INSECT HERBIVORES DRIVE IMPORTANT INDIRECT EFFECTS OF EXOTIC PLANTS ON NATIVE COMMUNITIES

Abstract

Exotic plant species can affect native plant species both directly via competition and indirectly by modifying native species' interactions with other organisms in the community. Both direct and indirect effects can have strong fitness impacts on the native species and can result in cascading effects throughout the invaded community. Many exotic plant species escape from the herbivores with which they co-evolved, but in our study system, the exotic Egyptian alfalfa weevil (Hypera brunneipennis) has been introduced to California and is able to feed both on the exotic plant Medicago polymorpha and on the California native Lotus wrangelianus. Because these two plant species share this common herbivore, the possibility of strong herbivore-mediated indirect interactions between these introduced and native plant species is plausible. We used observational studies and manipulative field experiments to determine whether exotic Medicago affects herbivory on the native Lotus and whether Medicago reduces the fitness of Lotus through direct competitive effects and/or herbivore-mediated indirect effects. Both observational and experimental data indicate that Medicago increases weevil herbivory on the native Lotus. Additionally, Medicago reduced Lotus fitness through direct as well as indirect mechanisms. In 2002, Medicago reduced Lotus fitness even when herbivores were experimentally reduced, suggesting direct competitive effects of Medicago on the native Lotus. In contrast, in 2003, Medicago reduced Lotus fitness only in the presence of herbivores, indicating that the fitness effects in that year were largely indirect and mediated by herbivores. Our results demonstrate that the net fitness consequences of indirect and direct effects of exotic plant species on native plants can vary temporally, depending on the abundance of members of the herbivore community. Also, rather than limiting the success of exotics, introduced herbivores that feed on exotic plants may provide additional, indirect avenues through which exotic plants can reduce the fitness of native species.