COS 8-1
Experimental exclusion of rabbits drives contemporary evolution of plant defenses and decreases plant phylogenetic diversity
Herbivores are thought to play important roles in shaping plant evolution and the diversity of plant communities. However, few studies have experimentally tested if, and how, the presence/absence of herbivores influences plant contemporary evolution, especially on multiple plant species within a community. And, while many studies have tested the impacts of herbivores on species diversity few have investigated if, and how, herbivores shape phylogenetic diversity of plant communities—a measure of diversity that accounts for evolutionary history of species. In the acid grasslands at Silwood Park, England, we collected seeds of 5 species from rabbit exclosures ranging from 1-30 years old and grew them in common gardens to test for evolution of a range of plant traits following rabbit exclusion. In a separate experiment we used community composition data from 21 year-old rabbit exclosures and a dated molecular phylogeny of all species in the community to test how rabbit exclusion shaped plant phylogenetic diversity.
Results/Conclusions
We found convincing evidence for contemporary evolution in 2 of 5 plant species studied following >20 years of rabbit exclusion. In the perennial herb Rumex acetosa we observed evolution of decreased growth rate and in the perennial grass Festuca rubra we observed evolution of decreased tolerance to herbivory and decreased leaf number. This suggests that the presence / absence of herbivores can shape evolution of plants but the outcomes are highly variable across species. In our other experiment we found that rabbits had no effect on plant species diversity but that plant phylogenetic diversity decreased by 3.5 standard deviation units in rabbit exclosures. This pattern seems to be the result of increases in the relative abundance of grasses in rabbit exclosures, which lead to communities of closely related species and thus lower phylogenetic diversity. Together these experiments demonstrate that grazing herbivores can be drivers of evolution on contemporary times scales and may play important roles in maintaining phylogenetic diversity within plant communities.