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Diversity Among Ground-Dwelling Spider Assemblages: Habitat Generalists and Specialists

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Diversity Among Ground-Dwelling Spider Assemblages: Habitat Generalists and Specialists 2005. The Journal of Arachnology 33:101±109 DIVERSITY AMONG GROUND-DWELLING SPIDER ASSEMBLAGES: HABITAT GENERALISTS AND SPECIALISTS Rachael E. Mallis and Lawrence E. Hurd1: Department of Biology, Washington & Lee University, Lexington, Virginia 24450 USA. E-mail: [email protected] ABSTRACT. We sampled assemblages of ground-dwelling spiders with pitfall traps in six terrestrial habitats representing a successional gradient in southwestern Virginia, during the summer of 2002. Ap- proximately half of the 50 species trapped were habitat specialists with low abundance, found at only one of the sites, which is qualitatively consistent with the literature. Only four species, Schizocosa ocreata (Hentz 1844), Pirata insularis (Emerton 1885) Pirata aspirans (Chamberlain 1904) and Neoantistea mag- na (Keyserling 1887) were found at as many as four sites. A few species that were found in more than one study from disparate geographical communities, such as Trochosa terricola (Thorell 1856) tended also to be relatively abundant habitat generalists. In general, the majority of spider species found in studies such as ours that examined multiple sites were habitat specialists and had low abundance. For our sample sites, there was no relationship between any measure of spider diversity (S, H', J') and successional age. Our results, and those of most other published studies, are consistent with the hypothesis that spider assemblages do not undergo succession and except for a few very common generalist species the com- position of these communities is unpredictable, and may depend more on stochastic colonization and speci®c resource requirements of specialists following immigration than on any predictable association with successional parameters. Keywords: Cursorial spiders, habitat specialization, spider diversity, succession The importance of predators in the structure old ®eld, and forest litter communities (Hurd and function of natural ecosystems is becom- & Eisenberg 1990; Riechert & Bishop 1990; ing increasingly well documented (Terborgh Moran et al. 1996; Lawrence & Wise 2000). et al. 2001). Spiders are widespread and di- Given their demonstrated importance to the verse predators that are part of terrestrial ar- structure and function of many communities, thropod assemblages (Wise 1993) and arthro- it is important to gather information on the pods comprise more than half of known distribution and abundance of cursorial spider species (Wilson 1992). Cursorial spiders in species. Often it has been dif®cult to deter- particular are the dominant arthropod preda- mine what features of an environment deter- tors in many terrestrial communities, e.g., mine which, or how many, species of cursorial grasslands (Weeks & Holtzer 2000) and forest spiders will be present. For example, spider ¯oor litter (Uetz 1979). Their position in tro- diversity may not follow a trend toward in- phic structure of communities often is com- creasing diversity with increasing succession- plex: spiders in forest litter belong to both the al age (Hurd & Fagan 1992; Aitchison & decomposition and the grazing food webs be- Sutherland 2000; Buddle et al. 2000) that has cause they consume detritivores/fungivores been a traditional expectation for species of and herbivores (Uetz 1975; Wise et al. 1999). plants and animals during terrestrial succes- As larger species of wolf spiders mature, they sion (Odum 1969). prey more on herbivores that are part of the Spiders have legendary powers of dispersal grazing food web (Uetz 1975; McNabb et al. and often are among the ®rst colonizers of dis- 2001). Spiders have been experimentally dem- turbed sites (Hodkinson et al. 2001); the ®rst onstrated to exert important effects on the known colonist of Krakatoa was a spider populations of other arthropods in a variety of (Spiller et al. 1998). Many spiders have the experimental systems, including agricultural, ability to disperse by ``ballooning'' with silk at some point in their life cycles (Hodkinson 1 Corresponding author. et al. 2001). Lycosids and gnaphosids balloon 101 102 THE JOURNAL OF ARACHNOLOGY Figure 1.ÐMap of the Science Park on the campus of Washington & Lee University. The habitats used for sampling sites are described in the text: CG 5 cedar grove; OF 5 old ®eld; DR 5 disturbance recovery; LH 5 lowland hardwoods; UH 5 upland hardwoods; PW 5 Pine woods. as juveniles, while many linyphiids retain the der guilds is far from being answered, and will capability throughout adulthood (Mrzljak & require the accumulation of much more data Wiegleb 2000). However adept they are at ini- (Uetz et al. 1999). tially getting to new sites, cursorial spiders We report here on a ®eld study in which six should have speci®c habitat preferences that ®eld sites, representing different successional dictate which species will become established seres, are compared with respect to ground- and how abundant they will be. Some species dwelling spider species captured by pitfall are more particular than others: along a suc- trap sampling. We compared diversity among cessional gradient in Delaware, the lycosid sites, and the extent of habitat specialization Pirata insularis (Emerton 1885) was found (relative number of habitats in which species abundantly in all four communities examined, were found in this, and in previous studies) in whereas the gnaphosid Zelotes hentzi (Bar- these spiders. rows 1945) was rare and con®ned to the youn- gest successional site (Hurd & Fagan 1992). METHODS The structure of vegetation and some physico- Study Sites.ÐThe study sites we used for chemical habitat parameters may determine a this study are located in the 35ha Science Park spider's habitat choice (Mrzljak & Wiegleb of Washington & Lee University in Rock- 2000). Along forest litter gradients and in bridge Co, Virginia, USA (Fig. 1). The sites agroecosystems, lycosids manifest microhab- were chosen to represent different habitat itat preferences possibly based upon leaf litter, types and stages of secondary terrestrial suc- herbaceous vegetation and available moisture cession typical of this region, as described be- (Weeks & Holtzer 2000). The question of low in order of successional age. what determines the structure of cursorial spi- 1. Disturbance recovery (DR): This site is MALLIS & HURDÐCURSORIAL SPIDER DIVERSITY 103 a 0.5ha patch of level ground, surrounded by placed by hardwoods. The ground is mostly woods, that is being used in a long-term study in shade, with small patches of sunlight most of community succession. The ground had of the day. The soil is sandy and well-drained. been stripped of all native top soil prior to The litter layer consists of a # 1.0cm layer of 2001, leaving a bare, hard clay surface. Dur- dead cedar needles. We considered this site to ing fall and winter 2001 ground leaf mulch be the earliest forest community, between OF was applied to the bare clay and tilled into the and PW in age. upper 2±3cm, after which seeds of 19 native 4. Pine woods (PW): This site is in deep forb species and ®ve native grasses were shade, provided by an overstory of tall planted. The plant assemblage that sprouted ($10m) white pines (Pinus strobus) and during spring and summer 2002 constituted a mixed species of smaller hardwoods, espe- dense mixture of these intentionally planted cially red maple (Acer rubrum) and white ash species and incidental species that were pres- (Fraxinus americana), with an understory of ent as seed in the mulch (an especially con- saplings and shrubs. This species mix repre- spicuous member of the second category dur- sents an alternative intermediate stage of suc- ing late summer was ragweed, Ambrosia cession to the assemblage in CG; the relative artemisiifolia). The vegetation, which reached height of the hardwoods in this stand indicated a height of approximately 0.5m, was dense that PW may be the older of the two, at least enough to provide shade at the soil surface. 40 years old. The site is located on a ¯ood There was no organic litter layer on the soil, plain, with relatively most soil. The litter layer owing to the absence of plants on the site dur- is mainly dead needles, but somewhat thicker ing the previous year. This site is surrounded ($1cm). by a wire fence to keep out deer, and is sur- 5. Upland hardwoods (UH): This site is rounded by woods. within the woodlot boundary of the DR site. 2. Old-®eld (OF): This site is a third year The overstory is of mixed hardwoods 10±15m successional sere, which was previously sub- high, especially white ash and tulip poplar jected to mowing once or twice annually. It is (Liriodendron tulipifera), with several species fully open to the sun, and consists of grasses of oaks (Quercus spp.), and red maple in the and forbs growing up to approximately 0.5m understory. This species mix is typical of the high typical of early successional old-®elds in community that replaces coniferous species, this region, including patches of emergent (ca i.e., perhaps 10±15 years older than site PW 1.5m high) late-season goldenrod (Solidago (i.e., 50±55 years old). The soil here is well- spp.), teasel (Dipsacus sylvestris), and rag- drained and rich in organic matter, with a rel- weed (Ambrosia artemisiifolia). Plant height atively deep (3±4cm) leaf litter layer. and soil shading at this site were similar to 6. Lowland hardwoods (LH): This is a ma- those at site DR (see above), but here there ture hardwood forest that has been subjected was a sparse and shallow (#0.5cm depth) lit- to little disturbance for at least the past 70 ter layer consisting of dead plant stalks and years, with canopy trees (tulip, maple, white leaves from the previous year's growth. An ash, and mixed oak) $ 30m. This site is deep- abundant and diverse arthropod assemblage ly shaded and is on a ¯oodplain at the bottom inhabits this site, the most conspicuous of of a slope, downward from site UH.
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