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Effects of long-term cattle exclosure on vegetation and rodents at a desertified arid grassland site
ARTICLE IN PRESS Journal of Arid Environments
Journal of Arid Environments 61 (2005) 161–170 www.elsevier.com/locate/jnlabr/yjare
Effects of long-term cattle exclosure on vegetation and rodents at a desertified arid grassland site T.J. Valonea,, P. Sauterb a
Department of Biology, Saint Louis University, 3507 Laclede Avenue, Saint Louis, MO 63103, USA b Department of Biology, California State University Northridge, Northridge, CA 91330, USA Received 3 December 2003; received in revised form 13 May 2004; accepted 28 July 2004 Available online 7 October 2004
Abstract Arid grasslands are often presumed to exist in one of two alternate stable states: grassland or desertified shrubland. While the conversion to shrubland can occur rather rapidly following intense overgrazing, the recovery of perennial grasses is often presumed to be difficult or impossible even with livestock removal. We examined vegetation and rodent communities at a desertified shrubland site from which livestock had been removed for more than four decades. Total shrub cover was similar but differed in composition across the grazing fence. Larrea tridentata had significantly higher cover outside while Parthenium incanum had significantly higher cover inside the fence. Basal perennial grass cover was significantly higher inside the fence. Rodent diversity was significantly higher inside the fence due to higher abundance and diversity of pocket mice. These data suggest that recovery of perennial grasses at severely desertified sites is possible but may require several decades and that rodent diversity responds positively to such recovery. r 2004 Elsevier Ltd. All rights reserved. Keywords: Alternate stable state; Arizona; Livestock; Overgrazing; Perennial grass; Time lag
Corresponding author. Tel.: +1-314-977-4090; fax: +1-314-977-3658.
E-mail address: [email protected] (T.J. Valone). 0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.011
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1. Introduction Many arid grasslands around the world have experienced similar changes in vegetation over the past 200 years: perennial grass cover has declined while shrub density and cover has increased (e.g. Walker et al., 1981; Skarpe, 1990; Wiegand and Milton, 1996; Moleele and Perkins, 1998; Van Auken, 2000). Although these changes, symptomatic of desertification, have been attributed to many possible causes (Neilson, 1986; Humphrey, 1987; Archer et al., 1995; Brown et al., 1997; Weltzin et al., 1997) overgrazing by livestock is thought by most workers to have played a major role (e.g. Bahre, 1991; Vavra et al., 1994; Hodgson and Illius, 1996; Van Auken, 2000). Desertified arid grassland sites have stimulated much research on the stability and resilience of vegetation assemblages. Restoration attempts have often involved removal of livestock but typically perennial grass recovery has not been observed even after 20 years following livestock removal (e.g. Brown, 1950; Gardner, 1950; Glendening, 1952; Smith and Schmutz, 1975; West et al., 1984; Roundy and Jordan, 1988; Skarpe, 1990; Laycock, 1991; Kelt and Valone, 1995; Whitford et al., 1995). These observations have led to the development of alternate stable state models that predict no recovery of perennial grasses over time-scales relevant to management following livestock removal because of presumed changes in soil properties (Walker et al., 1981; Westoby et al., 1989; Schlesinger et al., 1990; Friedel, 1991; Lockwood and Lockwood, 1993; Archer, 1996; Rietkirk et al., 1996, 1997; Rietkirk and van de Koppel, 1997). In contrast to the numerous studies of vegetation, fewer studies have examined how desertification, and its concomitant vegetation changes, affect animals (but see Grant et al., 1982; Germano et al., 1983; Heske and Campbell, 1991; Hayward et al., 1997). Much early work was conducted at one large site (Bock et al., 1984, 1990; Bock and Bock, 1988; Jepson-Innes and Bock, 1989; Jones et al., 2003) but, over the past few years, there has been growing interest in better documenting faunal responses to grazing-induced vegetation changes (Whitford, 1997; James, 2003; Jones et al., 2003; Tabini and Ojeda, 2003). Most such work has focused on mammals and has culminated in a model that predicts how grazing induced vegetation change affects rodent community diversity in arid south-western North America (Jones et al., 2003). Recently, Valone et al. (2002) reported significant increases in perennial grass cover at a desertified arid grassland site from which livestock had been removed for 39 years. This site contains two shrub-dominated plant associations which occur on different soils. Valone et al. (2002) reported results from the Flourensia cernuadominated plant association portion of that site that occurs on deep sandy soil. The majority of the site, however, contains a Larrea tridentata-dominated plant association that occurs on shallow soils (Chew, 1982). In this paper, we first examine the vegetation inside and outside the grazing fence at this site in the Larrea portion to determine whether perennial grasses have also increased significantly on the shallow soil. Second, we examined the response of rodents to the vegetation changes that have occurred inside the grazing fence. Specifically, we evaluate the
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model of Jones et al. (2003) who hypothesize that variation in percent ground cover dictates the relative abundance of various rodent taxa.
2. Material and methods The 9.3 ha site was fenced in 1958 to exclude livestock with year-round open range grazing by cattle continuing outside the fence. The site is located in south-eastern Arizona, USA in the northern half of township T17S R31E (Sections 2 and 3). Two lines of evidence suggest the site is a historic arid grassland. First, the site contains numerous abandoned Dipodomys spectabilis mounds (Chew and Whitford, 1992). D. spectabilis prefers open grassy habitats and rarely occurs when shrub cover exceeds about 20% (Hoffmeister, 1986; Chew and Whitford, 1992; Krogh et al., 2002; Waser and Ayers, 2003). Second are data from the US General Land Office which surveyed the site in 1884 (Valone et al., 2002). The 38 vegetation descriptions (one description for each 1.2 km surveyed) from the northern half of the township indicate the site was a grassland: 27 (71.1%) of the descriptions were ‘‘good grass’’ with the summary statement ‘‘[The township] is well adapted for grazing purposes; grass in most parts good’’. None of the descriptions for the township mentioned ‘‘greasewood’’ (L. tridentata) (Buffington and Herbel, 1965), although greasewood was described in a few of the 1884 descriptions of T16S R31E about 3 km north of the site. Shrubs likely invaded the site around 1900 (Barnes, 1936; Chew, 1982; Valone and Kelt, 1999). At present, the site contains two shrub-dominated plant associations. The majority of the site contains a L. tridentata-dominated association that occurs on shallow soils (Kimbrough Series, Petrocalcic Calciustolls) (Vogt, 1980). The remainder of the site contains a F. cernua-dominated plant community that lies on deep, well drained gravelly sandy loam soil (Eba Series, Typic Haplargid) (Vogt, 1980). In 1958, the entire site was dominated by woody shrubs and grass canopy cover was approximately 1% (Chew and Chew, 1965; Chew, 1982). In 1977, the shrub-dominated vegetation differed little across the grazing fence (Chew, 1982). To census the vegetation of the Larrea-association, we established 12 pairs of 25 m transect lines on opposite sides of the grazing fence. Each pair of lines began 5 m from the grazing fence and both ran perpendicular to it. On each line, we surveyed the vegetation at 10 cm intervals (250 survey points per transect) using the lineintercept method to record canopy intercepts for shrubs and basal intercepts for perennial grasses. We recorded basal rather than canopy intercepts for grasses to minimize potential biases due to the grazing of grass canopies outside the grazing fence. The vegetation was censused in December 1999 and 2000. Results were nearly identical and so we report data from 2000 only. We report the results of Wilcoxon matched-pairs tests for all comparisons. Nomenclature follows Kearney and Peebles (1960). We censused rodents inside and outside the fence that surrounds the entire site by establishing five pairs of trap grids around the grazing fence. Three pairs of 7 7 grids were established in Larrea habitat while two pairs of 5 5 grids were
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established in Flourensia habitat. Inter-trap distance was 10 m. All grids were spaced by at least 10 m. Rodents were censused during four trapping sessions: July 24–25, 1999, December 30–31, 1999, July 12–13, 2000, and December 29–30, 2000. During each session, traps were baited with millet and set on each trap line for a single night. Captured individuals were identified, sexed, weighed and uniquely marked with toe tags. During both censuses in 1999, rodent populations were very low. Thus for statistical analysis, we combined all data into one analysis and used a chi-square test to compare unique individuals captured inside versus outside the grazing fence over all trapping sessions. Because data were collected around one grazing fence at the site, statistical inferences are limited to the study area (Wester, 1992).
3. Results A total of 27 perennial plant species were recorded, included 10 grasses. Dominant shrubs included L. tridentata, Parthenium incanum and Zinnia pumila. Bouteloua eriopoda, Muhlenbergia porteri and Aristida spp. were the most common tall-statured grasses observed while, B. curtipendula, Andropogon barbinodis, Hilaria mutica, Sporobolus cryptandrus, and Trichachne californica were uncommon. Vegetation cover differed significantly across the grazing fence. Total cover was significantly higher inside compared to outside the grazing fence (Fig. 1). While total shrub cover was similar, L. tridentata had significantly higher cover outside the fence in grazed habitat while P. incanum had significantly higher cover in ungrazed habitat inside the fence (Fig. 1). Grass cover differed dramatically at the site. Basal grass cover outside the fence was 0% but nearly 5% inside. As such, the basal cover of both the most abundant grass, B. eriopoda, and total grass cover were significantly higher inside the fence (Fig. 1).
Fig. 1. Mean (7 se) percent cover in grazed and ungrazed habitat around the grazing fence and results of Wilcoxon signed-ranks test for each group. *po0.05, **po0.01.
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Table 1 Number of unique rodents captured in ungrazed and grazed habitat surrounding the grazing fence over four censuses and p-values testing the null hypothesis of equal abundance Family
Heteromyidae
Muridae
Species
Dipodomys merriami D. ordii Chaetodipus baylei C. penicillatus Perognathus flavus Neotoma albigula Onychomys torridus O. leucogaster Reithrodontomys megalotis
Total rodents
Habitat Ungrazed
Grazed
p
74 2 7 11 1 4 4 1 1 105
77 1 0 1 0 3 3 0 0 85
ns — 0.008 0.006 — ns ns — — ns
ns indicates non-significance.
A total of 191 unique individual rodents comprising nine species were captured in 3152 trap-nights (not including five individuals captured equally often on both sides of the fence). Dipodomys merriami was the most abundant rodent captured, followed in abundance by Chaetodipus penicillatus, C. baileyi, Onychomys torridus, Neotoma albigula, D. ordii, O. leucogaster, Reithrodontomys megalotis and Perognathus flavus. Both total rodent captures and total kangaroo rat (Dipodomys) numbers did not differ across the fence (Table 1). However, pocket mice (Chaetodipus and Perognathus) were significantly more abundant inside the grazing fence in ungrazed habitat (19 vs. 1 individual, po 0.001) and this resulted in significantly higher abundance and diversity of non-Dipodomys species inside the exclosure (29 vs. 7 individuals, po0:001; and 7 vs. 3 species, respectively; Table 1).
4. Discussion Many desertified arid grassland sites exhibit little recovery in perennial grass cover during the first two decades following removal of livestock grazing (e.g. Smith and Schmutz, 1975; Roundy and Jordan, 1988; Laycock, 1991; Valone et al., 2002). Indeed, this was true for the site studied here: in 1977, 20 years after establishment, the cover of tall perennial grasses was similar across the fence (Chew, 1982). However, since 1977, perennial tall grass cover has increased significantly at this site on two soil types inside the grazing fence while grass cover outside the fence remains sparse (Valone et al., 2002; this study). These results parallel the work of Fuhlendorf et al. (2001) who report significant increases in mid-grass basal cover beginning 25 years following the cessation of
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livestock grazing from a site in Texas and recent work in Africa which suggests that desertification may be reversible (Rasmussen et al., 2001). Such studies are important for two reasons. First, they begin to place a time-scale on the dynamics of perennial grass recovery in grasslands desertified by livestock grazing. It appears that at least 20 years are required to observe significant increases in perennial grass cover following the removal of livestock grazing. Second, they are consistent with recent theoretical work suggesting that desertification need not always be irreversible, depending on site-specific properties (Rietkirk et al., 1997). Additional work, however, will be required to more formally evaluate the reversibility of vegetation change in desertified systems. We suggest that long time lags in grass recovery exist for two possible reasons. First, perennial grass seed production and establishment are episodic in the arid south-west (e.g. Neilson, 1986, Frasier, 1989). Given that specific environmental conditions are required for both seed production and establishment, it is not surprising that more than two decades are required to observe significant increases in grass cover following the removal of livestock. Second, trampling by livestock reduces microtopographic soil structure which can then retard grass seedling establishment (Nash et al., 2003). Perhaps more than 20 years are required for trampled soils to re-establish sufficient microtopographic structure to facilitate perennial grass seedling establishment. While grass recovery was relatively modest, from 1% canopy to more than 5% basal cover, it was ecologically significant because it affected rodent abundance and diversity. Jones et al. (2003) proposed that percent ground cover strongly affects the composition of rodent communities in the arid south-west. They proposed that sites with less than 25% cover will be dominated by kangaroo rats (Dipodomys) whereas sites with between 25% and 75% cover will be dominated by pocket mice (Chaetodipus and Perognathus) and sites with greater than 75% cover will be dominated by rodents in the family Muridae (Jones et al., 2003). Our data are generally consistent with these predicted effects of ground cover: outside the grazing fence, ground cover was o25%, the rodent community was dominated by kangaroo rats (90% of all individuals captured) and pocket mice were rare (1.1% of individuals captured). Inside the grazing fence, ground cover was over 30% and, while the rodent community was still dominated by kangaroo rats, there were significantly more pocket mice (18.1% of individuals captured, Table 1). This dramatic effect on pocket mice along with differences in the number and diversity of other rodents inside the fence resulted in a significantly more diverse rodent community inside compared to outside the grazing fence. These differences reflect habitat preferences of the species. Kangaroo rats prefer more open habitats while pocket mice prefer habitats with moderate grass cover (Hoffmeister, 1986). The model of Jones et al. (2003) predicts that if ground cover continues to increase inside the grazing fence, the abundance of Murid rodents in the region such as Reithrodontomys, Sigmodon and Baiomys should increase (see Valone and Brown, 1995). Our data suggest that rodent diversity increases as desertified habitat recovers perennial grass cover. At first glace, our data appear to contradict Whitford (1997)
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who concluded that rodent diversity is highest in desertified areas. However, closer inspection reveals that Whitford (1997) compared shrub-invaded grassland habitat with shrub-free arid grassland habitat and found higher diversity in the former. Our ungrazed habitat inside the grazing fence corresponds to Whitford’s shrub-invaded grassland in that it contains a mixture of grass and shrub cover. Thus, both studies found highest diversity in mixed shrub-grass habitat because species with both shrubland and grassland habitat affinities were present (Whitford, 1997). Thus, it appears that habitats that are in either the initial stages of desertification (shrub invasion of grasslands) or the initial stages of recovery (grass recovery in shrublands) are likely to have higher rodent diversity than either pure arid grassland or pure shrubland habitat. At this site, both deep and shallow soil portions have experienced significant, although somewhat different, increases in perennial grass cover. On the shallow soil, B. eriopoda is the only common grass (Fig. 1). While B. eriopoda was also the most abundant species on the deep soil portion of the site, both Aristida spp., and A. barbinodis were also common and exhibited significantly higher cover inside the fence (Valone et al., 2002). Thus, total grass cover on the deep soil portion of the site was twice that of the shallow soil portion (9.6% vs. 4.8%, respectively) (Valone et al., 2002). These differences might result from differences in soil properties per se or from differences in shrub composition because F. cernua dominated the deep soils while L. tridentata dominated shallow soils. Additional work is required to tease apart these, or alternative, possibilities. Both the significant increase in perennial grass cover and the current grass-shrub habitat matrix inside the grazing exclosure are not predicted by most alternate stable state models (e.g. Rietkirk and van de Koppel, 1996). Such models are based on presumed changes in soil properties that occur during desertification that inhibit the re-establishment of grasses (e.g. Schlesinger et al., 1990). Evidence suggests that this historic arid grassland site became desertified at the end of the 19th century (Valone and Kelt, 1999). While no data document changes in soil properties during desertification at this site, any changes that did occur apparently did not prevent the re-establishment of perennial grasses on two very different soil association, although such changes may account for the 20+ year time lag in recovery following livestock removal (Nash et al., 2003). Such observations suggest that perennial grass recovery may be possible at a variety of sites (e.g. Rietkirk et al., 1997) and call for additional empirical studies to better understand relationships between desertification, soil properties and perennial grass recovery.
Acknowledgments We thank R. Arechiga, S.K.M. Ernest, S.E. Nordell, and M. Sauter for help with data collection and J.H. Brown and R.M. Chew for logistical support and intellectual stimulation. This research was partially supported by National Science Foundation grants DEB 9707406 and DEB 0211069.
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Journal of Arid Environments 61 (2005) 161–170 www.elsevier.com/locate/jnlabr/yjare
Effects of long-term cattle exclosure on vegetation and rodents at a desertified arid grassland site T.J. Valonea,, P. Sauterb a
Department of Biology, Saint Louis University, 3507 Laclede Avenue, Saint Louis, MO 63103, USA b Department of Biology, California State University Northridge, Northridge, CA 91330, USA Received 3 December 2003; received in revised form 13 May 2004; accepted 28 July 2004 Available online 7 October 2004
Abstract Arid grasslands are often presumed to exist in one of two alternate stable states: grassland or desertified shrubland. While the conversion to shrubland can occur rather rapidly following intense overgrazing, the recovery of perennial grasses is often presumed to be difficult or impossible even with livestock removal. We examined vegetation and rodent communities at a desertified shrubland site from which livestock had been removed for more than four decades. Total shrub cover was similar but differed in composition across the grazing fence. Larrea tridentata had significantly higher cover outside while Parthenium incanum had significantly higher cover inside the fence. Basal perennial grass cover was significantly higher inside the fence. Rodent diversity was significantly higher inside the fence due to higher abundance and diversity of pocket mice. These data suggest that recovery of perennial grasses at severely desertified sites is possible but may require several decades and that rodent diversity responds positively to such recovery. r 2004 Elsevier Ltd. All rights reserved. Keywords: Alternate stable state; Arizona; Livestock; Overgrazing; Perennial grass; Time lag
Corresponding author. Tel.: +1-314-977-4090; fax: +1-314-977-3658.
E-mail address: [email protected] (T.J. Valone). 0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.011
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1. Introduction Many arid grasslands around the world have experienced similar changes in vegetation over the past 200 years: perennial grass cover has declined while shrub density and cover has increased (e.g. Walker et al., 1981; Skarpe, 1990; Wiegand and Milton, 1996; Moleele and Perkins, 1998; Van Auken, 2000). Although these changes, symptomatic of desertification, have been attributed to many possible causes (Neilson, 1986; Humphrey, 1987; Archer et al., 1995; Brown et al., 1997; Weltzin et al., 1997) overgrazing by livestock is thought by most workers to have played a major role (e.g. Bahre, 1991; Vavra et al., 1994; Hodgson and Illius, 1996; Van Auken, 2000). Desertified arid grassland sites have stimulated much research on the stability and resilience of vegetation assemblages. Restoration attempts have often involved removal of livestock but typically perennial grass recovery has not been observed even after 20 years following livestock removal (e.g. Brown, 1950; Gardner, 1950; Glendening, 1952; Smith and Schmutz, 1975; West et al., 1984; Roundy and Jordan, 1988; Skarpe, 1990; Laycock, 1991; Kelt and Valone, 1995; Whitford et al., 1995). These observations have led to the development of alternate stable state models that predict no recovery of perennial grasses over time-scales relevant to management following livestock removal because of presumed changes in soil properties (Walker et al., 1981; Westoby et al., 1989; Schlesinger et al., 1990; Friedel, 1991; Lockwood and Lockwood, 1993; Archer, 1996; Rietkirk et al., 1996, 1997; Rietkirk and van de Koppel, 1997). In contrast to the numerous studies of vegetation, fewer studies have examined how desertification, and its concomitant vegetation changes, affect animals (but see Grant et al., 1982; Germano et al., 1983; Heske and Campbell, 1991; Hayward et al., 1997). Much early work was conducted at one large site (Bock et al., 1984, 1990; Bock and Bock, 1988; Jepson-Innes and Bock, 1989; Jones et al., 2003) but, over the past few years, there has been growing interest in better documenting faunal responses to grazing-induced vegetation changes (Whitford, 1997; James, 2003; Jones et al., 2003; Tabini and Ojeda, 2003). Most such work has focused on mammals and has culminated in a model that predicts how grazing induced vegetation change affects rodent community diversity in arid south-western North America (Jones et al., 2003). Recently, Valone et al. (2002) reported significant increases in perennial grass cover at a desertified arid grassland site from which livestock had been removed for 39 years. This site contains two shrub-dominated plant associations which occur on different soils. Valone et al. (2002) reported results from the Flourensia cernuadominated plant association portion of that site that occurs on deep sandy soil. The majority of the site, however, contains a Larrea tridentata-dominated plant association that occurs on shallow soils (Chew, 1982). In this paper, we first examine the vegetation inside and outside the grazing fence at this site in the Larrea portion to determine whether perennial grasses have also increased significantly on the shallow soil. Second, we examined the response of rodents to the vegetation changes that have occurred inside the grazing fence. Specifically, we evaluate the
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model of Jones et al. (2003) who hypothesize that variation in percent ground cover dictates the relative abundance of various rodent taxa.
2. Material and methods The 9.3 ha site was fenced in 1958 to exclude livestock with year-round open range grazing by cattle continuing outside the fence. The site is located in south-eastern Arizona, USA in the northern half of township T17S R31E (Sections 2 and 3). Two lines of evidence suggest the site is a historic arid grassland. First, the site contains numerous abandoned Dipodomys spectabilis mounds (Chew and Whitford, 1992). D. spectabilis prefers open grassy habitats and rarely occurs when shrub cover exceeds about 20% (Hoffmeister, 1986; Chew and Whitford, 1992; Krogh et al., 2002; Waser and Ayers, 2003). Second are data from the US General Land Office which surveyed the site in 1884 (Valone et al., 2002). The 38 vegetation descriptions (one description for each 1.2 km surveyed) from the northern half of the township indicate the site was a grassland: 27 (71.1%) of the descriptions were ‘‘good grass’’ with the summary statement ‘‘[The township] is well adapted for grazing purposes; grass in most parts good’’. None of the descriptions for the township mentioned ‘‘greasewood’’ (L. tridentata) (Buffington and Herbel, 1965), although greasewood was described in a few of the 1884 descriptions of T16S R31E about 3 km north of the site. Shrubs likely invaded the site around 1900 (Barnes, 1936; Chew, 1982; Valone and Kelt, 1999). At present, the site contains two shrub-dominated plant associations. The majority of the site contains a L. tridentata-dominated association that occurs on shallow soils (Kimbrough Series, Petrocalcic Calciustolls) (Vogt, 1980). The remainder of the site contains a F. cernua-dominated plant community that lies on deep, well drained gravelly sandy loam soil (Eba Series, Typic Haplargid) (Vogt, 1980). In 1958, the entire site was dominated by woody shrubs and grass canopy cover was approximately 1% (Chew and Chew, 1965; Chew, 1982). In 1977, the shrub-dominated vegetation differed little across the grazing fence (Chew, 1982). To census the vegetation of the Larrea-association, we established 12 pairs of 25 m transect lines on opposite sides of the grazing fence. Each pair of lines began 5 m from the grazing fence and both ran perpendicular to it. On each line, we surveyed the vegetation at 10 cm intervals (250 survey points per transect) using the lineintercept method to record canopy intercepts for shrubs and basal intercepts for perennial grasses. We recorded basal rather than canopy intercepts for grasses to minimize potential biases due to the grazing of grass canopies outside the grazing fence. The vegetation was censused in December 1999 and 2000. Results were nearly identical and so we report data from 2000 only. We report the results of Wilcoxon matched-pairs tests for all comparisons. Nomenclature follows Kearney and Peebles (1960). We censused rodents inside and outside the fence that surrounds the entire site by establishing five pairs of trap grids around the grazing fence. Three pairs of 7 7 grids were established in Larrea habitat while two pairs of 5 5 grids were
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established in Flourensia habitat. Inter-trap distance was 10 m. All grids were spaced by at least 10 m. Rodents were censused during four trapping sessions: July 24–25, 1999, December 30–31, 1999, July 12–13, 2000, and December 29–30, 2000. During each session, traps were baited with millet and set on each trap line for a single night. Captured individuals were identified, sexed, weighed and uniquely marked with toe tags. During both censuses in 1999, rodent populations were very low. Thus for statistical analysis, we combined all data into one analysis and used a chi-square test to compare unique individuals captured inside versus outside the grazing fence over all trapping sessions. Because data were collected around one grazing fence at the site, statistical inferences are limited to the study area (Wester, 1992).
3. Results A total of 27 perennial plant species were recorded, included 10 grasses. Dominant shrubs included L. tridentata, Parthenium incanum and Zinnia pumila. Bouteloua eriopoda, Muhlenbergia porteri and Aristida spp. were the most common tall-statured grasses observed while, B. curtipendula, Andropogon barbinodis, Hilaria mutica, Sporobolus cryptandrus, and Trichachne californica were uncommon. Vegetation cover differed significantly across the grazing fence. Total cover was significantly higher inside compared to outside the grazing fence (Fig. 1). While total shrub cover was similar, L. tridentata had significantly higher cover outside the fence in grazed habitat while P. incanum had significantly higher cover in ungrazed habitat inside the fence (Fig. 1). Grass cover differed dramatically at the site. Basal grass cover outside the fence was 0% but nearly 5% inside. As such, the basal cover of both the most abundant grass, B. eriopoda, and total grass cover were significantly higher inside the fence (Fig. 1).
Fig. 1. Mean (7 se) percent cover in grazed and ungrazed habitat around the grazing fence and results of Wilcoxon signed-ranks test for each group. *po0.05, **po0.01.
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Table 1 Number of unique rodents captured in ungrazed and grazed habitat surrounding the grazing fence over four censuses and p-values testing the null hypothesis of equal abundance Family
Heteromyidae
Muridae
Species
Dipodomys merriami D. ordii Chaetodipus baylei C. penicillatus Perognathus flavus Neotoma albigula Onychomys torridus O. leucogaster Reithrodontomys megalotis
Total rodents
Habitat Ungrazed
Grazed
p
74 2 7 11 1 4 4 1 1 105
77 1 0 1 0 3 3 0 0 85
ns — 0.008 0.006 — ns ns — — ns
ns indicates non-significance.
A total of 191 unique individual rodents comprising nine species were captured in 3152 trap-nights (not including five individuals captured equally often on both sides of the fence). Dipodomys merriami was the most abundant rodent captured, followed in abundance by Chaetodipus penicillatus, C. baileyi, Onychomys torridus, Neotoma albigula, D. ordii, O. leucogaster, Reithrodontomys megalotis and Perognathus flavus. Both total rodent captures and total kangaroo rat (Dipodomys) numbers did not differ across the fence (Table 1). However, pocket mice (Chaetodipus and Perognathus) were significantly more abundant inside the grazing fence in ungrazed habitat (19 vs. 1 individual, po 0.001) and this resulted in significantly higher abundance and diversity of non-Dipodomys species inside the exclosure (29 vs. 7 individuals, po0:001; and 7 vs. 3 species, respectively; Table 1).
4. Discussion Many desertified arid grassland sites exhibit little recovery in perennial grass cover during the first two decades following removal of livestock grazing (e.g. Smith and Schmutz, 1975; Roundy and Jordan, 1988; Laycock, 1991; Valone et al., 2002). Indeed, this was true for the site studied here: in 1977, 20 years after establishment, the cover of tall perennial grasses was similar across the fence (Chew, 1982). However, since 1977, perennial tall grass cover has increased significantly at this site on two soil types inside the grazing fence while grass cover outside the fence remains sparse (Valone et al., 2002; this study). These results parallel the work of Fuhlendorf et al. (2001) who report significant increases in mid-grass basal cover beginning 25 years following the cessation of
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livestock grazing from a site in Texas and recent work in Africa which suggests that desertification may be reversible (Rasmussen et al., 2001). Such studies are important for two reasons. First, they begin to place a time-scale on the dynamics of perennial grass recovery in grasslands desertified by livestock grazing. It appears that at least 20 years are required to observe significant increases in perennial grass cover following the removal of livestock grazing. Second, they are consistent with recent theoretical work suggesting that desertification need not always be irreversible, depending on site-specific properties (Rietkirk et al., 1997). Additional work, however, will be required to more formally evaluate the reversibility of vegetation change in desertified systems. We suggest that long time lags in grass recovery exist for two possible reasons. First, perennial grass seed production and establishment are episodic in the arid south-west (e.g. Neilson, 1986, Frasier, 1989). Given that specific environmental conditions are required for both seed production and establishment, it is not surprising that more than two decades are required to observe significant increases in grass cover following the removal of livestock. Second, trampling by livestock reduces microtopographic soil structure which can then retard grass seedling establishment (Nash et al., 2003). Perhaps more than 20 years are required for trampled soils to re-establish sufficient microtopographic structure to facilitate perennial grass seedling establishment. While grass recovery was relatively modest, from 1% canopy to more than 5% basal cover, it was ecologically significant because it affected rodent abundance and diversity. Jones et al. (2003) proposed that percent ground cover strongly affects the composition of rodent communities in the arid south-west. They proposed that sites with less than 25% cover will be dominated by kangaroo rats (Dipodomys) whereas sites with between 25% and 75% cover will be dominated by pocket mice (Chaetodipus and Perognathus) and sites with greater than 75% cover will be dominated by rodents in the family Muridae (Jones et al., 2003). Our data are generally consistent with these predicted effects of ground cover: outside the grazing fence, ground cover was o25%, the rodent community was dominated by kangaroo rats (90% of all individuals captured) and pocket mice were rare (1.1% of individuals captured). Inside the grazing fence, ground cover was over 30% and, while the rodent community was still dominated by kangaroo rats, there were significantly more pocket mice (18.1% of individuals captured, Table 1). This dramatic effect on pocket mice along with differences in the number and diversity of other rodents inside the fence resulted in a significantly more diverse rodent community inside compared to outside the grazing fence. These differences reflect habitat preferences of the species. Kangaroo rats prefer more open habitats while pocket mice prefer habitats with moderate grass cover (Hoffmeister, 1986). The model of Jones et al. (2003) predicts that if ground cover continues to increase inside the grazing fence, the abundance of Murid rodents in the region such as Reithrodontomys, Sigmodon and Baiomys should increase (see Valone and Brown, 1995). Our data suggest that rodent diversity increases as desertified habitat recovers perennial grass cover. At first glace, our data appear to contradict Whitford (1997)
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who concluded that rodent diversity is highest in desertified areas. However, closer inspection reveals that Whitford (1997) compared shrub-invaded grassland habitat with shrub-free arid grassland habitat and found higher diversity in the former. Our ungrazed habitat inside the grazing fence corresponds to Whitford’s shrub-invaded grassland in that it contains a mixture of grass and shrub cover. Thus, both studies found highest diversity in mixed shrub-grass habitat because species with both shrubland and grassland habitat affinities were present (Whitford, 1997). Thus, it appears that habitats that are in either the initial stages of desertification (shrub invasion of grasslands) or the initial stages of recovery (grass recovery in shrublands) are likely to have higher rodent diversity than either pure arid grassland or pure shrubland habitat. At this site, both deep and shallow soil portions have experienced significant, although somewhat different, increases in perennial grass cover. On the shallow soil, B. eriopoda is the only common grass (Fig. 1). While B. eriopoda was also the most abundant species on the deep soil portion of the site, both Aristida spp., and A. barbinodis were also common and exhibited significantly higher cover inside the fence (Valone et al., 2002). Thus, total grass cover on the deep soil portion of the site was twice that of the shallow soil portion (9.6% vs. 4.8%, respectively) (Valone et al., 2002). These differences might result from differences in soil properties per se or from differences in shrub composition because F. cernua dominated the deep soils while L. tridentata dominated shallow soils. Additional work is required to tease apart these, or alternative, possibilities. Both the significant increase in perennial grass cover and the current grass-shrub habitat matrix inside the grazing exclosure are not predicted by most alternate stable state models (e.g. Rietkirk and van de Koppel, 1996). Such models are based on presumed changes in soil properties that occur during desertification that inhibit the re-establishment of grasses (e.g. Schlesinger et al., 1990). Evidence suggests that this historic arid grassland site became desertified at the end of the 19th century (Valone and Kelt, 1999). While no data document changes in soil properties during desertification at this site, any changes that did occur apparently did not prevent the re-establishment of perennial grasses on two very different soil association, although such changes may account for the 20+ year time lag in recovery following livestock removal (Nash et al., 2003). Such observations suggest that perennial grass recovery may be possible at a variety of sites (e.g. Rietkirk et al., 1997) and call for additional empirical studies to better understand relationships between desertification, soil properties and perennial grass recovery.
Acknowledgments We thank R. Arechiga, S.K.M. Ernest, S.E. Nordell, and M. Sauter for help with data collection and J.H. Brown and R.M. Chew for logistical support and intellectual stimulation. This research was partially supported by National Science Foundation grants DEB 9707406 and DEB 0211069.
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