Insect herbivore species often specialize on the host plants that they eat, evolving adaptations to use a plant's unique set of resources. However, specialization doesn't come without costs.
"A lot of evolutionary ecologists have pondered the advantages of being a specialist, and there are presumably tradeoffs," says Michael Singer of Wesleyan University. "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, he postulated, at 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. These results suggest that the 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. Many specialists use chemicals from their host plant's tissue to make themselves toxic. Others, like Singer's specialist caterpillars, might be more adept at camouflaging themselves by finding the best places to hide or to blend in.
Singer's experiment is the first to quantify bird predation on specialist and generalist herbivores, and he hopes it will spark further research. He 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 says.
Animals and plants communicate with one another in a variety of ways: behavior, body patterns, and even chemistry. In a series of talks at the Ecological Society of America's annual meeting, to be held August 3-7 in Albuquerque, New Mexico, ecologists explore the myriad adaptations for exchanging information among living things.
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