The ecological determinants of space use by a burrowing wolf spider in a xeric shrubland ecosystem

Journal of Arid Environments (1997) 37: 379–393

The ecological determinants of space use by a burrowing wolf spider in a xeric shrubland ecosystem

Samuel D. Marshall* Department of Zoology and Graduate Program in Ethology, University of Tennessee, Knoxville, TN 37996, U.S.A. (Received 1 February 1997, accepted 13 May 1997) The patterns of dispersion and habitat associations of a burrowing wolf spider were examined in a xeric shrubland ecosystem in central Florida, U.S.A. Geostatistical analyses of habitat features and burrow sites revealed that at the level of the scrub landscape burrow dispersion was correlated with patches of barren sand. This association was found to be the result of active choice in enclosure experiments. There was less prey available at burrow sites than at random sites within the scrub; thus, this microhabitat association is not explained by improved foraging in barren patches. Discriminant analysis of 18 features of burrow sites and non-burrow sites documented that there was no predictor of burrow placement other than barren sand. Burrows were aggregated within the open sand microhabitat. However, these aggregations were not explained by substratum moisture, prey, or the differential burrowsite tenure of aggregated vs. solitary spiders. ©1997 Academic Press Limited Keywords: dispersion; aggregation; spider; Geolycosa; shrubland; Florida scrub

Introduction Patterns of dispersion of animals may reflect any of a number of deterministic and stochastic factors. Determinants of patterns include (but are not restricted to): (1) mode of dispersal from the natal site, (2) the probability of encountering suitable habitat, (3) efficacy of habitat selection mechanisms, and (4) social spacing (e.g. territoriality). Depending on the dispersal mechanism employed (e.g. wind- or current-driven dispersal vs. directed movement through the landscape), the process of dispersal has the greatest potential to introduce an element of chance into the formation of patterns of dispersion. Windborne dispersal of winged seeds or ballooning spiders are prime examples of dispersal mechanisms which may result in settlement in unsuitable locations due to chance affects. In contrast, for vagile animals with welldeveloped sensory capabilities, dispersal may be a largely non-random process. Habitat selection occurs when dispersing individuals discriminate between alternative sites to forage or for the establishment of a territory or home range. This discrimination is based on environmental cues such as habitat structure or the presence of *Present address: Department of Zoology, Miami University, Oxford, OH 45056, U.S.A. 0140–1963/97/020379 + 15 $25.00/0/ae970287

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conspecifics. Because habitat selection may be assumed to be an adaptive process (Fretwell, 1972), the examination of animal dispersion patterns and how they correlate with the dispersion of pertinent habitat features should reveal an association between the organism and important resources (e.g. food, breeding sites). Alternatively, failure to find an adaptive explanation for observed patterns may either be the result of lack of a selective advantage to deterministic habitat selection, or phylogenetic constraints on the evolution of behavioural and morphological adaptations necessary for wide dispersal coupled with effective habitat selection. Dispersal behaviour may itself give rise to pattern in the absence of patterned resources (Kareiva, 1990). In the absence of long-range dispersal coupled with active habitat selection, apparently non-adaptive dispersal strategies (e.g. ‘poor’ dispersal) may in fact be an adaptive response to the landscape in which the organism is moving, rather than evidence of a failure to evolve effective dispersal mechanisms. For instance, dispersal rates are predicted to be inversely correlated with stability and availability of the microhabitat sought (Southwood, 1962; Greenstone, 1982). Thus, limited dispersal may be an adaptive strategy that keeps habitat specialists in the appropriate microhabitat, rather than wandering into potentially hostile environments. Spiders that spin a web or dig a burrow lend themselves to the study of patterns of dispersion better than vagile animals as they are sessile and their site choice is unambiguous. Studies of habitat use by spiders have presented evidence that site choice is not the result of random processes; both habitat selection and social spacing influence patterns of dispersion (Turnbull, 1964; Riechert, 1976; Kronk & Riechert, 1979; Gillespie, 1981, 1987; Rypstra, 1985; Hodge, 1987a, b; Henschel et al., 1992; Ward & Lubin, 1993). This paper examines the behavioural mechanisms and ecological determinants leading to the pattern of dispersion exhibited by the burrowing wolf spider Geolycosa xera archboldi McCrone (hereafter referred to as G. xera). Geolycosa wolf spiders are obligate burrowers that live in close association with a burrow that they construct themselves. Geolycosa forage by sitting at the burrow mouth waiting for prey to pass within striking range, only leaving the burrow long enough to make sorties at passing prey. At no time are immature or adult female Geolycosa vagrant hunters. Only adult males give up their burrows to wander in search of sexually receptive females. Female Geolycosa tend eggs and newly-hatched young in the burrow. Spiderlings disperse from the female’s burrow by walking. Geolycosa xera is territorial, and nearest-neighbour distances approximating 30 cm are mediated by social spacing behaviour (Marshall, 1996). Because they are sessile, Geolycosa wolf spiders are a particularly useful animal system for studies of spatial ecology, due to the following characteristics: burrows are easily located and marked, dispersion patterns are entirely two-dimensional, and above-ground activity is closely restricted to the vicinity of the burrow mouth, which means that studies of burrow placement are studies of space use. In this study the pattern of dispersion and habitat associations of G. xera at two spatial scales are examined: (1) the level of the landscape and (2) the level of the microhabitat patch. The nested-scale approach is used as studies of dispersion conducted at different spatial scales may reveal contrasting patterns, reflecting qualitatively different interactions between the organism and its environment (Pielou, 1977; Wiens, 1989; Levin, 1992). Landscape-level studies are typically conducted in areas measured in hectares, however this study is conducted at a much smaller scale (square metres), commensurate with the small body size and limited activity area of the focal organism (Marshall, 1995a,b, 1996). Geolycosa xera is only found in the xeric shrubland ecosystems of central Florida (known as ‘Florida scrub’). Casual observation indicates that G. xera is associated with areas of barren sand. However, this microhabitat association has never been quantified. To this end habitat associations of G. xera are quantified first and then alternative hypotheses tested to explain any apparent association. This study examined whether habitat-selection behaviour or prey

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availability might lead to the burrow dispersion patterns observed. Within open sand patches, G. xera burrows appeared to be non-randomly dispersed. Burrow dispersion was quantified within areas of open sand and it was determined whether habitat structure, substratum moisture, or food were correlated with burrow placement. Burrow site tenure of individual spiders was also quantified and related to the population density in the vicinity of the burrow to see if aggregation itself conferred advantages on the aggregated individuals. Evidence is presented that patterns of dispersion exhibited by G. xera are explained by both habitat specialization and limited dispersal, and that dispersal behaviour may be an adaptive strategy given the landscape in which this spider evolved.

Methods Study site Research was conducted at Archbold Biological Station, a private research facility approximately 10 km south of Lake Placid in Highlands County, Florida, U.S.A. Geolycosa x. archboldi is endemic to the scrub habitat in Highlands County (Marshall, 1995a). Scrub is a xeric shrubland ecosystem found on sandy uplands in Florida, surrounded by mesic or hydric low-lying habitats. The scrubs of Highlands County are an ancient ecosystem on the top of a relict dune system that is the Lake Wales Ridge (Abrahamson et al., 1984; Myers,1990).

Landscape-level habitat associations An area of 1296 m2 (36 by 36 m) was selected for determination of local patterns of dispersion and habitat associations. The site is in a scrub type classified as scrubby flatwoods (dominated by the scrub oaks Quercus inopina and Q. geminata; Abrahamson et al., 1984). This site was selected because it afforded a large population of G. xera dispersed across a complex array of distinct habitat patches. The study area was mapped by hand onto sheets of graph paper in August 1990. This map contained information on substrata type, burrow location, and large-scale vegetation features (e.g. clones of scrub oak, palmettos). For the present habitat analysis barren sand alone was analysed, as the resolution of the map precluded analysis of vegetation type. (Vegetation is examined further in the analyses which follow.) Two elements of this map (burrows and barren sand substrata) were entered seperately into spreadsheets in raster format (raster maps are rows and columns of numbers representing values for features measured at each location). This was done by scoring the presence or absence of barren sand substrata (as opposed to sand covered with vegetative debris) or burrows in each map grid unit (0·09 m2) and summing them for adjacent blocks of nine (0·81 m2). These summations were necessary for statistical analyses which rely on the quantification, as well as the location, of features analysed. As a result of this summation, values in individual locations in the substratum raster map ranged from 0 to 9 for substratum type. In the burrow raster map burrow counts were summed for all nine units. Geostatistics were developed to specifically address the problem of identifying patterns in mapped data (Isaaks & Srivastava, 1989), and have only recently been applied to ecological data (e.g. Rossi et al., 1992; Liebhold et al., 1993). Geostatistics (rather than traditional pattern analyses) were chosen as they do not rely on any assumptions regarding the statistical distribution of the data. Geostatistics were used to look for patterns in the landscape-level dispersion of burrows and to quantify the observed association of G. xera with open sand substrata. For these analyses, the

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variogram and correlogram were used. The variogram examines variation in the dispersion of values across the mapped area by comparing values between adjacent sampling units and then between sampling units at increasing distances apart (referred to as lag distances). A correlogram will give a picture of how closely, and at what spatial scale, any two variables under consideration co-occur. The correlogram is calculated in a similar way to the variogram except that a correlation coefficient is calculated between variables in the sampling units for each lag distance across all lag distances. A variogram was generated for burrow dispersion to look for non-random patterns. A correlogram was generated to quantify the association of burrows with sand. Both the variogram and the correlogram were calculated using lag increments of one (90 cm), a maximum lag distance of 20 (18 m) and lag directions in both north–south and east– west directions using the software package GSLIB (Deutsch & Journel, 1992).

Habitat selection Replicate enclosure experiments were performed in August 1990 to determine whether the observed association of G. xera burrows with open sand patches was the result of active choice. Enclosures were surrounded by vertically-set 30-cm wide sheet aluminum in an area of uniform, barren sand. There were five enclosures, each 1·0 by 2·0 m. A 1·0 by 1·0 m half of each enclosure was randomly selected and covered to a depth of 3·0 cm with leaf litter. Each evening 10 marked spiders were introduced at random locations in each enclosure. Burrow sites were recorded the next morning and which substratum type each spider selected for burrow construction was scored.

Landscape-level variation in prey availability Most spiders are generalist predators of small arthropod prey. Thus, any estimate of small arthropod abundance should be correlated with prey availability. In this study prey availability was assessed using sticky traps; 15 cm diameter white paper plates coated with Stickum Special® (a non-toxic petroleum based adhesive; Seabright Industries, Emeryville, CA, U.S.A.), staked flush to the ground surface. These sticky traps were passive traps of surface active and flying arthropods which came into contact with the trap surface. The intent of the trap design was to assess potential prey coming into contact with the ground surface in the vicinity of a hypothetical burrow. Because the prey trapped are mostly members of vagile taxa, and the surface area of the traps was small, no depletion of local arthropod populations by the traps was assumed. This is an important point as successive days were used as replicates in the analyses, as it is likely that changing weather conditions would have a far greater influence on daily variation in arthropod activity in the vicinity of the traps than depletion of local arthropod abundance. To examine the availability of prey at burrow sites vs. randomly-selected non-burrow sites 15 sticky traps were placed at burrow sites (covering the burrow) and 15 at random sites selected using the ‘random walk’ technique (Southwood, 1966). The burrow sites were along the shoulder of a fire lane (an anthropogenic habitat feature), and the random non-burrow sites were in the scrub off the sides of the same fire lane. Most of the randomly-placed traps were in areas covered in leaf litter. All traps were covered with poultry netting (2·5 cm mesh) to prevent capture of larger vertebrates. Traps were left for 1 week and checked each evening, at which time captured arthropods were collected and recorded. Only prey in the size range of G. xera ( ≤ 1·0 cm body length) was considered. Trapping took place in March, July and August 1990. The effect of sticky trap location on insect capture was tested using a Chi-square contingency table with days as replicates and burrow site vs. random site trap location as treatments.

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Patch-level patterns of dispersion For analysis of burrow dispersion patterns at the level of the patch five of the largest patches of open sand in the area mapped for the macrohabitat analysis above and an additional four sand patches in an area of rosemary scrub (a variant of sand pine scrub dominated by Florida rosemary, Ceratiola ericoides) were selected at the north end of Archbold Biological Station. The sites in the rosemary scrub were used in addition to the sites in the area mapped for geostatistical analysis both to increase the sample size and to include G. xera populations from a different plant association. Here a ‘patch of sand’ is defined as an area where the surface of the sandy soil is entirely barren of a leaf litter layer, or vegetable debris of any kind (e.g. twigs, dead leaves, or detritus). Barren areas such as these typically have a distinct defining boundary of leaf litter and are easily identified as the sand is almost pure white in colour. Each patch was mapped to estimate area and nearest-neighbour distances measured among all burrows during July 1991. Each patch was chosen because it had both a well-defined edge and a large enough spider population for statistical analysis ( ≥ 19 G. xera, based on the minimum expected frequencies for the Chi-square test with at least four nearest-neighbour distance categories). The Pielou goodness-of-fit nearest-neighbour statistic was used as a significance test for non-random dispersion as this statistic is calculated from the frequency distribution of the nearest-neighbour distances rather than the mean, as does the widely used Clark–Evans test (Clark & Evans, 1954; Pielou, 1977; Campbell, 1990). Because G. xera is territorial (Marshall, 1996) the resulting overdispersion of burrows at a fine spatial scale will confound the Clark–Evans statistic by reducing the frequency of small nearest-neighbour distances (Campbell, 1990). The Pielou test is sensitive to any deviation from randomness, even if the pattern includes overdispersion and aggregation of burrows at different spatial scales. The Clark–Evans index of aggregation was also calculated for its descriptive value.

Microhabitat associations To test for fine-scale environmental determinants of burrow placement, habitat data were collected on burrow sites along a north–south 30 cm line transect centred on each of 60 arbitrarily-selected burrows in scrubby flatwoods and rosemary scrub at three different sites at Archbold in August 1990. A 30 cm line transect was selected because this is the mean nearest-neighbour distance mediated by territorial behaviour and thus should reflect the diameter of a circular area used by the spider in its daily movements (Marshall, 1996). A non-burrow site was selected for comparison with each burrow site by taking a parallel transect 30 cm east of each burrow transect with the centre 30 cm south of the burrow (42·4 cm from the burrow). This was done in order to have a simple rule for collecting data on non-burrow sites in the same general area as active burrow sites. The presence or absence of different habitat variables were scored for each of ten 3-cm intervals along the 30 cm transect. The categorical variables were substratum type (e.g. sand, leaf litter) and vegetation (species). Quantitative variables for each burrow were: distance to vegetation taller than 50 cm, distance to the edge of the sand patch (recorded as 0 if outside of the sand patch), nearest-neighbour size (as estimated by the burrow mouth diameter, which is closely correlated with body size; Marshall, 1995a), and nearest-neighbour distance. Discriminant analysis was here used for exploratory analysis of habitat associations. None of the variables were distributed normally. Because discriminant analysis assumes multivariate normality, univariate non-parametric tests (Mann-Whitney U) were also conducted on each variable rather than the customary F statistics. Alpha levels were adjusted (because multiple tests on the same data set were being

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conducted) using the Bonferroni adjustment (alpha = 0·05 divided by the number of tests; Chandler, 1995).

Patch-level patterns of prey availability To test the hypothesis that aggregations form at sites of high prey availability, arthropod abundance was sampled on three dates over a 2-year period using sticky traps. On each date traps were set at 30 sites in areas of barren sand: 15 high density G. xera population sites and 15 sites lacking G. xera. The criterion for trap site selection was that at least 10 spiders must be within a 1 m diameter circle centred on the trap for high density sites, and no spiders within a 1 m diameter circle for low density sites. The traps were set in June and July 1991 and April 1992. In 1992 a lower criterion of seven spiders was used for high density sites. This was necessary because the trapping date was earlier in 1992 and recruitment of the young of that year had not yet increased the overall population densities.

Substratum moisture While G. xera can obtain moisture from its prey, it is also dependent on soil capillary water to meet its moisture requirements. During the early spring dry season in the scrub the sandy soils become somewhat droughty (Menges & Gallo, 1991) and G. xera might be water limited. Both microtopography and the proportion of humic material in the sand affects water retention (Abrahamson et al., 1984). Water content of sand was quantified during the spring dry season (on 15 April 1993) after an extended period with no rain. The hypothesis that aggregations might persist at sites that retain water better was tested by gravimetric measure of the moisture content of sand at the centre of each of 10 high- and low-population density sites. These sites were paired within sand patches. A standard volume of sand was collected from each of three depths: 5·0, 15·0, and 25·0 cm. This range of depths was selected to represent the range of depths of G. xera burrows (mean depth in cm ± 1 S.D.: 16·6 ± 3·2, N = 25). Sand samples were immediately weighed, oven-dried, and weighed again and the proportion of water by weight calculated. A paired t-test was performed on the arc sine square root transform of these values, paired within patches.

Site tenacity and population density If there were an ecological benefit associated with aggregation, then aggregations of burrows might form by the longer burrow-site tenure of aggregated spiders when compared to solitary spiders. This differential site tenacity might be due either to some habitat variable not quantified, or any density-dependent ecological factor. To test for an effect of dispersion on survivorship, spiders in marked burrows were censused and the influence of grouping on burrow-site tenure by spiders was examined. Open burrows were used as an indication of active foragers and closed burrows as an indication of abandoned sites or closure for undetermined maintenance activity (e.g. moulting). It was assumed that extended closure ( > 2 weeks) was synonymous with abandonment (over 90% of burrow closure periods are shorter than 14 days; Marshall, 1995a). Abandoned burrows collapse within days (Marshall, 1995a). Thirty-seven spiders within 15 habitat patches were randomly selected and the number of neighbours within 1 m of the focal spider’s burrow on the first day was recorded as an estimate of burrow-site population density. In those sand patches with more than one focal individual test burrows were restricted to a distance of at least 2 m

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apart. Each marked burrow was checked on approximately alternate days between 28 May and 18 July 1991 and at each census it was noted whether the burrow was open or closed. It was predicted that extended burrow closure (taken as evidence of burrow abandonment) by focal animals in aggregations would be evidence of a negative affect of aggregation and vice versa. Data were analysed by taking the density estimate and comparing it with the number of days the burrow was open using Spearman’s Rho. Results Landscape-level habitat associations The area mapped had 37 patches categorized as barren sand (Fig. 1). The patches ranged in size from 0·1 to 29·5 m2 with an average size of 4·2 ± 5·9 m2 (mean ± 1 S.D.) and made up 12% of the area mapped. There were 422 burrows in the area mapped. The variogram shows that the variation in the pattern of burrow dispersion is small at a fine spatial scale, increases rapidly, and reaches an asymptote at approximately 3·0 m (Fig. 2). The correlogram shows that the correlation between spiders and sand is highest at short lag distances and that this relationship decays rapidly to approximately 3·0 m (Fig. 3). Habitat selection The morning after release, all spiders (10 in each of five enclosures) had built burrows of the end of the enclosure free of leaf litter. These results demonstrate that the observed habitat associations are the result of avoidance of areas covered with leaf litter rather than sampling bias or fortuitous association.

Figure 1. Map of patches of open sand in a 1296 m2 area of scrub habitat mapped in an area of scrubby flatwoods at Archbold Biological Station, Highlands County, Florida. Scale bar is 3 m. Arrow indicates North. Defined outlines are areas of open sand, all other substrata are areas in which the sand is covered with vegetative debris or dead leaves.

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Landscape-level variation in prey availability Burrow sites were prey-poor compared to random sites (Table 1(a)). More potential prey were trapped at random sites in the scrub than at burrow sites along the fire lane. This difference was highly significant, with traps at random sites catching approximately two to four times as much prey as traps at burrow sites. Most of the random 0.7

Moment of inertia

0.6

0.5

0.4

0.3

2.7

5.4

8.1 10.8 Lag distance (m)

13.5

16.2

Figure 2. Variogram of Geolycosa xera burrow dispersion in a 1296 m2 area of scrub habitat mapped at Archbold Biological Station, Highlands County, Florida. Note the initial sharp increase in variation at increasing lag distances, leveling off at approximately 3·0 m. (d) = East– West direction; (n) = North–South direction. 0.3

Correlation

0.2

0.1

0.0

–0.1 0.0

1.8

3.6

5.4

7.2 9.0 10.8 Lag distance (m)

12.6

14.4

16.2

18.0

Figure 3. Correlogram of Geolycosa xera burrows and open sand in a 1296 m2 area of scrub habitat mapped at Archbold Biological Station, Highlands County, Florida. Note the initial sharp decline in correlation at increasing lag distances. This indicates co-occurrence of sand and burrows at a small spatial scale. (h) = East–West direction; (r) = North–South direction.

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sites were in areas covered with leaf litter, which may explain the higher insect abundances.

Patch-level patterns of dispersion The dispersion pattern in three of nine G. xera populations was significantly nonrandom (Table 2) as reflected by the distribution of nearest-neighbour distances. The results of Fisher’s combinatorial test on significance values (Sokal & Rohlf, 1981) also indicates that the overall pattern is significantly non-random (Chi-square = 44·81, df. = 18, p = 0·0004). The Clark–Evans index shows that this is explained by aggregation in all but one case (Table 2).

Microhabitat associations Of all the variables tested, discriminant analysis identified barren sand alone as the best predictor of a burrow site (at alpha = 0·0028, Mann-Whitney U test; Table 3). Leaf litter was also strongly (but not significantly) associated with the non-burrow sites. Sand and leaf litter substrata are mutually exclusive. This is congruent with the findings at the level of the macrohabitat: open sand is a prerequisite for G. xera burrow placement.

Table 1. Results of sticky traps set to assess arthropod abundance (N=15 traps at each category site); (a) traps set to assess potential prey abundance at Geolycosa xera burrow sites and random sites; (b) traps set to assess potential prey at high Geolycosa xera population density sites and at low population-density sites

(a) Landscape-level variation in prey availability Total prey trapped Date 20–26 March 1990 17–24 July 1990 27 July–2 August 1990

Burrow sites

Random sites

Chi-square

df.

p

120 164 518

220 855 1,147

12·6 567·1 362·9

6 6 6

0·05 ≈0 ≈0

6 6 6

0·51 0·11 0·40

(b) Patch-level patterns of prey abundance

9–15 June 1991 23–29 July 1991 1–7 April 1992

High populationdensity sites

Low populationdensity sites

494 565 260

477 482 228

5·30 10·24 6·25

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Table 2. Summary of Pielou goodness-of-fit test for non-random dispersion and the Clark–Evans index of aggregation using nearest-neighbour distances between Geolycosa xera burrows found in nine microhabitat patches at Archbold Biological Station, summer 1991

Habitat type*

Area (m2)

N

SF SF SF SF SF RS RS RS RS

15 17 8 11 29 10 18 25 6

21 20 23 23 89 19 60 54 19

Density Mean (spider per NND† m2) (Cm) 1·5 1·2 2·9 2·1 3·1 1·9 3·3 2·2 3·2

30·2 37·9 28·7 29·1 33·1 32·4 24·4 33·3 28·6

χ2

df.

p

R‡

12·55 3·60 1·17 7·43 28·64 9·00 9·33 10·44 1·84

3 3 3 3 9 3 9 9 3

0·006 0·308 0·759 0·059 0·001 0·029 0·407 0·316 0·606

0·697 0·801 0·503 0·823 1·153 0·867 0·883 0·711 0·990

*SF=scrubby flatwoods; RS=rosemary scrub. †NND=nearest-neighbour distance. ‡Index of aggregation, values <1 indicate aggregation.

Table 3. Results of discriminant analysis of habitat variables at Geolycosa xera burrow sites vs. non-burrow sites. Variables are habitat features and plant species. Variables ranked using F scores and attributed to burrow site/non-burrow site on the basis of the sign of the canonical coefficient

Variable Burrow sites Sand Vegetative debris Ceratiola ericoides Lyonia spp. Nearest-neighbour distance Licania michauxii Distance to vegetation >0·5 m tall Quercus spp. Non-burrow sites Leaf litter Nearest-neighbour size Light leaf litter Serenoa repens and Sabal etonia Selaginella arenicola Distance to patch edge Paronychia chartacea Lechea deckertii Vaccinium myrsinites Aristida stricta

F

Significance level*

13·6186 2·3988 1·0000 0·8722 0·5261 0·3362 0·0973 0·0282

0·0023* 0·1241 0·3194 0·3522 0·4697 0·5631 0·7557 0·8669

7·6144 3·2544 3·2254 2·1790 1·0874 0·7324 0·5204 0·3233 0·0400 0·0075

0·0066 0·0738 0·0751 0·1426 0·2992 0·3938 0·4721 0·5707 0·8418 0·9314

*The test statistic is the Mann-Whitney U test on raw data for burrow sites vs. non-burrow sites, N=120. Based on an adjusted alpha level of p=0·0028 to account for multiple analyses, only sand is significant.

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Patch-level patterns of prey availability There was no significant difference in prey abundance at high vs. low population density sites for any of the three trapping periods (Table 1(b)). Geolycosa xera aggregations are not associated with sites of high prey abundance. Substratum moisture There was no significant difference in the soil water content between high and low population density sites (t-test for each depth: 5 cm, t = 0·69, df. = 9, p = 0·25; 15 cm, t = 1·22, df. = 9, p = 0·17; 25 cm, t = 1·57, df. = 9, p = 0·08; Fig. 4). There is no indication that differences in moisture retention of the sand at these sites explain the presence of aggregations. Site tenacity and population density Focal animals had between two and 12 neighbours within 1 m (mean ± S.D.: 5·9 ± 2·7, N = 37). There was no significant correlation between the number of census days the spider was active and the number of neighbours within 1 m of the focal spider’s burrow (Spearman’s Rho = 0·07, Z = 0·43, p = 0·67). Discussion When examined at the level of the scrub landscape, G. xera has a clumped distribution that is explained by the availability of open sand substrata. The correlation between burrow location and open sand is close at a fine spatial scale, and deteriorates rapidly

Mean proportion water by weight

0.04

0.03

0.02

0.01

0.00

5

15 Depth (cm)

25

Figure 4. Moisture content of sand (collected on 15 April 1993) at three depths at both low (h) and high (j) Geolycosa xera population density sites (mean ± 1 S.D., N = 10) at Archbold Biological Station, Highlands County, Florida.

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at greater lag distances, reflecting the observation that burrows were found in open, sandy areas and that all such patches mapped had a population of G. xera. The experimental test for habitat selection shows that the association of G. xera with barren sandy substrata is due to avoidance of areas covered in leaf litter when selecting a burrow site. The preference that G. xera exhibits for open, sandy microhabitats is not explained by prey availability, as burrow sites were prey-poor compared with random sites in the landscape. The association of G. xera with a relatively prey-poor microhabitat (open patches of barren sand) may be due to phylogenetic history. Geolycosa wolf spiders in the eastern U.S.A. fall into one of two ecotypes: those which are found in barren, sandy areas and those found in areas covered in leaf litter. Geolycosa spp. in the latter category generally build a turret of leaves and silk at the entrance to its burrow. Possession of this turretbuilding behaviour evidently allows the spiders to clear fallen leaves from the burrow mouth (Marshall, 1994). Geolycosa xera does not build a turret and will abandon burrows which are covered with leaf litter. At Archbold Biological Station there is a turret-building species, G. hubbelli, which is found in areas covered in leaf litter. Geolycosa hubbelli was present in the area mapped in this study in areas of deep leaf litter where G. xera was not found. Why G. xera and other Geolycosa species associated with barren substrata (e.g. G. escambiensis, G. patellonigra, G. pikei) are dependant on barren substrata for burrow construction is unknown, and perhaps unknowable. Geolycosa xera uses its burrow as a refuge from diurnal visual predators (e.g. birds, lizards, wasps) and thermal and hydric stress. Having this refuge, G. xera forages for most of the day. Thus it can make up for foraging opportunities lost by being restricted to barren areas by foraging for more hours of each day than do its vagrant confamilials which forage more actively, but only at night. Predation risk and extreme abiotic conditions are factors which may limit the activity of sympatric, surface-active, vagrant wolf spiders (e.g. Hogna spp., Arctosa spp.) to after dark. Animal dispersion patterns are assumed to reflect adaptive site choice decisions made in response to habitat cues or conspecifics. Within the sand patches the combinatorial test of Pielou test results reveals that the overall pattern of dispersion of G. xera is non-random, and the Clark–Evans index of aggregation indicates that this is explained by a tendency to aggregate. While the G. xera population within all but one of the patches tends towards aggregation, the indications of the Pielou statistic are not consistent between patches. Because there are no environmental factors dictating burrow placement within the barren sand patches, variation in the patterns of dispersion of the burrows of G. xera between individual habitat patches may reflect the fact that each barren sand patch has a unique history, with different mechanisms of formation, different times since colonization by G. xera, and a different G. xera population density. Four potential correlates of grouping were examined: habitat structure, prey abundance, substratum moisture, and site tenacity. No evidence was found that any of these were associated with G. xera population density. The results of the discriminant analysis again indicate that the only correlate of burrow placement within patches is the open sand itself, which is congruent with the results of the geostatistical analysis performed on a broader spatial scale. Among web spiders the aggregative response to locally abundant prey is well established (Riechert, 1974; Uetz, 1990). This is the basis of my prediction that groups of G. xera would be correlated with locally abundant prey. However, no evidence for any relationship between prey abundance and spider density was found. Even though the scrub habitat can be quite dry, there was no evidence that the sand in the vicinity of aggregations is moister than the sand at low population density sites. Thus, enhanced survivorship of individual spiders at sites which better retained moisture during periods of drought does not explain aggregation formation. Finally, there was no correlation of aggregation itself with site tenacity. Thus, no benefits associated with grouping can explain the pattern observed. The overall lack of

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a predictor of burrow placement, other than sand, may be a result of the fact that these microhabitat patches are generally barren, meaning that there are no cues for site selection. While mutual attraction may explain aggregation in some taxa, G. xera does not exhibit conspecific attraction (Marshall, 1995b). Dispersal strategies alone can generate pattern in the absence of patterned resources (Kareiva, 1990). Within patches of barren sand non-random dispersion patterns of G. xera burrows are generated by the dispersal behaviour of the spiderlings, which hatch in the maternal burrow and disperse a short distance before digging their own burrows (mean distance from the maternal burrow to hatchling burrows ± 1 SE for five cohorts; 55·5 ± 8·9 cm; Marshall, 1995b). This limited dispersal behaviour is opposed by the territorial behaviour of G. xera (Marshall, 1995b, 1996). In addition, spiders forced to relocate travel relatively short distances before digging a new burrow. It is found that dispersal distances between release site and the new burrow site are remarkably short (mean ± 1 SE; 43·9 ± 4·7 cm, N = 68; Marshall, 1995b). Geolycosa xera behaves more like a plant than an animal in that spiderling settlement in the vicinity of the maternal burrow is similar to seed fall near a parent plant. However, unlike plants, G. xera is capable of rapid directed movement across the sand. Why is G. xera such a conservative disperser? In the present study evidence is presented that there are no apparent ecological correlates with the aggregations observed. Geolycosa xera has restrictive habitat requirements and so its limited dispersal may be de facto habitat selection. The barren areas used by G. xera are predictable microhabitats. There are other taxa in the Florida scrub which have evolved a dependence on these barren patches (e.g. plants, Hawkes & Menges, 1996; insects, Deyrup, 1989). The area mapped for geostatistical analysis had not burned since 26 September 1984 (E. Menges, pers. comm.). Moreover, most of the barren areas mapped have been stable for at least the last 6 years (1990–1996). Thus, it is likely that there has been open sand available at the mapped study area for at least 12 years (1984–1996). These barren patches do not represent an early successional stage, but are distinctive components of the scrub ecosystem (Abrahamson et al., 1984; Hawkes & Menges, 1996). There is little net movement of sand by wind or water to create new open habitats. The Florida scrub, like most shrublands, is a pyrogenic ecosystem (Myers, 1990) and only fire can create the larger barren patches. However, it is vegetation which determines the limits of the barren areas which persist between fires. In scrubby flatwoods leaf fall soon covers the burned-over sand. Wind clears the sand of leaves in areas lacking vegetation (mostly shrub oaks) to anchor the leaf fall and what barren areas persist do so because the oaks are extremely slow to invade. In rosemary scrub factors other than wind and oak placement explain the maintenance of open areas including the growth habit and sparse leaf fall of Florida rosemary (Hawkes & Menges, 1996). Because many areas of barren sand in both scrubby flatwoods and rosemary scrub are temporally predictable islands of habitat in a spatially unpredictable landscape, the limited dispersal of G. xera may be an adaptive strategy (Southwood, 1962; Greenstone, 1982). Human activities can have a major impact on G. xera populations. These spiders will readily use anthropogenic barren areas such as sand roads and fire breaks. Alternatively, fire suppression will facilitate the succession of scrub to a xeric oak hammock (Laessle, 1968; Menges et al., 1993). In the later stages of this sere canopy closure occurs, areas of barren sand are covered with leaf fall, and G. xera populations disappear. For this reason, prescribed burns are an important habitat management technique for G. xera (and other taxa dependant on open, sandy areas). Geolycosa xera exhibits a specialized strategy of burrowing in barren, relatively preypoor patches of sand. Despite being a habitat specialist (or perhaps because of it), G. xera is locally abundant, and attains the highest population densities recorded for any Geolycosa wolf spider (Marshall, 1995a). At the level of the landscape the dispersion pattern of G. xera is determined by the pattern of patches of barren sand, which are in

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turn created by fire (and other unpredictable events) and defined by vegetation. This habitat association is the product of an evolved association of G. xera with barren substrata. Within the patches of barren sand the pattern of dispersion exhibited is a combination of territorial behaviour and the extremely limited dispersal of G. xera, rather than non-random dispersion of resources. The limited dispersal behaviour of G. xera may be adaptive, given the patchy, but stable landscape in which it has evolved. This work benefited from critical review by C. Boake, G. Burghardt, T. Crist, A. Echternacht, D. Etnier, J. Halaj, J. Henschel, M. Hodge, E. Menges, S. Riechert, A. Rypstra, D. Wise, and the members of the spider groups at the University of Tennessee and Miami University. The comments of anonymous reviewers also improved the manuscript. The research was funded by NICHD Training Grant (T32-HD-07303), and by grants from the Theodore Roosevelt Memorial Fund, Sigma Xi, and Archbold Expeditions.

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