by cattle and a few horses (the Babacomari and Empire ranches), (2) on the undeveloped Appleton-Whittell Research Ranch, a sanctuary of the National Audubon Society that has been ungrazed since 1968 (Bock and Bock, 2000), (3) on exurban properties whose owners did not keep livestock, and (4) on exurban properties whose owners grazed small numbers of either horses (six sites), cattle (four sites), both cattle and horses (one site), or sheep (one site). We further balanced our sampling design such that each group of 12 plots included four in open grassland, four in mesquite savanna, and four in oak savanna. Finally, 12 of the 24 exurban plots included at least one house, while the remaining 12 were embedded in developments but did not include any houses. Landscaping was restricted to the immediate vicinity of home sites, so most of the land in the exurban
35
neighborhoods supported natural grassland and savanna vegetation. A large wildfire (the Ryan Fire) burned about 85% of the Research Ranch and parts of the adjacent Babacomari Ranch in April of 2002, 1 year prior to the start of our butterfly surveys. Because of the extent and location of this burn, we had to place 8 of the 12 plots on the Research Ranch, including all four grassland plots and three of four mesquite plots, in areas that had burned in this fire. While vegetation had substantially recovered prior to the start of our surveys in May 2003, lack of pre-burn data made it impossible for us to quantify possible residual fire effects on butterflies. 2.2. Butterfly surveys We surveyed butterflies four times on each plot, during the last week of May and last week of July 2003, and during the last week of July and second week of September 2004. The two co-authors with extensive field experience with butterflies in southeastern Arizona (RAB and DWD) worked together on each survey, walking systematically through all parts of each 200 m diameter circle, counting and identifying all individuals to species. Each survey occurred between mid-morning and early afternoon of a relatively calm, clear, and warm day, and took approximately 30 min. We make no assumption that results represented actual densities, or that all species present were detected. However, because effort was equivalent for all plots over the two summers, we have interpreted count results as accurately reflecting differences in relative abundance and relative species richness among landscapes and habitats. Categorization of the different butterfly species in terms of number of broods per year (voltinism) and degree of larval dietary specialization were based on information provided by Tilden and Smith (1986) and Stewart et al. (2001), confirmed and in some cases modified by personal experience in the study area and elsewhere in southeastern Arizona. Mobility is another attribute important to consider in any study of butterfly landscape ecology. We found no published indices for mobility of the species in our study area, and possible surrogates such as wing or body size have proven unsatisfactory elsewhere (Cowley et al., 2001). Therefore, we adopted the approach used in previous studies (Cowley et al., 2001; Komonen et al., 2004), and ranked each of our study species in terms of relative mobility based on personal experience in the study area. 2.3. Vegetation measurements Between January and March of 2003, we visually estimated canopy cover of mesquite and/or oak trees to the nearest 10% on forty 10 m diameter circles spaced at 20 m intervals along eight 100 m transects radiating at 45◦ intervals from each plot center point. We averaged results from the 40 circles to generate a single tree canopy coverage estimate for each of the 48 plots. In July–August of 2003 and 2004, we sampled ground vegetation at 400 points spaced at 1 m intervals along four 100 m transects radiating in cardinal compass directions from each plot center point. We recorded the presence and identity of any grass,
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forb, succulent, or shrub whose canopy covered each point, and averaged these frequency data to generate percent canopy cover estimates for grasses, forbs, shrubs and succulents, and bare ground on each plot. We collected data only in areas of natural vegetation by making adjustments in the locations of both tree canopy and ground vegetation sampling transects in those few cases where they would have crossed structures or landscaping on some of the exurban plots. Cover data for planted non-native vegetation would have been valuable as well, but landowners were understandably reluctant to permit us to obtain such information by trampling on their landscaping. 2.4. Data analyses Butterfly species richness was calculated as the number of species detected cumulatively during the four surveys on each plot. We estimated relative abundance of species in two ways, first as the average number counted during the four sampling events, and second as the maximum number on any one of the four surveys. We then summed these values for each family
Fig. 1. Mean (+S.E.) species richness and abundance of butterflies on 48 plots in southeastern Arizona, that were in exurban vs. undeveloped landscapes, grazed vs. ungrazed by livestock, and in three different habitats. There was a significant interaction between landscape and habitat for species richness, it being positively related to development in grasslands, and negatively related to development in oak savannas (see Table 1).
and for butterflies as a whole. Results of the two abundance estimates were highly positively correlated for all species combined (r = 0.963, n = 48, P < 0.0001), and for each family individually (r > 0.90 in each case). We chose to present and analyze average rather than maximum count data, because they more accurately reflected species population sizes over the 2-year duration of the study. However, given the high correlations between the two abundance indices, results would have been the same using either approach. We square-root transformed count data prior to all analyses, which is an appropriate way of reducing heterogeneity of variances (Zar, 1999). Resulting abundance and species richness data were sufficiently homogeneous to permit analyses of variance, given our balanced experimental design (Zar, 1999). We compared butterfly abundance and species richness among plots that were exurbanized versus undeveloped (n = 24 per treatment), grazed versus ungrazed (n = 24 per treatment), and for which habitats were either grass or mesquite savanna or oak savanna (n = 16 per habitat), using three-way analyses of variance. We also used three-way analysis of variance to compare butterfly abundance and richness between exurban plots that included at least one house (n = 12) with those exurban plots that did not include houses (n = 12), with grazing and habitat as the second and third independent variables.
Fig. 2. Same as Fig. 1, but for exurban plots only, with landscape difference being presence vs. absence of homes on the plots. There were no significant differences related to landscape type, presence vs. absence of grazing, or habitat type (see Table 1).
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
Presence versus absence of livestock grazing had no statistically significant relationships to butterfly abundance or species richness, either independently or interactively with habitat or exurban development. Therefore, for clarity of data presentation and analysis, we limited comparisons of the different butterfly families and ecological groupings, and of plot vegetation, to the landscape and habitat variables alone, using two-way analyses of variance. We computed product moment correlations (r) to quantify the strengths of relationships between butterfly abundance and species richness and various measures of plant species richness and vegetative canopy cover. We computed chi-square contingency (χ2 ) statistics to test for independence of attributes among the different butterfly species, including dietary specialization, mobility, voltinism, and family membership. All statistical tests were performed in Statview 5.0.1 (SAS Publishing, 1999), with P < 0.05 considered significant, and 0.05 < P < 0.10 as marginally significant. 3. Results 3.1. Butterfly species richness and abundance We recorded 70 butterfly species in the Sonoita Valley (Appendix A). Species richness was unrelated to presence ver-
37
sus absence of livestock grazing, to exurban versus undeveloped landscapes, or to habitat type taken as individual variables (Fig. 1, Table 1). However, there was a significant interaction between landscape and habitat, attributable to the fact that species richness was positively associated with development in grasslands, but negatively associated with development in oak savannas, with mesquite savanna intermediate (Fig. 1). Butterfly abundance was unrelated to livestock grazing or landscape context, but it differed among habitats, being higher in mesquite than in grassland or oak (Fig. 1, Table 1). The interaction between landscape and habitat was not significant (Table 1), but the trend was similar to that for species richness, with abundance being higher in exurban than in undeveloped grasslands, but higher in undeveloped than in exurban oak savannas (Fig. 1). Both average species richness and abundance were higher in ungrazed than in grazed grasslands in undeveloped landscapes (Fig. 1). However, these apparent differences did not approach statistical significance, even in one-way analyses of variance with grazing as the lone independent variable (species richness: F1,6 = 0.95, P = 0.37; abundance: F1,6 = 2.41, P = 0.17). There were no differences in butterfly species richness or abundance between exurban plots that included at least one home site versus those embedded in exurban developments that did not include houses, regardless of habitat or livestock grazing (Fig. 2, Table 1). This was despite the fact that the butterfly
Table 1 Results of three-way analysis of variance of data on butterfly species richness and abundance: (1) on 48 plots in southeastern Arizona that differed in terms of presence vs. absence of exurban development, presence vs. absence of livestock, and habitat type (Fig. 1), and (2) on the 24 exurban plots that differed in terms of presence vs. absence of homes on the plots (Fig. 2) Comparison
Dependent variable
Independent variable
d.f.
F (P)
Species richness
A. Exurbanization B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.40 (0.532) 0.42 (0.535) 1.23 (0.304) 0.33 (0.570) 4.67 (0.016) 0.18 (0.833) 1.04 (0.363)
Abundancea
A. Exurbanization B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.16 (0.690) 1.39 (0.246) 3.83 (0.031) 0.01 (0.959) 2.32 (0.113) 1.17 (0.322) 0.72 (0.492)
Species richness
A. Homes on plots B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
0.10 (0.753) 0.84 (0.378) 0.72 (0.506) 0.75 (0.405) 0.01 (0.998) 0.32 (0.731) 0.83 (0.459)
Abundancea
A. Homes on plots B. Grazing C. Habitat A×B A×C B×C A×B×C
1 1 2 1 2 2 2
1.57 (0.335) 1.69 (0.218) 2.70 (0.108) 0.02 (0.897) 0.18 (0.841) 0.57 (0.580) 1.28 (0.314)
Exurban vs. undeveloped areas
Exurban plots with vs. without homes
a
Abundance data square-root transformed prior to analysis.
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surveys included landscaped areas when there were homes on the plots. 3.2. Differences among butterfly groups The distribution and abundance of butterflies in the Sonoita Valley appeared to be positively affected by development in grasslands, but negatively affected by development in oak savannas, with mesquite intermediate (Fig. 1). Forty-one of the 70 species contributed to this general pattern by being at least 1.5 times more abundant in exurban than in undeveloped grasslands, or 1.5 times more abundant in undeveloped than in exurban oak savannas, or both (Appendix A). These 41 species were nonrandomly distributed among butterfly families and among other groupings based on natural history. First, the pattern was significant for both species richness and abundance of the families Hesperiidae and Lycaenidae, but not for the families Papilionidae, Pieridae, or Nymphalidae (Table 2). Second, the pattern was strongly developed among 28 less mobile butterflies as a group (Table 3), but it was only marginally present among 42 relatively mobile species. Third, the affinity for exurban grass-
lands, coupled with avoidance of development in oak savannas, was stronger among 52 species with three or more broods per year than among 18 species with only one or two broods per year (Table 3). Finally, species richness and abundance of 37 dietary generalists fit the pattern, but there were no significant interactions between landscape and habitat for 33 dietary specialists whose larvae are known to feed on only one or a group of closely related plant species (Table 3). Butterfly mobility was statistically independent of either voltinism (Appendix A; χ2 = 2.44, d.f. = 1, P = 0.12) or dietary breadth (χ2 = 1.87, d.f. = 1, P = 0.17). However, there was a marginally significant positive association between voltinism and diet breadth, with 31 of 37 diet generalists having three or more broods, but only 21 of 33 diet specialists having three or more broods (Appendix A; χ2 = 3.71, d.f. = 1, P = 0.053). Family membership was independent of voltinism (χ2 = 5.91, d.f. = 4, P = 0.20) or diet breadth (χ2 = 1.22, d.f. = 4, P = 0.87), but species in the families Pieridae, Papilionidae, and Nymphalidae were more likely to be highly mobile than species of Hesperiidae or Lycaenidae (see Appendix A; χ2 = 15.77, d.f. = 4, P = 0.003).
Table 2 Mean (S.E.) and results of two-way analysis of variance comparing species richness and abundance per survey for four butterfly families on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for different effectsa
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Hesperiidae Richness
Undeveloped Exurban
2.6 (0.7) 5.1 (0.7)
4.9 (0.8) 5.1 (0.8)
7.4 (1.1) 5.0 (0.9)
0.03 (ns)b
3.65 (0.035)
4.06 (0.024)
Abundance
Undeveloped Exurban
1.4 (0.6) 3.6 (0.7)
2.8 (0.9) 3.2 (0.9)
4.9 (1.0) 3.1 (0.8)
0.52 (ns)
2.17 (ns)
3.73 (0.032)
Richness
Undeveloped Exurban
2.4 (0.7) 6.0 (0.2)
4.6 (0.7) 3.6 (0.4)
4.3 (0.5) 2.5 (0.5)
0.47 (ns)
1.51 (ns)
15.62 (0.001)
Abundance
Undeveloped Exurban
3.8 (2.2) 7.2 (1.1)
10.8 (1.7) 9.4 (2.1)
7.3 (1.5) 2.5 (0.5)
0.06 (ns)
7.36 (0.002)
6.82 (0.003)
Richness
Undeveloped Exurban
0.4 (0.2) 0.5 (0.3)
0.6 (0.3) 1.3 (0.2)
0.8 (0.3) 0.8 (0.3)
1.42 (ns)
1.93 (ns)
0.83 (ns)
Abundance
Undeveloped Exurban
0.2 (0.1) 0.3 (0.1)
0.7 (0.3) 1.7 (0.6)
0.3 (0.1) 0.3 (0.2)
2.70 (ns)
7.52 (0.002)
2.04 (ns)
Richness
Undeveloped Exurban
6.5 (0.6) 6.6 (0.2)
5.9 (0.5) 6.1 (0.5)
7.1 (0.7) 5.6 (0.6)
0.77 (ns)
0.60 (ns)
1.73 (ns)
Abundance
Undeveloped Exurban
8.4 (1.4) 13.3 (1.5)
13.5 (3.1) 13.6 (1.4)
11.6 (2.4) 11.5 (3.0)
1.28 (ns)
0.90 (ns)
1.14 (ns)
Richness
Undeveloped Exurban
3.6 (0.4) 4.3 (0.6)
4.3 (0.6) 4.9 (0.4)
5.1 (0.7) 5.1 (0.4)
0.92 (ns)
2.48 (0.10)
0.23 (ns)
Abundance
Undeveloped Exurban
7.8 (2.2) 5.9 (1.0)
8.7 (1.3) 8.7 (1.5)
6.8 (1.2) 6.3 (1.1)
0.41 (ns)
1.60 (ns)
0.13 (ns)
Lycaenidae
Papilionidae
Pieridae
Nymphalidae
a b
d.f.: 1, 2, 2, 42 for each ANOVA; abundance data were square-root transformed prior to analysis. ns: not statistically significant (P > 0.10).
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
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Table 3 Mean (S.E.) and results of two-way analysis of variance comparing species richness and abundance per survey for butterfly groups with different attributes, on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for different effectsa
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Low mobility Richness
Undeveloped Exurban
2.1 (0.7) 5.6 (0.7)
5.3 (1.1) 4.5 (0.7)
6.1 (1.1) 3.4 (0.5)
0.60 (ns)b
0.91 (ns)
7.78 (0.001)
Abundance
Undeveloped Exurban
2.1 (1.3) 4.4 (0.6)
4.4 (1.3) 2.9 (0.7)
4.1 (1.1) 2.8 (0.6)
0.48 (ns)
0.35 (ns)
3.82 (0.030)
Richness
Undeveloped Exurban
13.4 (1.2) 16.6 (1.1)
15.3 (1.5) 16.5 (1.3)
18.5 (1.6) 15.5 (1.5)
0.19 (ns)
1.11 (ns)
2.80 (0.072)
Abundance
Undeveloped Exurban
18.7 (4.2) 26.2 (2.4)
31.2 (4.7) 33.7 (3.1)
26.7 (4.1) 20.2 (4.3)
0.23 (ns)
4.49 (0.017)
2.53 (0.092)
Richness
Undeveloped Exurban
2.3 (0.4) 3.2 (0.5)
2.6 (0.6) 2.6 (0.3)
5.3 (0.7) 4.4 (0.7)
0.01 (ns)
10.13 (0.001)
Abundance
Undeveloped Exurban
4.3 (1.3) 3.3 (0.7)
4.1 (1.1) 5.8 (1.1)
6.5 (1.0) 5.9 (2.2)
0.00 (ns)
1.39 (ns)
0.78 (ns)
Richness
Undeveloped Exurban
9.3 (1.5) 14.6 (1.1)
13.6 (1.8) 14.5 (1.4)
14.4 (1.4) 10.8 (1.3)
0.57 (ns)
1.18 (ns)
5.01 (0.011)
Abundance
Undeveloped Exurban
17.3 (5.1) 26.8 (3.0)
32.2 (4.4) 30.6 (3.2)
24.1 (4.5) 17.7 (2.7)
0.19 (ns)
4.95 (0.012)
2.89 (0.067)
Richness
Undeveloped Exurban
4.6 (0.6) 6.3 (0.7)
6.1 (1.0) 6.6 (0.8)
8.1 (1.4) 6.4 (1.0)
0.03 (ns)
1.74 (ns)
1.46 (ns)
Abundance
Undeveloped Exurban
3.9 (0.9) 4.7 (1.2)
5.9 (1.1) 7.5 (1.8)
6.1 (1.6) 6.0 (1.6)
0.20 (ns)
1.65 (ns)
0.14 (ns)
Richness
Undeveloped Exurban
10.9 (1.3) 16.0 (0.7)
14.1 (1.4) 14.4 (0.8)
16.5 (1.2) 12.4 (1.2)
0.22 (ns)
0.46 (ns)
8.16 (0.001)
Abundance
Undeveloped Exurban
17.7 (5.3) 25.6 (2.6)
30.6 (4.2) 29.1 (3.0)
24.7 (3.6) 18.3 (3.8)
0.05 (ns)
3.93 (0.027)
2.86 (0.069)
High mobility
Broods 1–2 1.48 (ns)
Broods > 2
Diet specialists
Diet generalists
a b
d.f.: 1, 2, 2, 42 for each ANOVA; abundance data were square-root transformed prior to analysis. ns: not statistically significant (P > 0.10).
3.3. Plot vegetation
3.4. Correlations between butterflies and vegetation
Plant species richness was highest in oak savanna and lowest in grassland, with mesquite savanna intermediate (Table 4). Plant species richness was higher in exurban mesquite and grassland plots compared to their undeveloped counterparts, a marginally significant effect attributable to differences in species richness of forbs rather than of grasses or woody plants (Table 4). Percent canopy cover of grasses, shrubs and succulents, and trees did not differ between exurban and undeveloped landscapes, whereas forb cover was marginally significantly higher in exurban landscapes (Table 4). Percent canopy of exotic lovegrasses (Eragrostis spp.) averaged <5% across all 48 plots, while no other exotic species comprised >1% canopy. There were no differences among landscape, grazing, or habitat categories in canopy cover of any exotic plant species.
Butterfly species richness was positively correlated with four measures of plant species richness, and with percent canopy cover of trees and forbs, among plots in undeveloped landscapes (Table 5). However, there were no correlations between butterfly species richness and any of the same vegetation measurements on exurban plots. Correlations between butterfly abundance and vegetation generally were weaker than those involving butterfly species richness, but those that were significant (P < 0.05) or marginally significant (P < 0.10) all occurred among undeveloped plots (Table 5). The positive correlation between butterfly species richness and total plant species richness in undeveloped landscapes was driven by low richness values in grasslands and relatively high values in oak woodlands, with mesquite intermediate (Fig. 3). By
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C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
Table 4 Mean (S.E.) and results of two-way analysis of variance comparing various measures of plant species richness and vegetative canopy cover, on 48 plots evenly divided among three habitats and two landscape settings in the Sonoita Valley of southeastern Arizona Dependent variable
Landscape
F (P) for independent variablea
Mean (S.E.) per habitat Grassland
Mesquite
Oak
Landscape
Habitat
Interaction
Plant species richness All species
Undeveloped Exurban
46.5 (3.3) 55.8 (3.9)
56.3 (2.8) 66.1 (4.3)
76.9 (2.9) 73.4 (1.8)
3.81 (0.058)
27.15 (0.001)
2.66 (0.082)
Grasses
Undeveloped Exurban
16.6 (1.2) 15.3 (1.0)
18.5 (1.1) 19.3 (1.2)
23.1 (1.3) 22.1 (1.8)
0.26 (ns)b
13.47 (0.001)
0.39 (ns)
Forbs
Undeveloped Exurban
25.4 (2.5) 37.1 (2.9)
31.4 (2.6) 38.4 (4.1)
41.1 (2.6) 41.3 (2.9)
6.74 (0.013)
5.74 (0.006)
1.94 (ns)
Woody or succulent
Undeveloped Exurban
4.5 (0.7) 3.4 (1.1)
6.4 (0.8) 8.5 (1.4)
12.6 (1.4) 10.0 (1.8)
0.28 (ns)
17.33 (0.001)
1.88 (ns)
Grasses
Undeveloped Exurban
76.5 (2.6) 83.5 (7.7)
69.6 (2.7) 76.2 (7.0)
76.0 (4.9) 80.3 (1.9)
2.14 (ns)
1.09 (ns)
0.05 (ns)
Forbs
Undeveloped Exurban
16.9 (2.5) 26.5 (3.4)
20.4 (4.3) 27.6 (3.4)
20.0 (2.4) 22.5 (4.3)
2.95 (0.093)
0.20 (ns)
0.31 (ns)
Shrubs and succulents
Undeveloped Exurban
2.8 (0.8) 4.0 (1.3)
3.4 (1.0) 5.8 (2.1)
6.0 (1.3) 8.7 (2.5)
2.53 (ns)
3.10 (0.054)
0.12 (ns)
Trees
Undeveloped Exurban
0.0 (0.0) 0.3 (0.3)
5.1 (1.3) 5.5 (0.9)
18.1 (2.4) 13.5 (2.8)
0.94 (ns)
47.46 (0.001)
1.57 (ns)
Vegetative canopy (%)
a b
d.f.: 1, 2, 2, 42 for each ANOVA. ns: not statistically significant (P > 0.10).
contrast, most grassland plots in exurban landscapes supported relatively high numbers of both butterfly and plant species, such that there was no correlation between these variables across the exurban plots as a whole (Fig. 3).
4. Discussion Results of our study suggest that low-density exurban housing developments in the Sonoita Valley had relatively minor
Table 5 Product-moment correlations (and associated probabilities) between butterfly species richness or butterfly abundance, and various measures of plant species richness and vegetative canopy cover, on 24 grassland and mesquite/oak savanna plots in the exurbanized portions of Sonoita Valley of southeastern Arizona, and on 24 plots in undeveloped portions of the valley, in 2003–2004 Butterfly variable
Environmental variable
Exurban
Undeveloped
Species richness
Plant species richness Grass species richness Forb species richness Woody plant species richness Percent bare ground Percent grass canopy Percent forb canopy Percent shrub/succulent canopy Percent tree canopy
0.04 (ns)a 0.05 (ns) 0.04 (ns) 0.02 (ns) 0.02 (ns) 0.01 (ns) −0.10 (ns) 0.17 (ns) 0.09 (ns)
0.57 (0.004) 0.57 (0.004) 0.37 (0.072) 0.59 (0.002) 0.10 (ns) −0.12 (ns) 0.42 (0.041) 0.18 (ns) 0.59 (0.002)
Total countb
Plant species richness Grass species richness Forb species richness Woody plant species richness Percent bare ground Percent grass canopy Percent forb canopy Percent shrub/succulent canopy Percent tree canopy
0.01 (ns) −0.21 (ns) 0.19 (ns) −0.14 (ns) −0.19 (ns) 0.07 (ns) 0.14 (ns) −0.06 (ns) −0.11(ns)
0.38 (0.065) 0.39 (0.063) 0.30 (ns) 0.29 (ns) 0.13 (ns) −0.35 (0.093) 0.56 (0.004) −0.05 (ns) 0.22 (ns)
a b
ns: not statistically significant (P > 0.10). Count data square-root transformed prior to calculation of correlation coefficients.
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
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ness and cover in undeveloped landscapes, but these correlations were not present in exurban areas. We consider possible explanations for these two patterns in the following sections. 4.1. Positive and negative impacts of exurban development
Fig. 3. Scatter plots of butterfly species richness vs. plant species richness in three different habitats, on 24 plots in southeastern Arizona that were in undeveloped landscapes, and on 24 plots in exurban landscapes.
impacts on the abundance and variety of butterflies, compared to the negative effects of habitat loss and landscape alterations that have been documented for butterflies in highly developed urban and suburban areas (Blair and Launer, 1997; Hogsden and Hutchinson, 2004; Koh and Sodhi, 2004). Nevertheless, two general patterns in our data suggest that conversion of ranchland to exurban development in the Sonoita Valley has not been without consequence to butterflies. First, there were significant exurban effects involving interactions between habitat and type of butterfly, independent of livestock grazing. Relatively immobile species with multiple broods and/or generalized diets usually were strongly positively associated with development in grasslands, unaffected in mesquite savannas, and often negatively associated with development in oak savannas. Richness and abundance of other sorts of butterflies differed little between exurban and undeveloped landscapes. The second general pattern is that both abundance and variety of butterflies were positively correlated with various measures of plant species rich-
Butterfly habitat requirements include food plants for larvae that often are quite specific (Scott, 1986), and a different suite of more general but no less important resources for adults, including shade, water, and especially nectar (Weiss et al., 1988; Scott, 1986; Wood and Samways, 1992; Schultz and Dlugosch, 1999; Fleishman et al., 2005). Our study involved surveys of adult butterflies. However, their abundances on our plots likely reflected larval as well as adult habitat requirements, to the degree that adult presence was linked to the larval host plants on which they emerged and to which females returned to lay their eggs. This may explain in part why numbers and variety of adult butterflies were positively associated with various measures of native plant species richness and abundance on plots in undeveloped landscapes (Table 5). Adding houses and landscaping to a treeless southwestern grassland brings increased resources for adult butterflies (water, shade, and nectar) to an otherwise arid and essentially two-dimensional landscape. Therefore, exurban development of grasslands in our study area probably increased habitat quality primarily for adult butterflies. However, exurban parts of our study area also supported an increased variety and abundance of native forbs, perhaps the result of disturbances associated with roads and paths, and these could have been of direct benefit to certain larvae. Therefore, the positive effects of exurban development probably involved both adults and larvae. Increased resources linked to exurbanization apparently more than compensated for the small amounts of native grassland lost to development in the Sonoita Valley, particularly for butterflies that were dietary generalists, multivoltine, and/or relatively immobile. These results are consistent with those from urban and suburban areas, where dietary generalists with multiple broods have fared better than other sorts of butterflies, probably because of their abilities to take advantage of increased floral diversity and longer growing seasons associated with landscaping (Blair and Launer, 1997; Kitahara et al., 2000; Hogsden and Hutchinson, 2004). We found no previous studies that have examined the mobility of butterflies or differences among families in relation to anthropogenic landscapes. In our study area, butterflies with relatively low mobility, mostly in the families Hesperiidae and Lycaenidae, particularly benefited from the addition of houses in grasslands—perhaps because it provided them access to water and nectar resources in closer proximity than was the case in undeveloped landscapes. However, butterfly species richness and abundance did not differ between exurban plots that included a home site and those embedded in exurban developments that did not include a home. This suggests that the benefits of exurban development in grasslands operated at scales beyond the immediate vicinity of the houses. Mesquite savannas in the Sonoita Valley included more water, shade, and structural diversity than grasslands, which may
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explain why housing development had little or no impact on butterflies in this habitat. It is more difficult to explain why the same groups of species that apparently benefited from development in grasslands often suffered from it in oak savannas. Housing densities did not differ among the habitats, and there were no obvious features of the built environment in exurban oak savannas that did not occur in grasslands or mesquite. However, insectivorous birds were more than two times more abundant in exurban than in undeveloped parts of the study area, regardless of habitat (unpublished data). There also may have been increased use of pesticides in exurban areas, although we have no evidence this was the case. One untested possibility is that avian predation and/or pesticides negatively impacted butterflies in all exurban areas, but that the additional resources associated with development overcompensated for resulting losses in grasslands but not in oak savannas. Livestock grazing can have major effects on the structure, function, and biological diversity of southwestern grassland and savanna ecosystems (Jemison and Raish, 2000). However, it appears that differences in the height and percentage of vegetative ground cover associated with grazing in our study area had little or no impact on butterflies. Fire is another significant environmental variable in many southwestern ecosystems, and one effect of fire in the grasslands of southeastern Arizona can be a temporary increase in the abundance and variety of forbs (McPherson, 1995). It is possible that the relatively high species richness and abundance of butterflies on our ungrazed and undeveloped grassland plots (Fig. 1) was a consequence of increased abundance of forbs resulting from the April 2002 Ryan wildfire that burned all of these plots 1 year prior to our study. Elsewhere, fires have had positive, negative, or little effects on butterflies, depending upon the nature of the fire and its effects on vegetation, and the sorts of butterflies involved (Swengel, 1996, 2001; Fleishman, 2000). Whatever the effects of the Ryan fire in our study area, it cannot explain the apparent benefits of exurbanization to butterflies in grasslands, because the effect was even stronger in grazed areas (Fig. 1), none of which burned. 4.2. Relationships between butterflies and vegetation Various studies have documented positive correlations between butterfly species richness and/or abundance and the abundance and variety of plants (Thomas and Mallorie, 1985; Ries et al., 2001; Simonson et al., 2001). Implicit in these correlations is the possibility of a cause-and-effect relationship, given butterflies’ well-documented associations with particular larval host plants (Scott, 1986). Our results for undeveloped parts of the Sonoita Valley are consistent with this general pattern, but we found no such correlations between butterflies and vegetation in exurban areas. Much of the positive correlation in undeveloped areas was driven by low species richness of both butterflies and plants in grasslands relative to savannas, whereas species richness of both groups were relatively high in exurban grasslands (Fig. 3). However, plant species richness on our exurban plots still differed by a factor of more than two-fold, and yet there was no sign of any correlation between plants and butterflies among these plots.
The lack of correlation between butterfly and plant species richness and abundance in exurban areas suggests that butterflies in arid environments may be more limited by resources such as nectar and water than by the availability of larval host plants. The uncoupling of associations between butterflies and vegetation in exurban areas indicates that positive correlations elsewhere may not be entirely the result of cause-and-effect relationships. Instead, it could be that butterflies and plants on our undeveloped plots responded positively to a set of shared resources, which in arid environments would include factors such as water and shade associated with topographic heterogeneity. The association between butterfly and plant species richness might then have been weaker in exurban environments, first because mobility would permit butterflies to take advantage of widely scattered resources associated with home sites, and second because of a possible (but as yet undocumented) increased predation/pesticide risk near houses. 4.3. Implications for land use planning Perennial surface waters were much more abundant in the southwestern United States prior to watershed alterations caused by heavy livestock grazing in the 1890s (Bahre, 1991). Exurban development returns surface water and associated resources to otherwise arid places, perhaps creating landscapes similar to those of the early 19th Century. Results of our study show that butterflies can benefit from this circumstance, at least in grasslands. This finding is consistent with other studies documenting the conservation value of low-density rural “countrysides” (Daily et al., 2001; Horner-Devine et al., 2003; Rosenzweig, 2003; Mayfield and Daily, 2005). Housing density in most of the Sonoita Valley is zoned at about 2 ha per home, while actual density in the developed area we studied was about 5.2 ha per home (Sonoita Crossroads Community Forum, 2002). Our results suggest that such a landscape can sustain a rich abundance and variety of butterflies, if lands beyond the immediate vicinity of the home sites are left in natural vegetation. However, losses of native habitat resulting from further development eventually would overwhelm the benefits of increased resources such as water and shade, as demonstrated by various studies of butterflies in urban and suburban areas (Blair and Launer, 1997; Hogsden and Hutchinson, 2004). It remains to be determined just where such a transition might occur in southwestern landscapes, along a gradient from rural to urban housing densities. Acknowledgments We thank M. Donaldson and J. Donaldson, K. Simms, S. McFarlin, and the U.S. Bureau of Land Management for hosting our work on the Empire Ranch, and D. Ruppel, B. Brophy, and A. Gibson for allowing us to work on the Babacomari Ranch. L. Kennedy and B. Branan generously made available the lands and facilities of the Appleton-Whittell Research Ranch. We are grateful to the following individuals for granting access to their individual home sites in exurban parts of the Sonoita Valley: M. Bartol, D. Beal and H. Beal, J. Church, S. Clark and J. Clark, S. Dinham, B. Cook and N. Cook, J. Donaldson and B. Donald-
C.E. Bock et al. / Landscape and Urban Planning 80 (2007) 34–44
son, M. Donaldson and B. Donaldson, M. Douglas, J. Dowling and E. Dowling, B. Eifrig and G. Eifrig, S. Franklin, T. Hanson and C. Hanson, P. Hoffman and F. Hoffman, M. Johnson, J. Kolbe, V. Michael and J. Michael, D. Reilly, S. Shields, G. Sill, S. Strom and K. Strom, D. Sturges, L. Wilkening, J. Woods, and P. Workizer and K. Workizer. We thank the following individuals for field assistance: B. Audsley, K. Bishop, T. Crook, D. Goodheim, B. Loomis, L. Jones, L. Kennedy, J. MacAdam, A. Marshall, L. Schevets, C. Venot, and C. Wonkka. Y. Linhart and two anonymous reviewers provided helpful comments on the manuscript. This study was supported by the Ecology Program and the Research Experience for Teachers Program of the National Science Foundation. Appendix A Species of butterflies recorded on 48 plots in grasslands and mesquite/oak savannas in the Sonoita Valley of southeastern Arizona, 2003–2004.
Family and species
Mobilitya
Dietary specialist?b
Multiple broods?c
Hesperiidae Amblyscirtes aenus Amblyscirtes eosd Amblyscirtes exoteriad Amblyscirtes nereusd,e Amblyscirtes nysad Amblyscirtes oslarid,e Amblyscirtes texanaef Atrytonopsis edwardsi Cogia hippalusd Copaeodes aurantiacus Erynnis funeralisd Erynnis tristis Hesperia pahaska Hylephila phyleusd,f Lerodea eufalad,f Pholisora catullusd Piruna cingod,e Polites carusf Pyrgus albescensd Pyrgus philetasd Staphylus ceosd Systasea zampad Thorybes drusiusd Thorybes pyladesd
Low Low Low Low Low Low Low Low High High High High High High Low Low Low Low High Low High Low High High
No Yes Yes Yes No Yes Yes Yes Yes No No No No Yes No No Yes Yes No No No No Yes No
No Yes No No Yes No No No Yes Yes Yes Yes No Yes Yes Yes No Yes Yes Yes Yes Yes No No
Papilionidae Battus philenor Papilio polyxenes Papilio multicaudatusd
High High High
Yes No No
Yes Yes No
Pieridae Colias cesoniad Colias eurythemed Eurema mexicanumd Eurema nicippe Eurema proterpia Kricogonia lysided Nathalis ioled Phoebis agarithe Phoebis sennae
High High High High High High High High High
No No No Yes Yes Yes No Yes No
Yes Yes Yes Yes Yes Yes Yes Yes No
43
Appendix A (Continued) Mobilitya
Dietary specialist?b
Multiple broods?c
High High
No No
Yes Yes
Lycaenidae Apodemia mormo Apodemia palmeriie Atlides halesus Brephidium exiled Calephelis arizonensisd,e Euphilotes ritad,f Hemiargus ceraunusd Hemiargus isolad Leptotes marinad Ministrymon ledad Mitoura sivad,f Plebejus acmond Strymon melinusd
Low Low High Low Low Low Low Low High High Low High High
Yes Yes Yes No Yes Yes No No No Yes Yes No No
No Yes Yes Yes Yes No Yes Yes Yes Yes No Yes Yes
Nymphalidae Adelpha bredowiid Agraulis vanillaef Asterocampa celtisd Chlosyne laciniad Cyllopsis pyracmond,e Danaus gilippus Danaus plexippus Dymasia dymase Euptoieta claudia Junonia coenia Junonia nigrosuffusaf Libytheana carinenta Megisto rubricata Poladryas arachne Texola eladae Thessalia theonad,e Vanessa atalantad,f Vanessa cardui Vanessa virginiensisd,f
High High Low High Low High High Low High High High High Low High Low Low High High High
Yes Yes Yes No No Yes Yes Yes No No No Yes No Yes Yes No Yes No No
No Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes No Yes Yes Yes Yes Yes
Family and species Pieris protodiced Pieris rapae
a
Species were divided into two groups, based on extensive personal observations of their relative mobility in the study area and elsewhere in southeastern Arizona. b Dietary specialists were those species whose larval foods were restricted to one or a few closely related plant species, based on information in Stewart et al. (2001), modified in some cases by personal experience in the study area. c Multiple brooded species were those with three or more annual broods, according to Tilden and Smith (1986) and/or personal experience in the study area. d These species were >1.5 times more abundant in exurban than in undeveloped grassland, and/or >1.5 times more abundant in undeveloped than in exurban oak savanna. e These species were found only on undeveloped plots. f These species were found only on exurban plots.
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