Temporal variation in positive and negative interactions between marsh herbivores mediated by changes in plant traits

Studies on bidirectional benefactor−beneficiary interactions between organisms have generally neglected the importance of this feedback in trophic levels other than plants. The burrowing crab Neohelice granulata aids the development of larvae of the stem-boring moth Haimbachia sp. nov. within the stems of Spartina alterniflora. In our research, we evaluated whether the stem-boring moth subsequently influences crab feeding on these marsh plants. Surveys and experiments in a tidal marsh of the SW Atlantic coast (36° 22′ S) showed that at the beginning of the stem-boring moth attack there was no difference in crab herbivory between plants with or without larvae of the stem-boring moth. However, after 3.5 mo, crabs foraged more on plants without larvae than on those with larvae. Plant tissue analyses showed a decrease in leaf tissue carbon concentrations in plants with larvae. This change in the nutritional quality of leaves, caused by construction of the stem-boring moth galleries, could explain the segregation in plant use between both herbivores. Unlike an allelochemical response, the non-specificity of the induced nutritional change could impair a wide variety of herbivores regardless of their feeding modes or taxonomic proximity. These effects could propagate bottom-up through the food-web, leading to more diffuse interspecific effects. Thus, here we show how the benefactor−beneficiary feedback between herbivores can be important for the maintenance of species coexistence and the functioning of communities.