Mesic (or wet) flatwood habitats are typically dominated by slash pine (Pinus elliottii Engelm.; Pinales: Pinaceae) or longleaf pine (P. palustris Mill.; Pinales: Pinaceae), and because of the poor drainage of their soils they experience seasonal inundation with water (Harms et al. 1998). Historically these habitats stretched across the Atlantic and Gulf of Mexico coastal plains of the United States, but increasingly they are threatened by anthropogenic activities and, in recent decades, their range has been greatly reduced (Harms et al. 1998). In addition to their importance economically as a source of timber and fiber, mesic flatwoods are known to support diverse communities of macrofauna (Geneva & Roberts 2009). However, relatively few quantitative data exist on the arthropod assemblages that inhabit mesic flatwoods.

Given that a distinguishing feature of mesic flatwoods is their periodic flooding, the arthropod assemblages found in these habitats could be considerably different from those in nearby xeric or even hydric habitats. Indeed, classical intermediate disturbance theory (Grime 1973; Connell 1978) suggests that such periodic flooding in mesic flatwoods could even lead to higher arthropod diversity than in habitats with more consistent hydrological regimes, and thus they may host a relatively unique arthropod community. Collecting baseline data on the arthropod communities in these habitats is therefore important for several reasons. For example, because many arthropod taxa often are sensitive to changes in environmental conditions they can be utilized as indicators of pollution or other anthropogenic disturbances (Kremen et al. 1993; Schweiger et al. 2005), which could be useful for the management of mesic flatwoods. Furthermore, by determining the associations between different arthropod groups in a community at a coarse taxonomic level, it could also be possible to predict changes in the assemblages based on the loss of certain taxa or the type of disturbance.

Spiders (Araneae) in particular have frequently been utilized as indicator taxa because of their abundance and the relative ease with which they can be collected (Wise 1993; Shochat et al. 2004; Buchholz 2010). In addition to being utilized as indicator taxa, spiders can play an important role in many ecosystems because of the top-down effects they exert on their arthropod prey (Riechert & Bishop 1990; Wise 2004), which often includes pest species. Given that exhaustive species surveys of sites often are impractical for taxa such as spiders, using species richness estimators to extrapolate the true species richness for a site could be more useful (Colwell & Coddington 1994). Perhaps most importantly, the species accumulation curves could be useful for informing decisions on sampling effort at other sites with similar habitat.

Accordingly, we surveyed arthropods at 2 relatively undisturbed mesic flatwood sites in Hillsborough County, Florida, with a further focus on ground-surface spiders. Our objectives were as follows: 1) to collect baseline data on the richness and abundance of ground-surface spiders and other arthropods, 2) to examine the associations among arthropod taxa in the community, and 3) to use species richness estimators to extrapolate the true species richness for ground-surface spiders at these sites.

MATERIALS AND METHODS

RESULTS

Arthropod Richness and Abundance

Including spiders, we collected a total of 3,101 arthropods (1,591 at BCH, 1,510 at ERA) from 11 taxonomic groups (Actindedida, Araneae, Coleoptera, Collembola, Diptera, Formicidae, Hemiptera, Homoptera, Hymenoptera (excluding Formicidae), Lepidoptera, and Orthoptera) (Fig. 1). Collembola was the most abundant taxa at both sites (58.5% of the total arthropods at BCH, 60.3% at ERA), followed by Formicidae (29.5% at BCH, 21.6% at ERA). With the exception of Diptera at ERA (9.3%), all other taxa comprised < 3% of the total number of arthropods each for both sites.

TABLE 1.

TOTAL NUMBERS OF SPIDERS COLLECTED AT BROOKER CREEK HEADWATERS PRESERVE (BCH) AND THE UNIVERSITY OF SOUTH FLORIDA ECOLOGICAL RESEARCH AREA (ERA) IN 2008.

Spider Species Richness Estimators

None of the species richness estimators we used reached a clear asymptote, however the curve generated using Michaelis-Menten means appeared to be closest to approaching this (Fig. 2). The Chao 2 estimator yielded the highest number of species (91), almost double the number generated by Michaelis-Menten means (53). The ICE estimator produced a species number that was slightly above the Michaelis-Menten means (67). We also found that there was always a greater number of uniques than duplicates, with uniques steadily increasing while the number of duplicates eventually started to decrease.

Fig. 1.

Percent abundance of different arthropod taxa at Brooker Creek Headwaters Preserve (BCH) and the University of South Florida Ecological Research Area (ERA).

Principle Coordinate Analysis

PCoA revealed that arthropod assemblages differed at both sites, particularly driven by variation in abundance of Diptera and to a lesser extent Orthoptera (Fig. 3). Ground-surface spiders were strongly negatively associated with Diptera, and only weakly associated with other common prey taxa such as Collembola and Homoptera. Formicidae also appeared to be negatively associated with Collembola and Homoptera.

Fig. 2.

Individual-based spider species rarefaction (S observed), estimated true richness (Chao 2, ICE [incidence-based coverage estimator], and Michaelis-Menten means), and uniques and duplicates.

Fig. 3.

Results of a principal coordinate analysis (based on Hellinger's distance) of the relationships among arthropods at Brooker Creek Headwaters Preserve (BCH) and the University of South Florida Ecological Research Area (ERA).

DISCUSSION

At the family level, ground-surface spider diversity and relative abundance were fairly similar to sandhill and xeric or hydric flatwood habitats elsewhere in Florida (Muma 1973; Corey & Taylor 1988; Corey et al. 1998). For example, Muma (1973) found Lycosidae to be the most dominant ground-surface spider family at a hydric pine flatwoods site in central Florida, comprising 64% of the total number of spiders. At another site in central Florida, Corey & Taylor (1988) examined more mesic flatwoods, and found that lycosids also dominated the habitat (47.2%), although not quite to the same extent as found by Muma (1973). Our results at the family level are even fairly similar to those of Corey et al. (1998), who examined ground-surface spider assemblages at several sandhill sites and found Lycosidae to be the most dominant family (75.2%). However, at the species level some differences become apparent between the communities we examined and those from previously published work by Muma (1973) and Corey & Taylor (1988). For instance, we identified 9 species of lycosids compared to 13 by Muma (1973), with only 3 species common to both studies (Hogna lenta [Hentz], Pirata suwaneus and Sosippus floridanus). Furthermore, of the 15 most common species found in mesic flatwoods by Corey & Taylor (1988), only 2 were present at our sites (Ctenus captiosus and S. floridanus). These observed patterns could represent true differences between the communities, but perhaps more likely result from the less intensive sampling we conducted compared to the previous studies of Muma (1973) and Corey & Taylor (1988).

None of our species accumulation curves reached an asymptote, which further suggests that we did not sample all of the ground-surface spider species present at our study sites. Additionally, there was considerable variation in the results produced by the different estimators, with the total number of species projected to be approximately 2 to 3 times greater than the number of species observed. Some of the curves also generated seemingly unrealistic results. For example, 39 species were projected to be collected after just 2 samples using the Michaelis-Menten means estimator. The overall performance of the estimators used was similar to the results of Toti et al. (2000), who examined the species richness of spider assemblages in Appalachian balds and found that the Michaelis-Menten means estimator performed best when judged by their indirect criteria (that the curve was close to reaching an asymptote with fewer samples than the observed species curve, and that the estimates were close to a visual extrapolation of the asymptote of the observed species curve). Based on the performance of the 3 estimators, it appears as though the true spider species richness is at least 50–70 species (where the ICE and Michaelis-Menten means curves were more closely grouped together). This number seems to be consistent with the more extensive previous surveys in flatwoods by Muma (1973) and Corey & Taylor (1988), who found 55 and 48 species respectively.

Our objective in the present study was to survey ground-surface spiders and other arthropods, and accordingly we deployed pitfall traps. Pitfall traps generally are effective at catching arthropods inhabiting the ground-surface, and this method has previously been used numerous times (e.g., Corey & Taylor 1988; Corey et al. 1998; Stiling et al. 2010). However, spiders from the wandering-active guild (particularly lycosids) tend to be overrepresented in samples from pitfall traps, while those from web-building guilds are often underrepresented because they spend less time on the ground (Uetz & Unzicker 1975; Green 1999; Hövemeyer & Stippich 2000). Consequently, it is likely that some occasional ground-foraging species were missed by our sampling procedure. There is also evidence suggesting that the use of pitfall traps can result in samples with biased sex and age ratios, which could further limit any inferences made from our survey (Topping & Sunderland 1992). To reduce the possibility of such biases, any future sampling in mesic flatwoods should incorporate other methods such as visual species counts in quadrats, or the use of a suction sampling device (Rohr et al. 2007). Further limiting factors in any sampling procedure are the duration of the survey and seasonality, which can strongly influence the spider species captured (Hatley & MacMahon 1980). Our sampling period covered the most active time of year for ground-surface spiders in Florida (Corey & Taylor 1988), which is also the wet season. Because of the seasonal changes in water levels in mesic flatwoods however, more extensive sampling in the dry season would likely yield a more accurate estimate of the total spider species diversity.

We found that ground-surface spiders were not strongly associated with typical prey groups, supporting the idea that environmental factors can have an important influence on spider distribution (Greenstone 1984). However, many other studies have found strong associations between spiders and their prey (Harwood et al. 2001), somewhat contrary to our results. Indeed, even within the same study sites, we have previously found 1 species of spider (S. floridanus) to track the abundance of their common prey (Jennings et al. 2010). Our findings could therefore potentially be attributed to the relatively low number of spiders collected, which could have limited our ability to detect such associations between spiders and their prey.

The associations between other groups of arthropods were not particularly surprising. Collembola and Formicidae were clearly the 2 most abundant taxa, as found in ground-surface habitats elsewhere in Florida (Stiling et al. 2010; Gill et al. 2011), and both are known to be mobile foragers in the soil and leaf litter. They were negatively associated with each other, which could have been a result of microhabitat differences. For example, standing water was found at our sites on several occasions during our sampling period and certain groups of Collembola have a high affinity for these types of conditions, which conversely could have been prohibitive to many Formicidae. Diptera was responsible for most of the difference in the arthropod assemblages between our study sites, and the most parsimonious explanation for this is because of seasonal differences. Both study sites are close to lentic water-bodies and they have a similar habitat structure, but the sampling at the ERA was conducted after BCH and therefore may have coincided with increased abundance for certain species, perhaps for breeding events.

CONCLUSIONS

Our study provides baseline data on the ground-surface spider and other arthropod assemblages of mesic flatwoods. The guild structure of spiders at the study sites was similar to previous work in other flatwood habitats, being dominated by wandering-active spiders (primarily Lycosidae). The ground-surface spider species accumulation curves indicated that more intensive sampling of these habitats would be required to comprehensively sample and identify all of the species present, but from a management perspective, our results appear to be relatively consistent with previous surveys elsewhere.