If related plant species share herbivores, then herbivore damage on a given plant species may be predictable based on its relatedness to other community members. We examined this hypothesis by assessing leaf damage in two communities: an eastern US hardwood forest and a Rocky Mountain montane community. Focal species included both native and invasive plants. Pairwise phylogenetic distances between focal species and the hundreds of other native species in each community were generated in Phylocom. We examined the influence of three measures of relatedness within each community: NND (nearest native neighbor), MPD (mean distance to the native species in the community), and a new metric, the sum of the inverse pairwise distances between the focal species and the natives in the community. This new metric does not privilege any one position within a phylogeny, and nor does it treat all co-occurring species as equally influential (as do NND and MPD, respectively). Rather, it assumes that the sharing of natural enemies between any two species increases in proportion to their relatedness, and that the aggregate effect of relatedness to the larger community can influence attack by natural enemies.
Results/Conclusions
Our study included estimates of herbivory for 44 plant species in a deciduous forest in New York (25 native and 19 exotic; 258 native species identified in the community as whole) and 41 plant species from Rocky Mountain meadows and forests (34 native and 7 exotic; 603 native species identified in the community as whole). Using simple regression models, we found that herbivore damage decreased with decreasing phylogenetic similarity of focal species to the identified native species in both sites, although the pattern was significant only for native focal species in the Rocky Mountains and exotic focal species in the New York forest. There was no significant relationship between NND and herbivory for any of the four site by origin combinations, although damage to New York exotics did decrease with increasing MPD. Our results support the hypotheses that the aggregate effect of relatedness to the larger community can influence attack by natural enemies, and that species escape attack in proportion to their phylogenetic dissimilarity to the larger community of native species.
