Invading plants may escape many of their native-range natural enemies, as predicted by the Enemy Release Hypothesis. This loss of enemies potentially distinguishes nonindigenous plants from natives, providing them with an advantage over native competitors; however, native herbivores and pathogens present in the invaded community can have significant new impacts on these invaders, reducing this advantage. Whether exotic plants avoid damage by these invaded-range enemies may depend on their phylogenetic distance from native plant species: enemies adapted to closely-related natives may be able to shift rapidly to exotics. We evaluated the influence of locally-occurring native relatives on escape from herbivorous insects at three taxonomic levels.
First, in common garden experiments using invaders drawn from a wide range of plant families, we found that the risk of attack by invertebrate herbivores was influenced by an invader's phylogenetic isolation from the local native plant community: herbivory was greater for species with closer familial relatives in the local flora. In contrast, within the family Asteraceae, field surveys indicated that damage by foliar herbivores and pre-dispersal seed predators did not decline with phylogenetic isolation from natives in the same family, though analyses that incorporated phylogenetic structure indicated that exotics consistently experienced lower levels of damage than related natives. Finally, for a single invasive native species within this family, insect damage to common ragweed (Ambrosia artemisiifolia) was significantly greater in artificial populations located closer to conspecifics. Together, these results suggest that the phylogenetic and physical proximity of an invasive plant to locally-occurring native relatives can in some cases affect its chances of experiencing enemy release. Consequently, the degree to which an exotic plant is "different" needs to be assessed in the context of the phylogenetic structure of the community it invades.