Biodiversity drives evolution of a community member through ecological functioning in indirect interaction webs

Background/Question/Methods

Despite growing interplay between ecological and evolutionary studies, the question of how biodiversity influences evolutionary dynamics within species remains understudied. We focus on indirect interaction webs in a plant-insect system in order to answer the question. Herbivore species induce phenotypic changes in host plants, which subsequently affect abundance and performance of other herbivore species. In other words, herbivory influences the spatio-temporal distribution of host plant traits, which provides a basis for indirect interactions among herbivores. Such trait-mediated links shape ecological networks as plant-centered indirect interaction webs where diverse herbivore species are embedded. Furthermore we have recently revealed that indirect interactions mediated through herbivore-induced plant responses can affect trait evolution of herbivorous insect species.

Here we refer to the spatio-temporal distribution of host plant traits regulated through herbivore-induced plant responses as ecological function of herbivory. Then we hypothesize that species diversity of herbivore governs the ecological function of herbivory, which drives trait evolution of a community member. To examine the hypothesis, we have focused on geographic variation in the induced plant responses, species diversity of herbivorous insects, and feeding preference of the leaf beetle Plagiodera versicolora in a willow-insect system, and have conducted field research and community-manipulation experiments.

Results/Conclusions

Willow trees produced new lateral shoots in response to attack by various herbivore species, resulting in regulation of the spatio-temporal distribution of willow new leaves in local environments. We found that the willow leaf beetle evolutionarily developed adaptive feeding preference for leaf-age-types according to the local intensity of the herbivore-induced plant regrowth. The leaf beetle populations at some local sites with intense willow regrowth displayed strong, exclusive preference for new, non-fully expanded leaves. Other leaf beetle populations occurring at sites with weak regrowth responses showed non-preferential feeding on new and mature leaves. The local species diversity of herbivorous insects predicted the local intensity of the induced plant regrowth and the feeding preference of the leaf beetle. Our community-manipulation also supported that variation in species diversity resulted in difference in plant regrowth intensity, leading to divergent evolution of the leaf beetle preference. This was likely to be because local species diversity could link with the probability of including multiple species which was a strong inducer of plant regrowth and provide a temporal insurance for maintaining a selection regime for evolution of the leaf beetle preference.