Master-of-one caterpillars dodge bird predation

Insect herbivore species often specialize on the host plants that they eat, evolving adaptations to use a plant’s unique set of resources.  But like any time you throw all your eggs in one basket, these caterpillars put themselves at risk.

Michael Singer of Wesleyan University gave a talk today at the ESA Annual Meeting that evaluated these tradeoffs in caterpillars. “A lot of evolutionary ecologists have pondered the advantages of being a specialist, and there are presumably tradeoffs,” he said. “Specialists have a smaller resource base, but they might be better adapted to their niche.”

Singer and his colleagues wondered if there could be other advantages to specialization than better utilization of host plants as food. Specialists might also be more adept than generalists at, for example, using their host plants for defense or refuge from predation, specifically by birds.

The team tested this idea by excluding birds from experimental plots in a temperate forest in Connecticut and surveying the density of generalist and specialist caterpillar species inside and outside the exclosures. In the exclosures, his team observed a surge in generalist density compared to natural areas. The number of specialists, however, only increased slightly. The conclusion? Bird predators were preferentially targeting generalists.

The difference is likely due to the specialists’ ability to take better advantage of their host plants, says Singer. Specialists can use chemicals from their host plant’s tissue to make themselves toxic.  While Singer’s caterpillars don’t do this, they might be more adept at camouflaging themselves by finding the best places to hide or to blend in.

Singer says that the interactions among the three trophic levels – plants, herbivores and predators – are the key to understanding the species’ ecology and evolution. “Food webs are complex, and that complexity is fundamental to understanding ecological specialization and diversity in natural ecosystems,” he said.