Evolution isn’t so simple

To steal a line from the paper that I’m about to talk about: “Herbivores have fed on plants for more than 400 million years”. I thought this was a pretty striking way to start a paper; it suggests that these interactions might play a really important role in how plants and herbivores evolve. And indeed countless papers have discussed the effects of herbivores in driving the evolution of plant defenses and diversification among plant taxa. If interactions between plants and herbivores are important on these really long evolutionary time scales, then it stands to reason that they should also be important on much shorter time scales (let’s say somewhere between one and a hundred plant generations). But the story turns out not to be so simple.

Here when we’re talking about “short time scales”, we’re talking about evolution on time scales on which we can actually observe evolution in action. Such rapid evolution is also important because it means it might affect the outcome of ecological interactions between species. Plant populations that quickly evolve a chemical defense against herbivores should experience less herbivory, right? So in that sense, if you want to understand how plant and herbivores interact ecologically, then it’s also important to understand the recent evolutionary history of those species. See some of the other posts on this blog about why rapid evolution is so important here and here and here.

But rapid evolution isn’t only important for interactions between plants and herbivores…it might also be important for interactions between plants and other species. Let’s imagine that plants evolve chemical defenses against herbivores. That comes at a cost to the plant. Because it’s expensive to produce a chemical defense, the plant might have less energy available to invest in other functions, like growth or reproduction. Well-defended plants might be poor competitors, for instance. So in that sense, if you want to understand competitive interactions among plants, you might also need to consider their evolutionary history with other competitors, but also their evolutionary history with herbivores!

Enter this interesting paper by Turley et al (The American Naturalist). Marc Johnson has always done cool research on evolution of plants on ecological time scales. Nash Turley is one of his grad students and following in some good footsteps. This special supplement of Am Nat from last month also deserves a shout-out. There’s a handful of really good papers about interactions between ecological and evolutionary processes, so I wouldn’t be surprised if you see some other papers from this issue show up on this blog.

Anyway, Turley et al. took advantage of various rabbit exclosures that had been imposed in different years near Imperial College in England. Plants in these plots had been released from rabbit herbivory for times ranging from 4 months to 26 years. The authors collected seeds from a common herb in these plots (common garden sorrel, Rumex acetosa) and planted them in the greenhouse to measure several traits of these plants. The expectation is that if rabbits are important for the evolution of plant traits, then plants that have been released from rabbit herbivory for longer should evolve different traits than those that have only been released for a short time.

Rumex acetosaThe absence of rabbit herbivory led to decreased plant growth rates. This suggests that plants evolve increased growth rates in response to rabbits. In some ways, this might make sense as an herbivore tolerance strategy. Because rabbits tend to eat most or all of the above ground tissue of a plant, plants that are able to quickly grow new tissue might have higher fitness ultimately. However, the authors formally quantified herbivore tolerance and found no evolution in this trait at all. In fact, they found a negative correlation between growth rate and herbivore tolerance! Moreover, there was no change in plant defenses over this same time period. Nor was there a change in competitive ability. So although rabbits appear to play some role in driving the evolution of plant traits, the only trait that evolves doesn’t appear to actually affect interactions with rabbits, or other species for that matter, in an obvious way.

Liz Schultheis adds that if you want to know more about herbivore resistance vs. tolerance, you should check out Herms & Mattson’s classic paper from 1992 in Quarterly Review of Biology (The dilemma of plants: to grow or defend).

Why this paper is cool:

These data show that plant traits can evolve quite quickly (<26 years in this case), but that the manner in which these traits affect ecological interactions is not necessarily straight-forward. We probably need to rethink some of the classic theory on how plant defenses evolve, especially when evolution occurs in a community context with lots of other species. Ultimately, this paper generates more questions than it answers, and I think that’s cool too. What exactly is going on here? Is rabbit exclusion correlated with some other environmental factor that’s actually driving evolution? Do rabbits have an indirect effect on growth rate? Is there a lack of genetic variation in the traits that do not evolve, or is there really no selection on those traits? Why don’t increased growth rates correlate positively with increased tolerance? These questions are all really interesting and the fact that we constantly generate new questions with every experiment, for me at least, is what makes science worth doing in the first place.

Turley, Nash E., Walter C. Odell, Hanno Schaefer, Georg Everwand, Michael J. Crawley, and Marc T. J. Johnson (2013). Contemporary evolution of plant growth rate following experimental removal of herbivores. The American Naturalist 181: S21-S34.

Casey terHorst

About Casey terHorst

Casey is a community ecologist interested in how species interactions are affected by genetic variation within species and evolution that occurs on ecological time scales. He is currently a post-doc in Jen Lau's lab at the Kellogg Biological Station. In the Fall, he will be starting as an Assistant Professor at California State University, Northridge.
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