TROPHIC CONNECTIONS BETWEEN STREAM AND TERRESTRIAL FOOD WEBS

Abstract

Most animals have complex life histories (CLH) in which an individual's niche shifts through ontogeny. These organisms often cross habitat or ecosystem boundaries as they develop from larvae to adults, coupling energy flow among food webs in separate ecosystems. As a result, ecological processes such as productivity and predation that govern the abundance of organisms during one stage of their life history can have effects that cascade beyond the boundaries of the focal ecosystem. However, empirical and theoretical studies often treat food webs as closed systems in which in situ ecological processes are the primary components regulating the structure and function of food webs. In my dissertation research, I examined how a group of CLH organisms, aquatic insects, couple stream and riparian food webs as they develop from aquatic larvae to terrestrial adults. I further examined how predation by fish on aquatic insects alters emergence of insects into terrestrial food webs.

Aquatic insects are ubiquitous in freshwater habitats where they spend the majority of their larval stages. During development in freshwater habitats aquatic insects occupy nearly every trophic level in aquatic food webs from herbivores (e.g. Trichoptera) to predators (e.g. Odonata). Most species of aquatic insects undergo metamorphosis during development in which they emerge from an aquatic pupal or nymphal stage to become winged adults in terrestrial habitats. When they emerge as winged adults from aquatic habitats, adult aquatic insects subsidize diets of terrestrial predators such as birds, spiders, lizards and bats. The importance of adult aquatic insects as subsidies in terrestrial food webs is ultimately determined by the abundance and biomass of the emerging insect assemblage, which in turn is driven by ecological interactions in aquatic habitats. The life cycle of aquatic insects and the strong environmental boundary between aquatic and terrestrial habitats offer an ideal setting to study the consequences of spatial connectivity among food webs in physically distinct habitats

In Chapter 1, I measured the contribution of adult aquatic insects to terrestrial food webs along three streams in Oklahoma. I made monthly collections of all winged insects in the terrestrial habitats along each stream and sorted insects according to larval origin (aquatic or terrestrial). Overall, adult aquatic insects comprised more than one-third of all winged insects. This contribution peaked along a permanent spring stream, reaching as high as 94% of abundance and 86% of biomass in winter. The majority of adult aquatic insects were taxa that do not feed as adults (non-consumers), whereas most adult terrestrial insects fed (consumers). This resulted in a strong negative relationship between the relative biomass of adult aquatic insects and the relative biomass of consumers in the overall insect assemblage. Because winged terrestrial insects are important prey for terrestrial predators like birds, spiders, and lizards, this study demonstrated that insects emerging from streams substantially elevate prey availability in a terrestrial food web. Neither prey availability nor insect trophic structure in terrestrial habitats could be accurately predicted based on terrestrial productivity alone.

In Chapter 2, I tested the hypothesis that predation by fish on larval aquatic insects alters insect emergence from aquatic mesocosms to terrestrial habitats. I tested the effects of predation by two fish species with different foraging strategies (Cyprinella lutrensis - water-column feeder; and Etheostoma spectabile - benthic feeder). Both fish reduced emerging insect biomass by nearly 50% relative to fishless pools. Fish effects were strongest on emergence of dragonflies (Pantala flavescens), which are predators as adults in terrestrial food webs. Therefore, insect assemblages emerging from pools with fish had less overall biomass and fewer predators than assemblages emerging from pools with fish, regardless of fish foraging strategy. These results demonstrate that predation in streams can cascade to terrestrial habitats, altering biomass and trophic structure of adult aquatic insect subsidies in terrestrial food webs.

In Chapter 3, I tested the hypothesis that fish species richness in aquatic mesocosms alters insect emergence to terrestrial habitats. I also measured the distributional response of a terrestrial consumer (tetragnathid spiders) to shifts in insect emergence. Three fish species (with complementary habitat domains were the predators in a factorial design using all possible combinations of fish. Pools with high fish richness reduced insect emergence by more than 30% relative to control pools. Tetragnathid spiders responded to reductions in insect emergence by shifting their distribution away from pools with high fish richness. Fish effects in the high richness treatments (three fish species) were generally stronger than predicted based on individual fish species performance, suggesting that interactions among fish species in high richness treatments were synergistic. These results show that the effects of fish species loss in streams can cascade to adjacent terrestrial systems. Additionally, the strength of these effects are driven by the habitat domain of the fishes, supporting the idea that the effects of fish species loss can be predicted based on the foraging ecology of the fish.

My dissertation research demonstrates the importance of spatial context in food web studies. I found that the abundance, biomass and trophic structure of winged insect assemblages in terrestrial habitats is driven by the relative productivities of the both aquatic and terrestrial habitats. In turn, the contribution of adult aquatic insects to terrestrial habitats is regulated by fish predation on larval insects in aquatic habitats. Fish reduce insect emergence, thereby reducing the amount of energy available to terrestrial predators, an effect that varies relative to fish species richness. These results show that ecological processes like predation have effects that cascade beyond the habitat of the predator, altering prey availability and the distribution of consumers in adjacent food webs.