Tradeoffs between plant strategies to sequester resources and withstand herbivory are key to understanding how plant species abundances change following perturbation of natural communities. In equilibrium competition theory, a competition-defense trade-off suggests that the abundance of a good nutrient competitor (low R*) decreases with the addition of nutrients, but increases with the removal of herbivores. On the other end of the spectrum, species profiting most from excess resources (high R*) are well-defended and respond negatively to herbivore removal. Thus herbivory maintains diversity by preventing dominance by nutrient competitors. In contrast, plant defense theory suggests a growth-defense tradeoff, wherein species can cause or be tolerant of low nutrient conditions and also successful in avoiding fitness loss due to herbivory (nasty tolerators), while other species are limited by both nutrient availability and herbivory (tasty growers). To empirically test the generality of competition-defense or growth-defense tradeoffs in natural communities we used data from 38 sites participating in NutNet, a globally distributed experiment manipulating nutrient levels and herbivory in herbaceous plant communities. A key feature of NutNet is that plant communities are surveyed using permanent plots, where abundance is recorded and compared to a pre-treatment baseline. This allows a strong evaluation of the response of individual species in their natural context, as the same physical location and plant patches are observed through time. We used the NutNet data to ask (1) whether the slope of species responses overall indicates any competition or growth tradeoff with defense; (2) whether slopes differed by plant functional group; and (3) whether slopes differed by site, and whether any differences could be explained by local environmental conditions. We used standardized major axis regression to quantify the relationship between response to nutrient addition and response to fencing in plant species.
We found strong positive covariance in species responses to both treatments, suggesting that a growth-defense tradeoff structures these communities. Thus, the species profiting most from the alleviation of nutrient limitation were also most vulnerable to consumers. Growth-defense tradeoff slopes were invariant across plant functional types, and in the strong majority of within-site slopes. In general our results support theoretical expectations of plant defense theory, and correspond with global analyses of leaf trait spectra. In addition, these results suggest grassland diversity is maintained at least in part by coexistence mechanisms not related to herbivory.