From Science:
Biologists studying organisms that rely on each other have traditionally focused only on a pair of species. Researchers have also tended to assume that each participant contributes equally to the relationship. But these models are limited, says ecologist Jordi Bascompte. That’s because most organisms depend on a network of species, to varying degrees. Until now, he says, quantitative models failed to take these complexities into account.
To address the issue, Bascompte and his colleagues at the Donana Biological Station in Sevilla, Spain, along with collaborators at the University of Aarhus in Denmark studied data from 26 communities of plants, seed dispersers, and pollinators all over the world. The researchers then calculated the extent to which dozens of species in each of these communities rely on one another. According to their results–published today in Science— these networks are dominated by asymmetric relationships: When a plant is highly dependent on a particular animal, that animal tends to rely only somewhat on the plant. The Cazorla violet of Spain needs a particular species of moth to disperse its seeds, for example, but the moth feeds on a variety of plants. This inequality may help interdependent groups survive even when species in the network are threatened. In an equal relationship, “if the plant declines, the animal will follow,” says Bascompte. “Because the animal is now also in trouble, the plant has no chances to recover.”
More here.