Plants put up with a lot – everyone wants to eat them! And, basically, there are two ways a plant might respond to being eaten. They can put energy into regrowing bits that get eaten, or they can put energy into making a lot of some nasty chemical, like the milky sap in milkweed. The trouble with the first option is obvious – it doesn’t do anything to stop the damage. But the trouble with the second is that, whenever plants evolve a new defensive strategy, herbivores evolve a way around it. Often, these herbivores do very well, because they can eat something no one else can – and they become specialists on their new favorite food.
Photo by Melete.
Evolutionary ecologists have been thinking about this plant-herbivore arms race ever since Darwin. Back in 1964, Paul Erhlich and Peter Raven proposed that plants and insects might go through alternating cycles of diversification [$-a] driven by the evolution of new plant defenses and insect counterdefenses. Now, in a new paper in last week’s PNAS, Anurag A. Agrawal (who is at the top of everyone’s reference list) and Mark Fishbein show that sometimes, plants just throw in the towel [$-a].
Agrawal and Fishbein examine the evolutionary history of milkweed, which has a number of interesting anti-herbivore defenses besides the eponymous sap – and a number of specialized herbivores, like the red milkweed beetle pictured here. Their analysis looks for long-term evolutionary trends in the degree to which milkweeds put their energy into defenses, and the degree to which they put energy into regrowth. Over evolutionary time, it seems that milkweeds have reduced their defenses, and increased their regrowth efforts.
A. A. Agrawal, M. Fishbein (2008). Phylogenetic escalation and decline of plant defense strategies PNAS, 105 (29), 10057-10060 DOI: 10.1073/pnas.0802368105