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. Over the past three years I have conducted a large series of field experiments to quantify the structure of the food webs surrounding the damselflies. 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 in each lake type experience substantially less mortality from coexisting predators, but Ischnura species utilize resources more effectively in both lake types. 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.

I am extending 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 coexist today because they are so similar ecologically.  Model results show that the time to competitive exclusion for the loser in a competitive fight over a niche position in a food web increases geometrically 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.

Patterns of species distributions and abundances suggest such a structure for littoral food webs.  My previous lab and field experimental results indicate that Enallagma species found in the same habitat are ecologically nearly identical to one another.  For example, the 12 Enallagma found coexisting in local fish lakes are all found in the same microhabitats, they have similar growth and mortality rates as larvae, they are behaviorally nearly identical, and differences in their relative abundances do not seem to correlate with any physical or biological variable I have measured.  I am currently testing whether the recent radiation of Enallagma species created a large set of ecologically identical species which are all now competing for the same niche with each lake type.  The great diversity of Enallagma species found at North American lakes may be largely the result of non-adaptive speciation processes, and thus the  food web found in the littoral zones of these lakes have a tremendous amount of species redundancy built into them.

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.

Kohler, S. L., and M. A. McPeek. 1989. Predation risk and the foraging behavior of competing stream insects. Ecology 70:1811-1825.

McPeek, M. A. 1989. Differential dispersal tendencies among Enallagma damselflies (Odonata: Coenagrionidae) inhabiting different habitats. Oikos 56:187-195.

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.

Werner, E. E., and M. A. McPeek. 1994. The roles of direct and indirect effects on the distributions of two frog species along an environmental gradient. Ecology 75:1368-1382.

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.

McPeek, M. A., and B. L. Peckarsky. 1998. Life histories and the strengths of species interactions: combining mortality, growth, and fecundity effects. Ecology 79:867-879.

Stoks, R., and M. A. McPeek. 2003. Predators and life histories shape Lestes damselfly assemblages along the freshwater habitat gradient. Ecology 84:1576-1587

McPeek, M. A., and R. Gomulkiewicz. 2005. Assembling and depleting species richness in metacommunities: insights from ecology, population genetics and macroevolution.  In Metacommunities (M. A. Leibold, M. Holyoak, and R. D. Holt, editors). University of Chicago Press.