The adaptive evolution of herbivory in freshwater systems

Background/Question/Methods The apparent advantages of omnivory and carnivory have motivated hypotheses of the evolution of these diets; however, hypotheses about the adaptive evolution of herbivory are lacking. Aquatic herbivores evolved from carnivorous ancestors, suggesting that herbivory is adaptive in some circumstances. To improve our understanding of the advantages of herbivory over other diets, we propose five adaptive hypotheses for diet evolution in freshwaters: 1) Intake-efficiency - herbivores use part of their food source as habitat, minimizing the energy/time spent searching for food and avoiding predators; 2) Suboptimal habitat - herbivory allows organisms to invade and establish populations in habitats that have high primary production but low abundance of animal prey; 3) Heterotroph facilitation - herbivory is adaptive because herbivores consume microbes associated with algae; 4) Lipid allocation - herbivory is adaptive because basal resources are rich in fatty acids, which fuel reproduction and storage; and 5) Disease avoidance - herbivory minimizes animal-facilitated disease transmission. We review the current aquatic herbivory literature and use examples from these works to provide support for the incorporation of these hypotheses to future studies.

Results/Conclusions These hypotheses are based on basic ecological principles that many studies have examined in detail, and with a few assumptions, these ideas could be implemented into current research programs. For example, the intake-efficiency hypotheses might be tested using a similar experimental design to Alvarez and Peckarsky’s (2013). They measured growth rates of caddisflies and mayflies, algal accrual rates and per capita effects of grazers on algae in chambers that differed in the presence of moss and predation risk. They found no differences in growth; however, when mayflies were subjected to predation, algae associated with moss accrued at a slower rate, suggesting that they are using moss as both habitat and a source of food in the presence of predators. Comparable experiments could be designed to include additional life history trait estimates (e.g. herbivore survival) and estimates of foraging energy expenditure vs energy gain (as in optimal foraging theory).

Herbivory has been the focus of many ecological studies and these contributions have improved our understanding of animal diet ecology. However, there is a significant gap in knowledge pertaining to the adaptive evolution of herbivory in nature. The incorporation of these hypotheses to the current literature will provide information about diet evolution, where it is currently lacking. These proposed hypotheses represent a starting point that will lead to more comprehensive studies of diet evolution in freshwater and other systems.