PORTLAND, OREGON--The amount of genetic variation within a plant species can influence which insects and spiders live on the plants, a new study finds. The research shows that diversity within a species can have some of the same effects as diversity among species.
It's long been clear to ecologists that the number of plant species can determine the makeup of an ecological community. For instance, places with more plant species tend to have more kinds of plant-eating insects, because many insect herbivores specialize on single species. But does diversity also get a boost simply from the genetic variation within single plant species?
Yes, wagered ecologist Marc Johnson of the University of Toronto. Figuring that individual plants would differ in their genetic resistance to insect attack, Johnson set out to test the idea with the evening primrose, Oenothera biennis, a plant that is home to more than 200 species of insects and spiders. Johnson planted clusters of primrose at field sites, each cluster consisting of eight plants. Some included eight genotypes, others two plants each of four genotypes, and the rest were all of the same genotype. Johnson let arthropods colonize the plants and monitored their diversity and abundance through the summer.
The most diverse patches hosted 17% more arthropod species than the monocultures, Johnson found. The abundance was due not to more plant-specific herbivores but to many types of spiders and other predators coming to feast on the insects. The genetically diverse clusters were also crawling with more individuals of predators and omnivores, although not of herbivores--suggesting that genetic resistance to herbivores cannot explain the difference. Because some effects appeared toward the end of the season as plants were flowering, it may be that arthropods coming for flowers and fruit are responsible for the pattern, Johnson suggested during his presentation of the results here on 2 August at the annual meeting of the Ecological Society of America.
Ecologist Michael Singer of Wesleyan University praises the study's experimental design and suggests that flowering may indeed explain the results. If different genotypes flower at different times, then mixed-genotype clusters contain flowers longer and could accumulate more insects over time, he says.