Foraging Trade-Offs Along A Predator-Permanence Gradient In Subalpine Wetlands - Statistical Data Included

Ecology, Sept, 1999 by Scott A. Wissinger, Howard H. Whiteman, Grace B. Sparks, Gretchen L. Rouse, Wendy S. Brown

INTRODUCTION

Top predators in temporary wetlands of North America are typically large aquatic insects (e.g., dragonflies, beetles) and amphibians (e.g., salamander larvae) that, through metamorphosis, can escape drying habitats (Batzer and Wissinger 1996, Wellborn et al. 1996). In contrast, the top predators in permanent wetlands are typically fish, which often eliminate large predatory invertebrates and salamanders. Wellborn et al. (1996) recently proposed a model for trade-offs among the different types of mortality risks for prey along predator-permanence gradients and noted that the great majority of experimental evidence for the model is based on studies conducted with larval anurans (also see review by Wilbur [1997]). These studies suggest that the behavioral, developmental, and morphological characteristics that promote rapid development and timely metamorphosis in temporary habitats are disadvantageous in permanent habitats where these same traits lead to increased vulnerability to fish predation. Conversely, the low activity rates and risk-sensitive foraging behaviors exhibited by anurans that coexist with fish predators are disadvantageous in temporary habitats, where selection is for rapid growth and metamorphosis (Skelly 1992, Werner and Anholt 1993, Skelly 1995, Wellborn et al. 1996, Wilbur 1997). Experimental studies with larval anurans have also demonstrated the importance of behavioral trade-offs between avoiding fish predators and avoiding invertebrates that are the top predators in fishless habitats (Werner and McPeek 1994).

Many taxa of aquatic invertebrates exhibit similar distributional trends with respect to the presence or absence of vertebrate predators. However, the degree to which differences among habitats are related to trade-offs between antipredatory behaviors vs. the timely completion of development is not well documented for invertebrates (Batzer and Wissinger 1996, Wellborn et al. 1996). McPeek and coworkers have shown that distributional patterns of Enallagma damsel flies in ponds are based largely on differences in activity rates and foraging behaviors that are alternatively effective in reducing vulnerability to fish and invertebrate predators (McPeek 1990a, b, 1996, McPeek et al. 1996). However, it is not clear whether differences in the activity rates among Enallagma or any other aquatic insects compromise their ability to complete development in temporary habitats.

In this paper we present evidence that distributional patterns of caddis fly (Trichoptera, Limnephilidae) larvae in subalpine wetlands in Colorado are the result of species-specific trade-offs between adaptations for avoidance of vertebrate predation in permanent habitats and rapid development/competitive ability in temporary habitats. Our study was stimulated by the observation that adjacent wetland basins with different hydroperiods contain dramatically different invertebrate assemblages (Wissinger et al. 1999). However, the causal relationships between hydroperiod and invertebrate community composition are confounded by covarying differences in the presence of aquatic stages of the salamander Ambystoma tigrinum nebulosum (Hallowell) (Wissinger and Whiteman 1992).

We first present data from field experiments that determined whether differences in caddisfly abundances among wetland basins were related to differential vulnerabilities to salamander predation and how such differences might lead to indirect effects that explain observed patterns of coexistence. Laboratory experiments were designed to quantify caddisfly activity levels, foraging behaviors, and antipredatory responses to the threat of salamander predation. The data presented here and in a previous paper (Wissinger et al. 1996) allowed us to integrate the importance of three factors that taken together could explain patterns of caddisfly distribution and abundance among wetland habitats in this region; (1) physiological and developmental effects of habitat drying, (2) direct and indirect effects of inter-habitat differences in salamander predation, and (3) asymmetric intraguild predation (IGP) between caddis flies (Wissinger and Whiteman 1992, Wissinger et al. 1996). We interpret our results in the context of previous work at this study site (Dodson 1970, 1974, Sprules 1972, Maly 1976) and consider the role of salamanders as keystone predators (sensu Paine [1966], Power et al. [1996]) in maintaining differences in community composition between permanent and temporary subalpine wetlands.

BACKGROUND AND NATURAL HISTORY

The study was conducted at the Mexican Cut Nature Preserve in central Colorado. The Preserve contains several subalpine (3400-3600 m elevation) fens with numerous open water basins that vary in size ([less than]5 to 4647 [m.sup.2]), water chemistry, and hydroperiod (Wissinger and Whiteman 1992, Wissinger et al. 1999). Basins can be categorized as permanent, temporary autumnal (drying and then refilling in autumn in most years), or temporary vernal (filled only after snowmelt and drying during early summer). There is considerable biological variation among basins, and even those separated by only a few meters can have dramatically different assemblages of planktonic and benthic invertebrates (Dodson 1970, 1974, Sprules 1972, Wissinger et al. 1999).