WETLANDS. Vol. 14, No. 3, September 1994, pp. 239-242 ~ 1994, The Society of Wetland Scientists

NOTE

HABITAT USE BY THE FISHING SPIDER D O L O M E D E S TRITON IN A NORTHERN EVERGLADES WETLAND

Frank Jordan ~,3, Howard L. Jelks 2,3, and Wiley M. Kitchens z,~ 1 Department of Zoology

2 Department of Wildlife & Range Sciences 3 National Biological Survey

Florida Cooperative Fish & Wildlife Research Unit University of Florida

117 Newins-Ziegler Hall Gainesvitte, FL 32611

Abstract: We investigated patterns of habitat use by adult fishing spiders Dotomedes triton in a seasonally dynamic freshwater wetland of the northern Everglades. Spiders were collected monthly (July 1990-January 1992) in adjacent wet prairie, alligator hole, and sawgrass habitats. Overall density of adult fishing spiders during the study period was 0.1 spiders per m 2, although there was extensive seasonal variation in abundance of these semi-aquatic arthropod predators. Densities were significantly higher in sawgrass stands than in wet prairies or alligator holes, which did not differ from one another. Sawgrass sites were characterized by shallower water depths, taller and denser canopy structure, more extensive litter deposits, and fewer potential avian and piscine predators. OveraLl, these data suggest that structural complexity and habitat heterogeneity influ- ence the demographics of the fishing spider in this Everglades marsh system.

Key Words': fishing spider, Dolomedes triton, habitat use, Everglades

INTRODUCTION

The vast freshwater wetlands of the Horida Ever- glades support a diverse aquatic and semi-aquatic ar- thropod fauna (Serrao 1980, Gunderson and Loftus 1992). One of the most conspicuous members of this arthropod fauna is the pisaurid fishing spider Do- lomedes triton (Walckenaer). These semi-aquatic, hunting arachnids are voracious predators of other ar- thropods, fishes, and anurans (Gudger 1925, Williams 1979, Zimmerman and Spence 1989). It 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 communities (Turnbull 1973, Foelix 1982, Nyf- feler and Benz 1987, Oraze and Grigarick 1989). In- deed, there are few published studies dealing with any aspect of the biology of spiders living in natural wet- land systems. Clearly, quantitative abundance and dis- tribution data are needed to increase our understanding of the ecology of fishing spiders in freshwater wetlands.

In this paper, we provide basic habitat use infor- mation about the fishing spider D. triton in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, a

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northern Everglades wetland system. Specifically, pat- terns of differential habitat use and seasonal variation in abundance were examined during an 18-month pe- riod.

STUDY AREA AND METHODS

Study Area

Research was carried out in the northern Everglades (Palm Beach County, Horida) at the Arthur R. Mar- shall Loxahatchee National Wildlife Refuge (hereafter the Refuge). The Refuge is a 57,234 hectare impounded marsh system historically comprised of emergent wet prairies, sawgrass stands, alligator holes, and elongate tree islands (Loveless 195 9). More detailed habitat de- scriptions are provided by Richardson et al. (1990).

The Refuge is the northern remnant of the Ever- glades, which historically stretched from the southern shores of Lake Okeechobee to the mangrove forests of northern Horida Bay. Currently, it is part of the vast south Horida water routing system and is operated as Water Conservation Area 1 by the South Horida Water Management District. The entire marsh is encircled by

240 WETLANDS, Volume 14, No. 3, 1994

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'~ 0.05

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alligator hole prairie sawgrass

Figure 1. Habitat use by the fishing spider in the Arthur R. Marshall Loxahatchee National Wildlife Refuge. Each bar is the mean number of spiders per m 2 _+ 1 SE of 306 throw trap samples. Means with different letters are significantly different at the p < 0.05 level (iterative loglikelihood anal- ysis).

a deep-water canal and dike system that drains an ap- proximately equal area of the Everglades Agricultural Area. The altered hydrologic regime, coupled with ag- riculturally derived nutrient effluents, has led to ex- tensive habitat conversion in the Refuge (Richardson et al. 1990). For example, approximately 2,400 ha of sawgrass-dominated habitat has been replaced by a virtual monoculture of cattail (Typha domingensis Pers.). Concern over the impact of habitat degradation on wildlife resources provided the primary impetus for this research. This study was carried out as part of a larger ecological characterization of the Refuge and its aquatic macrofaunal assemblages.

Sampling Methods

Aquatic macrofauna were collected by throwing an aluminum throw trap (100 × 100 × 75 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 et al. 1984, Chick et al. 1992). Above- ground vegetation was identified and counted (relative coverage), uprooted, and shaken to dislodge any mac- rofauna. A bar seine with 3.0-mm stretch mesh was then passed through the trap until three consecutive empty sweeps were obtained. Traps were visually in- spected for spiders following the clearing procedure. Fishing spiders were identified, counted, and released back into the surrounding habitat.

Several habitat variables were measured in addition to vegetation composition. Water depth and canopy height were measured to the nearest centimeter inside

each trap prior to removal of plants. Percentage dead sawgrass within each trap was visually estimated at the sawgrass sites. Combined (live and dead) above-grotmd vegetation was dried and Weighed to the nearest gram to obtain plant biomass estimates.

The study involved a stratified random design with blocking. Areas (blocks) were random coordinates lo- cated throughout the Refuge. Within an area, one site each ofsawgrass, wet prairie, and alligator hole habitat were visited. These sites were sampled as encountered, and in as close proximity to one another as possible. Within each of these habitats, three traps (subsamples) were randomly collected. Sampling was carried out monthly (except December 1991) from August 1990 through January 1992. Each month, six areas were randomly selected for sampling using a grid map of the Refuge and a random number table. Therefore, 54 trap samples (6 areas × 3 habitats x 3 subsamples) were collected during each of the 17 months (total n=918). This design was used to minimize airboat damage to vegetation associated with repetitive sam- pling of fixed positions.

Due to the large number of empty traps, we con- vetted spider densities to presence/absence data and used multiple logistic regression (loglikelihood analy- sis) to examine the relationship between spider pres- ence and the independent variables of habitat type, area, and month. This loglikelihood approach is an- alagous to the analysis of variance method used for continuous, interval data.

RESULTS

Sawgrass, wet prairie, and alligator hole sites were physically distinct from one another. Sawgrass sites were virtually monotypic, with Cladium jarnaicense Crantz comprising 94% of the plant cover at these sites. The wet prairie sites were dominated by the emergents Eleocharis spp. and Rhynchospora spp. Alligator holes were comprised primarily of Utricutaria spp. and N.vm- phaea odorata Alton. Wet prairies and alligator holes were generally deeper and more open, whereas sawgrass sites were relatively shallow (average depth = 22 cm) and had well developed canopies (average height = 145 cm) that provided significant shading and cover. Above-ground plant biomass (dry, weight) ranged from a low of 125 g/m 2 in alligator holes to a high of 757 g/m 2 in sawgrass, with wet prairie sites being inter- mediate at 284 g/mL Dead plant material on average comprised 44% of the total Ctadium coverage in sawgrass sites. The other two habitats did not have well developed litter deposits,

Fishing spiders were distributed non-randomly with respect to habitat ty. pe (loglikelihood analysis, p < 0.05; see Figure 1). Iterative removal of the different habitat

Jordan et aL, SPIDER HABITAT USE 241

types from the multiple regression model indicated that fishing spiders were significantly more abundant in sawgrass than in either wet prairies or alligator holes, which did not differ from each other. The blocking effect was not significant (loglikelihood analysis, p>0.05). Therefore, the observed patterns of differ- ential habitat use were consistent across the wetland landscape.

A total of 918 1-m 2 throw trap samples were col- lected during the study, yielding only 85 fishing spiders. Individual traps contained between 0 and 5 spiders. Overall density was 0.1 fishing spiders per meter square. However, there was considerable temporal variation in abundance estimates. Monthly densities ranged from 0.3 spiders per m 2 in May 1991, to no spiders during the months of February, March, and April of 1991. The month effect was not significant (loglikelihood analysis, p> 0.05). There were no clear seasonal trends in spider abundance, and interannual variation was great.

DISCUSSION

The overall abundance of the fishing spider in this freshwater marsh system seemed to be low. Compa- rable.quantitative abundance data are not available for this species, or for freshwater wetland spiders in gen- eral. Bleckmann and Rovner (1984) found an average of 0.3 fishing spiders per meter of shoreline for a lake in Ohio. Zimmerman and Spence (1992) reported peak densities of 1.1 fishing spiders per meter square in a small, man-made lake in Canada, although their es- timates were generally much lower (ca. 0.1-0.3 spiders per mZ). Smaller-sized, cursorial spiders are generally more abundant. For example, LaSalle and de la Cruz (1985) found higher densities ofLycosa watsoni Gertsch (9.5 spiders per m 2) and Pirata mayaca Gertsch (17.8 spiders per m s) in the vegetation of a Mississippi salt marsh. However, salt marsh ecosystems are highly pro- ductive compared to the historically nutrient-poor Ev- erglades (Richardson et al. 1990, Gunderson and Lof- tus 1992).

Spatial complexity is one of the most important fac- tors affecting spider ecology (Uetz 1991). Horizontal and vertical variation in habitat structure affects both population- and assemblage-level dynamics of spiders (Turnbull 1973, Abraham 1983, Riechert and Gilles- pie 1986, Uetz 1991). In this study, we found that the complex wetland mosaic comprising the northern Ev- erglades affected the distribution of the fishing spider. The finding of higher densities of fishing spiders in sawgrass is concordant with previous work by Bleck- mann and Rovner (1984). They found that the number of fishing spiders was highest in cattail habitat and decreased as a function of decreasing plato cover and

complexity. Relative to emergent vegetation of lesser stature, cattail and sawgrass provide similar habitat structure for fishing spiders: tall plant canopies with elevated levels of shading, enhanced protection from avian and piscine predators, and reduced wind sti- multi.

Fishing spiders are not web builders but rather rely on mechanoreception to detect prey that have become trapped on the water's surface or that are swimming just beneath it (Williams 1979, Bleckmann and Barth 1984, Bleckmann 1985, Bleckmann and Lotz 1987). The reduced amount of wind stimulii O.e., sensory noise) characteristic of denser and taller vegetation in- creases the ability of fishing spiders to detect vibratory signals generated by potential prey (Bleckmann and Rovner 1984).

Sawgrass stands of the Florida Everglades are rela- tively dense and spatially complex. For this reason, sawgrass is not commonly used as a foraging habitat by either avian or piscine predators (Gunderson and Loftus 1992). Fishing spiders would easily fall prey to either of these two predatory groups. For example, the gut of an immature little blue heron (Egretta caerulea L.) collected in Florida contained 32 fishing spiders (Carico 1973). The remains of semi-aquatic spiders have also been found in fish stomachs (F. Jordan, un- published data). Therefore, it seems that lowered pre- dation risk could help explain the observed patterns of habitat use by fishing spiders.

In addition to providing a "'quieter" and "safer" environment for fishing spiders, sawgrass may also provide enhanced foraging opportunities. The abun- dant sawgrass litter provides excellent ambush sites for these sit-and-wait predators. Sawgrass habitat in this portion of the Everglades supports a diverse assem- blage of potential prey (F. Jordan, unpublished data). Fish densities average about 15 per m 2, with the sur- face-dwelling topminnows (Fundutus chrysotus Giinth- er) and mosquitofish (Gambusia hotbrooki Girard) be- ing common in this habitat. Interestingly, the Everglades pygmy sunfish (Elassoma evergladei Jor- dan), which has been documented as prey of semi- aquatic spiders (Gudger 1931), is most abundant in sawgrass habitats. Various aquatic and semi-aquatic invertebrates (odonate larvae, coleoptera, hemiptera) and anuran larvae are common in this habitat.

Fishing spiders may play an important role in the trophic network of Everglades marshes (Gunderson and Loftus 1992). No research has examined the pred- atory role played by these spiders in natural wetlands. Zimmerman and Spence (1989) found that fishing spi- ders were voracious predators on semi-aquatic arthro- pods in the littoral zones of small lakes, but aquatic arthropods and vertebrates were relatively unimpor- tant dietary components. Oraze et al. (1989) found

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evidence for biological control of aquatic and semi- aquatic arthropods by the lycosid spider Pardosa ra- mulosa (McCook). However, our density estimates suggest that fishing spiders are unlikely to play im- portant roles in the marsh food web.

ACKNOWLEDGMENTS

Sean Coyne, Jane Jimeian, and other volunteers helped to collect spiders and other swamp creatures. Kim Babbitt, Peg Johnson, and Mark Stowe kindly reviewed the manscript. The staff of the Arthur R. Marshall Loxahatchee Refuge provided logistical sup- port and access to the Refuge. This study was made possible by funding from the U. S. Environmental Pro- tection Agency and the U. S. Fish and Wildlife Service. This paper is Contribution No. R-03889 of the Journal Series, Florida Agricultural Experiment Station, Gainesville, 32611.

IJTERATURE CITED

Abraham, B.J. 1983. Spatial and temporal patterns in a sagebrush steppe community (Arachnida: Aranae). Journal of Arachnology 1J:31-50.

Bleckmann, H. 1985. Perception of water surface waves: how sur- face waves are used for prey identification, prey localization, and intraspecific communication, p. 147-166. In D. Ottoson (ed.) Ad- vances in Sensory Physiology 5. Parey Vedag, Berlin, Germany.

Bleckmann, H. and F. G. Barth. 1984. Sensory ecology of a semi- aquatic spider (Dolomedes triton). I1. The release of predatory behavior by water surface waves. Behavioral Ecology and Socio- biology 14:303-312.

Bleckmann, H. and T. Lotz. 1987. The vertebrate-catching be- haviour of the fishing spider Dolomedes triton (Araneae, Pisaur- idae). Animal Behavior 35:641-651.

Bleekmann, H. and J. S. Rovner. 1984. Sensory ecology of a semi- aquatic spider (Dolomedes triton). I. Roles of vegetation and wind- generated waves in site selection. Behavioral Ecology and Socio- biology 14:297-301.

Carico, J. E. 1973. The nearctic species of the genus Dolomedes (Araneae: Pisauridae). Bulletin of the Museum of Comparative Zoology 144:435488.

Chick, L H., F. Jordan, J. P. Smith, and C. C. Mclvor. 1992. A comparison of four enclosure traps and methods used to sample fishes in aquatic macrophytes. Journal of Freshwater Ecology 7:353- 361.

Foelix, R. F. 1982. Biology of Spiders. Harvard University Press, Cambridge, MA, USA.

WETLANDS, Volume 14, No. 3, 1994

Freeman, B. J., H. S. Greening, and J. D. Oliver. 1984. Comparison of three methods for sampling fishes and macroinvertebrates in a vegetated freshwater wetland. Journal of Freshwater Ecology 2:603- 610.

Gudger, W.E. 1925. Spiders as fishermen and hunters. Journal of the American Museum of Natural History 25:261-275.

Gudger, W.E. 1931. Some more spider fishermen. Natural History 31:58-61.

Gunderson, L. H. and W, F. Loftus. 1992. The Everglades: com- peting land uses imperil the biotic communities of a vast wetland. p. 123-134. In W. H. Martin, S. G. Boyce, and A. C. Esternacht (eds.) Biotic Communities of the Southeastern United States. John Wiley and Sons, Ne~, York, NY, USA.

Kushlan, J. A. 1981. Sampling characteristics of enclosure fish traps. Transactions of the American Fisheries Society. 110:557- 562.

LaSalle, M. W. and A. A. De La Cruz. 1985. Seasonal abundance and diversity of spiders in two intertidal marsh plant communities. Estuaries 8:381-393.

Loveless, C. M. 1959. A study of the vegetation of the Florida Everglades. Ecology 40: 1-9.

Nyffeler, M. and G. Benz. 1987. Spiders in natural pest control: a review. Journal of Applied Entomology 103:321-339.

Oraze, M. J. and A. A. Grigarick. 1989. Biological control of aster lea~opper (Homoptera: Cicadella) and midges (Diptera: Chiro- nomidae) by Pardosa ramulosa (Araneae: Lycosidae) in California r~ce fields. Journal of Economic Entomology 82:745-749.

Richardson, J. R., W. L. Bryant, W. M. Kitchens, J. E. Mattson, and K. R. Pope. 1990. An evaluation of Refuge habitats and relationships to water quality, quantity, and hydroperiod: a syn- thesis report. Final Report to Arthur R, Marshall Loxahatchee National Wildlit~ Refuge, Boynton Beach, FL, USA.

Riechert, S. E. and R. G. Gillespie. 1986. Habitat choice and utilization in web-building spiders, p. 23--48. In W. A. Shear (ed.) Spiders: webs, behavior, and evolution. Stanford University Press, Stanford, CA, USA.

Serrao, J. 1980. Giant spiders of the Everglades. National Parks 54:12-13.

Turnbull, A.L. 1973. Ecology ofthe true spiders (Araneomorphae). Annual Review of Entomology 18:305-348.

Uetz, G.W. 1991. Habitat structure and spider foraging, p. 325-- 348. In S. S. Bell, E. D. McCoy, and H. R. Mushinsky, (eds.) Habitat structure: the physical arrangement of objects in space. Chapman and Hall, New York, NY, USA.

Williams, D. S. 1979. The feeding behaviour of New Zealand Dolomedes species (Aranae: Pisauridae). New Zealand Journal of Zoology 6:95-105.

Zimmerman, M. and J. R. Spence. 1989. Prey use of the fishing spider Dolomedes triton (Pisauridae, Araneae): an important pred- ator of the neuston community. Oecologia 80:187-194.

Zimmerrnan, M. and J. R. Spence. 1992. Adult population dy- namics and reproductive effort of the fishing spider Dolomedes triton (Araneae, Pisauridae) in central Alberta. Canadian Journal of Zoology 70:2224-2233.

Manuscript received 7 June 1993; revision received 16 May 1994; accepted 14 June 1994.