HABITAT USE by the FISHING SPIDER DOLOMEDES TRITON in a NORTHERN Arthropods, Fishes, and Anurans (Gudger 1925, Williams 1979, Zimmerman and Spence 1989)
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Published in Wetlands, Volume 14:239-242. INTRODUCTION The vast freshwater wetlands of the Florida Everglades support a diverse aquatic and semi-aquatic arthropod fauna (Serrao 1980, Gunderson and Loftus 1992). One of the most NOTE conspicuous members of this arthropod fauna is the pisaurid fishing spider Dolomedes triton (Walckenaer). These semi-aquatic, hunting arachnids are voracious predators of other HABITAT USE BY THE FISHING SPIDER DOLOMEDES TRITON IN A NORTHERN arthropods, fishes, and anurans (Gudger 1925, Williams 1979, Zimmerman and Spence 1989). It EVERGLADES WETLAND is plausible that fishing spiders are keystone predators, although there is little agreement concerning the general role that spiders play in structuring either aquatic or terrestrial prey Frank Jordan1,3, Howard L. Jelks2,3, and Wiley M. Kitchens2,3 communities (Turnbull 1973, Foelix 1982, Nyffeler and Benz 1987, Oraze and Grigarick 1989). 1-Department of Zoology Indeed, there are few published studies dealing with any aspect of the biology of spiders living in 2-Department of Wildlife & Range Sciences 3-National Biological Survey natural wetland systems. Clearly, quantitative abundance and distribution data are needed to Florida Cooperative Fish & Wildlife Research Unit University of Florida increase our understanding of the ecology of fishing spiders in freshwater wetlands. 117 Newins-Ziegler Hall Gainesville, FL 32611 In this paper, we provide basic habitat use information about the fishing spider D. triton in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, a northern Everglades wetland system. Specifically, patterns of differential habitat use and seasonal variation in abundance Abstract: We investigated patterns of habitat use by adult fishing spiders Dolomedes triton in a were examined during an 18-month period. seasonally dynamic freshwater wetland of the northern Everglades. Spiders were collected STUDY AREA AND METHODS monthly (July 1990-January 1992) in adjacent wet prairie, alligator hole, and sawgrass habitats. Study Area Overall density of adult fishing spiders during the study period was 0.1 spiders per m2, although Research was carried out in the northern Everglades (Palm Beach County, Florida) at the there was extensive seasonal variation in abundance of these semi-aquatic arthropod predators. Arthur R. Marshall Loxahatchee National Wildlife Refuge (hereafter the Refuge). The Refuge is Densities were significantly higher in sawgrass stands than in wet prairies or alligator holes, a 57,234 hectare impounded marsh system historically comprised of emergent wet prairies, which did not differ from one another. Sawgrass sites were characterized by shallower water sawgrass stands, alligator holes, and elongate tree islands (Loveless 1959). More detailed habitat depths, taller and denser canopy structure, more extensive litter deposits, and fewer potential descriptions are provided by Richardson et al. (1990). avian and piscine predators. Overall, these data suggest that structural complexity and habitat The Refuge is the northern remnant of the Everglades, which historically stretched from heterogeneity influence the demographics of the fishing spider in this Everglades marsh system. the southern shores of Lake Okeechobee to the mangrove forests of northern Florida Bay. Currently, it is part of the vast south Florida water routing system and is operated as Water Conservation Area 1 by the South Florida Water Management District. The entire marsh is in as close proximity to one another as possible. Within each of these habitats, three traps encircled by a deep-water canal and dike system that drains an approximately equal area of the (subsamples) were randomly collected. Sampling was carried out monthly (except December Everglades Agricultural Area. The altered hydrologic regime, coupled with agriculturally 1991) from August 1990 through January 1992. Each month, six areas were randomly selected derived nutrient effluents, has led to extensive habitat conversion in the Refuge (Richardson et for sampling using a grid map of the Refuge and a random number table. Therefore, 54 trap al. 1990). For example, approximately 2,400 ha of sawgrass-dominated habitat has been samples (6 areas x 3 habitats x 3 subsamples) were collected during each of the 17 months (total replaced by a virtual monoculture of cattail (Typha domingensis Pers.). Concern over the impact n=918). This design was used to minimize airboat damage to vegetation associated with of habitat degradation on wildlife resources provided the primary impetus for this research. This repetitive sampling of fixed positions. study was carried out as part of a larger ecological characterization of the Refuge and its aquatic Due to the large number of empty traps, we converted spider densities to macrofaunal assemblages. presence/absence data and used multiple logistic regression (loglikelihood analysis) to examine the relationship between spider presence and the independent variables of habitat type, area, and Sampling Methods month. This loglikelihood approach is analagous to the analysis of variance method used for Aquatic macrofauna were collected by throwing an aluminum throw trap (100 x 100 x 75 continuous, interval data. cm) into the desired habitat and pressing firmly into the substrate. This technique has been shown to provide quantitative estimates of macrofaunal abundance (e.g., Kushlan 1981, Freeman RESULTS et al. 1984, Chick et al. 1992). Above-ground vegetation was identified and counted (relative Sawgrass, wet prairie, and alligator hole sites were physically distinct from one another. coverage), uprooted, and shaken to dislodge any macrofauna. A bar seine with 3.0-mm stretch Sawgrass sites were virtually monotypic, with Cladium jamaicense Crantz comprising 94% of mesh was then passed through the trap until three consecutive empty sweeps were obtained. the plant cover at these sites. The wet prairie sites were dominated by the emergents Eleocharis Traps were visually inspected for spiders following the clearing procedure. Fishing spiders were spp. and Rhynchospora spp. Alligator holes were comprised primarily of Utricularia spp. and identified, counted, and released back into the surrounding habitat. Nymphaea odorata Aiton. Wet prairies and alligator holes were generally deeper and more Several habitat variables were measured in addition to vegetation composition. Water open, whereas sawgrass sites were relatively shallow (average depth = 22 cm) and had well depth and canopy height were measured to the nearest centimeter inside each trap prior to developed canopies (average height = 145 cm) that provided significant shading and cover. removal of plants. Percentage dead sawgrass within each trap was visually estimated at the Above-ground plant biomass (dry weight) ranged from a low of 125 g/m2 in alligator holes to a sawgrass sites. Combined (live and dead) above-ground vegetation was dried and weighed to the high of 757 g/m2 in sawgrass, with wet prairie sites being intermediate at 284 g/m2. Dead plant nearest gram to obtain plant biomass estimates. material on average comprised 44% of the total Cladium coverage in sawgrass sites. The other The study involved a stratified random design with blocking. Areas (blocks) were two habitats did not have well developed litter deposits. random coordinates located throughout the Refuge. Within an area, one site each of sawgrass, Fishing spiders were distributed non-randomly with respect to habitat type (loglikelihood wet prairie, and alligator hole habitat were visited. These sites were sampled as encountered, and analysis, p<0.05; see Figure 1). Iterative removal of the different habitat types from the multiple regression model indicated that fishing spiders were significantly more abundant in sawgrass 1986, Uetz 1991). In this study, we found that the complex wetland mosaic comprising the than in either wet prairies or alligator holes, which did not differ from each other. The blocking northern Everglades affected the distribution of the fishing spider. The finding of higher effect was not significant (loglikelihood analysis, p>0.05). Therefore, the observed patterns of densities of fishing spiders in sawgrass is concordant with previous work by Bleckmann and differential habitat use were consistent across the wetland landscape. Rovner (1984). They found that the number of fishing spiders was highest in cattail habitat and A total of 918 1-m2 throw trap samples were collected during the study, yielding only 85 decreased as a function of decreasing plant cover and complexity. Relative to emergent fishing spiders. Individual traps contained between 0 and 5 spiders. Overall density was 0.1 vegetation of lesser stature, cattail and sawgrass provide similar habitat structure for fishing fishing spiders per meter square. However, there was considerable temporal variation in spiders: tall plant canopies with elevated levels of shading, enhanced protection from avian and abundance estimates. Monthly densities ranged from 0.3 spiders per m2 in May 1991, to no piscine predators, and reduced wind stimulii. spiders during the months of February, March, and April of 1991. The month effect was not Fishing spiders are not web builders but rather rely on mechanoreception to detect prey significant (loglikelihood analysis, p>0.05). There were no clear seasonal trends in spider that have become trapped on the water's surface or that are swimming just beneath it (Williams abundance, and interannual variation was great. 1979, Bleckmann and Barth