Science: African Acacia trees reduce support for a mutualistic species of ant when they aren't experiencing herbivory [abstract only without subscription]. Normally, the whistling thorn tree (Acacia drepanolobium) enlists the help of an ant, Crematogaster mimosae, to fight off large herbivores and harmful insects. It works like this: The tree attracts ants by providing sugary nectar from glands at the base of its leaves and balloonlike growths called domatia (see photo), which the ants use for shelter. The ants attack anything that tries to eat the tree, for the very reasonable (and selfish) reason that it's also their nest. It seems like a mutually beneficial arangement, but no one has tested the hypothesis that, if the trees no longer need defense, they'll stop "paying" their ants to stick around.
Palmer et al. do exactly that by comparing ant provisioning on trees in plots that are fenced in (preventing access by big herbivores) with trees in control plots that aren't. After ten years inside the fence, they found that Acacia trees had reduced their nectar output and the rate at which they developed new domatia. The mutualistic ants, dependent on these rewards, were displaced by another species, C. sjostedti, which doesn't need nectar or domatia, but also doesn't defend the tree as much.
None of the changes in trees' provisioning for ants are the result of immediate natural selection - the time over which this happened is considerably less than one generation for Acacia. This is individual trees "judging" that they no longer need ant protection because they're not under attack, a response that is expected to evolve over long periods of balancing the need for protection against the cost of provisioning ants. Another ant species that uses Acacia nectar and domatia, C. nigriceps, didn't suffer from the lack of large herbivores, probably because it prunes the trees it occupies, which the authors think may be enough to make the tree "think" it's still being eaten.
Palmer T.M., M.L. Stanton, T.P. Young, J.R. Goheen, R.M. Pringle, and R. Karban. 2008. Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna. Science 319:192-5.