Food web structure
My previous work has shown that Enallagma species segregate between dragonfly-dominated and fish-dominated lakes because the two species groups are differentially vulnerable to the two predators. However, species in the genus Ischnura are abundant in both of these lake types. I have conducted a large series of field experiments to quantify the structure of the food webs surrounding these damselfly assemblages. The results of these experiments indicate that Enallagma and Ischnura species coexist in each lake type because they settle the fundamental tradeoff of avoiding predators and utilizing food resources very differently. Enallagma species have effective antipredator defenses against coexisting predators, but these defenses apparently come at the expense of the ability to effectively utilize resources. In contrast, Ischnura species are able to coexist with Enallagma in both lake types because their phenotypes make them effective at utilizing resources at the expense of predator avoidance.
Species in the genus Lestes are found in temporary ponds that may dry completely each as well as the permanent ponds and lakes where Enallagma and Ischnura are found. In fact, Lestes has specialist species in vernal ponds that dry each year, temporary ponds where dragonflies dominate, permanent ponds and lakes dominated by dragonflies, and permanent ponds and lakes dominated by fish. My work with Robby Stoks, Lab of Aquatic Ecology and Evolutionary Biology, KU Leuven, has shown that the segregation of Lestes species among these four pond types is enforced by their differential vulnerabilities to the key predators - just like Enallagma - and by differences in their life histories. The Lestes species inhabiting temporary ponds have a diapausing egg stage that allows them to survive when the pond dries, but the eggs of Lestes species inhabiting permanent ponds and lakes hatch directly in the summer.
I have extended this research to explore how these dynamical interactions may be shaped by the microevolutionary and macroevolutionary processes operating in component taxa. For example, up to twelve species of Enallagma coexist in lakes containing fish, with all twelve species being found at every lake in a local area. It is difficult to imagine that each of these twelve species has its own unique niche. I am now testing the possibility that these large numbers of coexisting species are effectively ecologically equivalent, and co-occur today because they are so similar ecologically. Model results show that the time to competitive exclusion for the loser in a competitive fight among species increases exponentially as the competing species become more similar to one another ecologically. This means that poorer competitors may persist in systems for vast expanses of time before they are finally driven extinct. If this is true, we may see a great degree of species redundancy in food web structure. We are currently testing these ideas in field and laboratory experiments.
McPeek, M. A., and P. H. Crowley. 1987. The effects of density and relative size on the aggressive behaviour, movement and feeding of damselfly larvae (Odonata: Coenagrionidae). Animal Behaviour 35:1051-1061.
McPeek, M. A. 1990. Determination of species composition in the Enallagma damselfly assemblages of permanent lakes. Ecology 71:83-98.
McPeek, M. A. 1990. Behavioral differences between Enallagma species (Odonata) influencing differential vulnerability to predators. Ecology 71:1714-1726.
McPeek, M. A. 1996. Tradeoffs, food web structure, and the coexistence of habitat specialists and generalists. American Naturalist 148:S124-S138.
McPeek, M. A. 1998. The consequences of changing the top predator in a food web: a comparative experimental approach. Ecological Monographs 68:1-23.
Stoks, R., and M. A. McPeek. 2003. Predators and life histories shape Lestes damselfly assemblages along the freshwater habitat gradient. Ecology 84:1576-1587
Stoks, R., and M. A. McPeek. 2003. Antipredator behavior and digestive physiology determine Lestes species turnover along a gradient. Ecology 84:3327-3338.