Just because plants are at the bottom of the food chain doesn't mean they're helpless. Scientists have discovered that plants have evolved multiple lines of defense against their predators, giving them a surprising amount of control of their surrounding ecosystem.
Most studies of the dynamics of food webs focus on how carnivorous predators control the populations of animal prey, which in turn allows plants to increase their own populations. This process is known as a trophic cascade, and its end result is the creation of more plant biomass in the given environment.
Researchers Anurag Agrawal and Kailen Mooney reversed the trend to look at predators by concentrating on sixteen species of milkweed and how these plants interact with their environment. Plants are known to have three major kinds of defenses against predators: they grow as quickly as they can, increasing their biomass; they employ actual physical protectionlike a toxic surface or prickly leaves; or they develop aesthetically in such a way that they attract animals that eat their attackers.
In general, plants can't develop all of these different lines of defense, so evolutionary trade-offs require that one kind of protection be selected instead of another. What Agrawal and Mooney's study found was that whichever defense develops impacts the structure of the entire ecosystem.
For instance, some of the milkweed eschewed built-in defenses in favor of the ability to gather carnivorous predators nearby. In order to accomplish this, the milkweed produced compounds known as sesquiterpenes, which bring in pest-hunters like ladybugs. The key, however, is that the plants were able to increase their biomass even if they were unable to attract protectors. The scientists surmise that the sesquiterpenes also stimulate a fast growth rate, which in turn determines the ratio of plant biomass to the contents of the ecosystem as a whole. In this way, the adaptive selections of the plants help determine the larger character of their environments.
The researchers point out the positive impact these findings could have on agriculture, as it might be possible to complement man-made pesticides with more robust versions of the defenses plants have evolved for themselves. Still, let's not ignore the real significance of this study - plants are clearly not to be messed with, and once again M. Night Shyamalan looks like a prophetic genius.