ITHACA, N.Y. Forget top-to-bottom only. New Cornell University evolutionary biology research shows how plants at the bottom of the food chain have evolved mechanisms that influence ecosystem dynamics as well. (Science, March 26, 2010.)
"The ecology and interactions of most organisms is dictated by their evolutionary history," said Anurag Agrawal, associate professor of ecology and evolutionary biology (EEB), the study's senior author.
In food webs, predators help suppress populations of prey by eating them; that frees species lower in the food chain, such as plants, to flourish, a dynamic called a "trophic cascade." Most trophic cascade studies have focused on the ability of predators to increase plant biomass by eating herbivores. Such studies typically find strong trophic cascades in aquatic environments, where big fish eat minnows, which eat the tiny algae-eating crustaceans called daphnia.
Agrawal, first author Kailen Mooney, who is a former Cornell postdoctoral researcher and now assistant professor at the University of California-Irvine, and colleagues studied trophic cascades in 16 milkweed species, famed for their interactions with monarch butterflies, and also fed upon by aphids.
Plants have evolved three main strategies for increasing their biomass as much as they can against the forces that limit their growth, said the researchers: They grow as quickly as possible; develop direct defenses, such as toxins or prickly leaves, against herbivores; and attract such predators as ladybugs that eat their pests.
But plants do not have the resources to develop all three defenses. Since Darwin, evolutionary biologists have hypothesized that over millions of years of evolution, plant species are subject to trade-offs, developing some defense strategies in lieu of others; a key finding of the new study is that these evolutionary trade-offs drive how modern ecosystems are structured.
In the case of milkweed,
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