When it comes to evolution, no plant stands alone.
For mustard plants, investing heavily in pest defense is a good idea if a different plant species lives next door, says Richard Lankau of the Illinois Natural History Survey in Champaign. If the nearest plant happens to be another mustard, however, then the investment in protection becomes a bloated military budget and the offspring suffer.
Plant communities influence evolutionary forces in very complex ways, a new study finds. Lankau and Sharon Strauss at the University of California, Davis have demonstrated that a plant’s surrounding community can boost, shrink, or even reverse evolutionary forces. The relationships are complex enough to make evolutionary outcomes unpredictable, says Lankau.
“This study is amazing,” says Anurag Agrawal of Cornell University. “It will make it more difficult to justify studying the evolution of a species in the absence of its real ecological context.”
The idea that communities influence evolution isn’t new. “Perhaps even Darwin would have argued that it must be so,” says Agrawal. However, as researchers begin to consider such influences in their studies, “the logistical, statistical, and conceptual hurdles are substantial,” he says.
Lankau and Strauss focused on black mustard plants. Like other members of the mustard family, Brassica nigra produces strong-flavored substances that can discourage indiscriminant grazers such as slugs. The black mustard produces most of its defense chemicals in the form of an amino acid called sinigrin. The researchers wanted to see whether the mustard’s neighbors would affect the evolutionary benefit of a plant’s sinigrin.
The researchers planted 512 pairs of plants in California’s Central Valley. Each pair included a black mustard plant with either high or low sinigrin concentrations. The second plant in each pair was either another black mustard or a different species of plant. By dusting aphids off some of the plants and adding slug-repelling collars to others, the researchers tested various pest assaults on the plant pairs.
At the end of the growing season, Lankau and Strauss sampled the seeds of the mustard plants. If a plant had plenty of nice, fat seeds for abundant offspring, it meant that natural selection favored that plant’s sinigrin strategy.
Examining the influences of the various other species on a plant’s seed quality produced a pattern that was “all over the place,” says Lankau. Effects of the neighbors, slugs, and aphids interacted in very complex ways, the authors write in an upcoming American Naturalist.
For example, a slug that bites into a plant with too much sinigrin will leave and chew on the plant’s neighbor. Aphids aren’t much bothered by sinigrin, so when choosing between two plants in the mustard family, they attack the larger one. Aphids therefore eliminate the benefit of slug protection, making sinigrin production potentially wasteful.