Forest disturbance effects on insect and bird communities: Insectivorous birds in coast live oak woodlands and leaf litter arthropods in the Sierra Nevada.

摘要

I studied the effects of different types of anthropogenic disturbance on arthropods and birds in two California forest ecosystems: oak woodlands in the San Francisco Bay Area and Sierra Nevada conifer forests. These studies highlight the many scales that are impacted by forest disturbance, and also how disturbance can impact trophic interactions.; Sudden oak death (SOD) is a disease that has resulted in stand-level dieback of oaks in California. I found that symptomatic coast live oaks harbor fewer "leaf-feeding" arthropods than asymptomatic oaks, and that oaks have more foliar arthropods than bay trees. Both chestnut-backed chickadees and oak titmice switched their foraging locations from oaks to a variety of other trees, but avoided bays, on sites heavily affected by SOD. There were few effects of SOD on nesting, but I found high dietary stability despite increased foraging effort on affected sites. As SOD spreads, the long-term prospect for these bird species is mixed. If bay trees dominate these woodlands as oaks die, many arthropods that rely on oaks may disappear. Oak titmice, which rely heavily on oaks, may be particularly affected, while chestnut-backed chickadees demonstrated greater foraging flexibility, and may be able to compensate.; In the Sierra Nevada, I examined the effects of prescribed burning and thinning practices on litter arthropods. In one of the few studies to examine arthropod diversity in the Sierras, I collected 278 beetle species, 24 ant species, and thousands of other arthropods. In a controlled, replicated experiment using burning, thinning, and combined treatments, I found few major effects on abundances of litter fauna. There were noticeable effects on beetle assemblage structure, brought about by changes in structural features of the forest.; These studies demonstrate how disturbances not only have impacts on a wide range of scales, from individual behavior to community structure, but also may have multi-tropic effects within a community, if vegetation changes impact herbivores and, ultimately, predators. Whether managers are attempting to mitigate for habitat disturbance caused by introduced pathogens, or using fire as a disturbance agent to restore habitat, their impacts on many scales and trophic levels must be considered.