Effects of environmental context on the susceptibility of Atriplex patula to attack by herbivorous beetles.

The susceptibility of plants to attack by insect herbivores often depends on local environmental conditions. This study documents variation in herbivore damage by the chrysomelid beetle Erynephala maritima to the annual forb Atriplex patula in two microhabitats within New England salt marshes: bare patches and dense matrix vegetation. Environmental conditions within bare patches differ from those within matrix vegetation in a number of ways. Bare patches are characterized by the absence of perennial grasses and rushes (matrix vegetation) and greater levels of physical stress, and are rapidly colonized by the fugitive annual, Salicornia europaea, a second host plant of these beetles. Surveys of herbivore damage across three marshes revealed that A. patula in bare patches had a greater proportion of leaves damaged by beetles than those within matrix vegetation. Presence or absence of matrix vegetation and presence or absence of S. europaea were experimentally manipulated to determine the proximate cause of this pattern. The presence of S. europaea significantly increased the susceptibility of A. patula to herbivory in experimental plots. Both the extent of herbivore damage to plants and the proportion of plants damaged through time were greater in treatments with S. europaea than in controls, regardless of the presence or absence of matrix vegetation. Plants in S. europaea addition treatments were also less likely to survive to reproduction. Decreased survival appears to result from increased herbivory, suggesting that the negative effect of S. europaea on A. patula is mediated indirectly through shared insect herbivores. These results support the hypothesis that indirect interactions between alternative host plants, mediated by insect herbivores, can be important in natural communities.