The ecological and evolutionary influences of belowground herbivory on plant defense and community structure

Most of the research conducted on interactions between plants and herbivores has focused on aboveground herbivory, even though belowground impacts are usually greater than similar aboveground rates of herbivory. This dissertation attempts to characterize the nature and scope of pocket gopher (Thomomys bottae) herbivory on California grassland communities on both ecological and evolutionary time scales. Pocket gophers specialize on consuming belowground plant parts and the coincidental effects of their burrowing and mound production have significant impacts on plant community structure and ecosystem processes.; The Channel Islands near Santa Barbara, with no current or fossil record of pocket gophers, and the adjacent mainland, with high gopher densities, were used to assess the role of root herbivory in the evolution of deterrence and tolerance in two native California forbs. Taken together, the results of three separate studies suggest that mainland populations of these forbs have adapted to deter and/or tolerate gopher herbivory. In addition, undamaged island forms of D. fasciculata had very high productivity; this is discussed with regard to a recent theory explaining the evolution of overcompensation.; The final study explores the role pocket gophers may play in the structuring of native California bunchgrass habitats. Bunchgrasses grow up to 700cm 2 and live 100+ years. Pocket gophers prefer forbs over grasses and segments of gopher feeding tunnels excavated in bunchgrass areas showed that pocket gophers focus their foraging efforts in the interstices between bunchgrasses over 8cm in diameter. Because forbs are adapted to germinating on disturbed soils, increased rates of gopher mound production may promote their community persistence, however, in bunchgrass areas there is both a net reduction in the space available for forb recruitment and increased chance of their attack by foraging gophers. The effects of pocket gophers on the competitive environment of the interstices between bunchgrasses are likely to generate complex interactions between native and exotic annuals in bunchgrass areas. Indeed, there is the potential that gophers may facilitate exotic annual invasion in California grasslands, as annual forb and grass dominated areas may support higher densities of pocket gophers than bunchgrass habitats.