When resistance to herbivory shows unexpected geographic patterns, one common explanation is that plant tolerance to herbivory probably varies in a related way. For example, if populations that experience lower levels of herbivory are also less able to tolerate damage, then a mismatch between resistance and herbivory rates can be explained. Previous research (Woods et al. 2012) demonstrated a surprising cline in common milkweed plants: northern populations were more resistant to herbivory though attack rates were low. One hypothesis for this result is that the per-unit impact of consumption on fitness is greater, or tolerance to herbivory is lower, in northern populations. We investigated geographic variation in milkweed growth responses to herbivory to test the hypothesis that there is a cline such that northern populations are less able to compensate for herbivory. Using genotypes collected by participants in a research and education network from ~16 different populations across the range of common milkweed, we conducted two greenhouse experiments in which we manipulated herbivory or damage to plants and measured plant growth responses.
Results/Conclusions
In the first experiment, monarch damage impacted milkweed mass, but aphid herbivory did not, and estimates of plant growth rate diverged across these two treatments, with plants growing more in response to monarchs and less in response to aphids. Although we were unable to detect genetic variation in plant response to herbivory in the first experiment, the second experiment did provide evidence for a latitudinal cline in compensation such that northern genotypes suffered greater reductions in biomass due to damage than did southern genotypes. High root:shoot ratio predicts growth compensation in our study, while final biomass and growth rate estimates do not. Although precipitation, not latitude, predicts root:shoot ratio in our dataset, we infer that geographic factors favoring investment in roots will also facilitate growth compensation in response to damage.