Large effects of consumer offense on ecosystem structure and function

Background/Question/Methods

Study of the role of within-species adaptation in ecological dynamics has focused largely on prey adaptations that reduce consumption risk (prey defense). Few, if any, studies have examined how consumer adaptations to overcome prey defenses (consumer offense) affect ecosystem structure and function. We manipulated two sets of genotypes of a planktonic herbivore (Daphnia pulicaria) in a highly productive ecosystem with abundant toxic prey (cyanobacteria). The two sets of consumer genotypes varied widely in their tolerance of toxic cyanobacteria in the diet (i.e., sensitive vs. tolerant).

Results/Conclusions

We found a large effect of tolerant D. pulicaria on phytoplankton biomass and ecosystem metabolism but no effect of sensitive genotypes, this result stemming from genotype-specific differences in population growth in the presence of toxic prey. The former effect was as large as effects seen in previous Daphnia manipulations at similar productivity levels. Thus, we demonstrated that the effect of consumer genotypes with contrasting offensive adaptations was as large as the effect of consumer presence/absence. In this system, adaptation can be as important a driver of ecosystem dynamics as species identity. This result has major implications for community and ecosystem ecology since adaptation (particularly by consumers) is usually ignored in studies of food-web interactions.