Positive interactions between leafrollers and other arthropods enhance biodiversity on hybrid cottonwoods

We examined the potential of a common herbivore to indirectly influence other diverse community members by providing habitat. Larvae of the leafroller Anacampsis niveopulvella commonly construct shelters by rolling leaves of cottonwood trees. These leaf rolls are later colonized by other arthropods. We first documented 4 times greater species richness and 7 times greater abundance on cottonwood shoots that contained a rolled leaf compared to adjacent shoots without leaf rolls. Second, with both removal and addition experiments, we showed that leaf rolls are responsible for these differences in arthropod assemblages. Leaf roll removal caused a 5-fold decline in richness and a 7-fold decline in abundance; leaf roll addition resulted in a 2.5-fold increase in richness and a 6-fold increase in abundance. Third, to determine whether rolled leaves are colonized for food or for shelter, we compared colonization of natural and artificial leaf rolls. Both richness and abundance were approximately 2-fold greater in artificial leaf rolls, indicating that leaf rolls are colonized primarily for shelter. Fourth, in a natural hybrid zone we found that leafroller densities were 2-fold greater on backcross hybrids than on F₁ hybrids. These differences are likely associated with genetically-based differences in leaf morphology and/or leaf chemistry. Ultimately, plant genotype affects positive indirect interactions that have the potential to affect community structure. This study and others demonstrate that shelter builders (i.e., leafrollers and gall formers) enhance biodiversity, while free-feeders are more likely to negatively affect biodiversity.