THE CONSEQUENCES OF CHANGING THE TOP PREDATOR IN A FOOD WEB: A COMPARATIVE EXPERIMENTAL APPROACH

Changing the top predator in a food web often results in dramatic changes in species composition at lower trophic levels; many species are extirpated and replaced by new species in the presence of the new top predator. These shifts in species composition also often result in substantial alterations in the strengths of species interactions. However, some species appear to be little affected by these changes that cause species turnover at other positions in the food web. An example of such a difference in species responses is apparent in the distributions of coenagrionid damselflies (Odonata: Zygoptera) among permanent water bodies with and without fish as top predators. Enallagma species segregate between ponds and lakes that do and do not support fish populations, with each lake type having a characteristic Enallagma assemblage. In contrast, species of Ischnura, the sister genus to Enallagma, are common to both fish and fishless ponds and lakes. Previous research has shown that Enallagma species segregate because they are differentially vulnerable to the top predators in each lake type: dragonflies in fishless lakes and fish in fish lakes. This paper reports the results of a series of laboratory and field experiments quantifying the mortality and growth effects of interactions in the food webs surrounding Enallagma and Ischnura species in both lake types. These results are compared to determine how features of the food web change to force segregation of Enallagma species between the lake types but permit Ischnura species to inhabit both. The results of experiments conducted in a fishless lake show that damselflies are not food limited in this lake type, but that they do strongly compete via interference mechanisms. Interference effects between the genera are symmetrical. Ischnura species have substantially higher growth rates than Enallagma species under all conditions in fishless lakes. Although both Enallagma and Ischnura experience substantial mortality from predation by dragonflies (Anax and Aeshna species, the top predators in fishless lakes), these dragonflies display a significant bias towards feeding on Ischnura. Mortality rates due to dragonfly predation are not density dependent. The results of experiments done in a fish lake indicate that damselflies are food limited and thus compete for resources in fish lakes. Ischnura growth rates are also substantially higher than Enallagma species in the fish-lake system. Dragonfly species that coexist with fish (Basiaeschna and Epitheca species) do not impose significant mortality on coexisting damselflies, but they do compete for resources with the damselflies, and they may also generate feeding interference in the damselflies. Fish impose significantly higher mortality on Ischnura species than on coexisting Enallagma species, and this mortality is negatively density dependent. The coexistence of Enallagma and Ischnura species is fostered in both lake types by trade-offs in their abilities to avoid predators and to utilize resources. Native Enallagma species are better at avoiding coexisting predators in each lake type, but these abilities come at the expense of the ability to utilize resources effectively and to avoid the predator found in the other lake type. In contrast, Ischnura are better at utilizing resources in both lake types, but these abilities come at the expense of effectively avoiding both fish and dragonflies. Understanding the trade-offs faced by species at similar trophic positions within a food web is critical to predicting changes in food webs following major environmental perturbations such as changing the top predator.