Vertebrates, arthropods, and invasive plants : an investigation of the top-down and bottom-up interactions of multiple herbivore guilds in a grassland system Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6t053j72m

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  • Humans are increasing the scale and frequency of many natural disturbances, as well as adding novel disturbances to ecosystems. This thesis uses the arthropod community as a metric to examine the multi-trophic responses to disturbances in California grasslands. Chapter 2 explores how the long-term exclusion of native vertebrate herbivores has impacted arthropod community and trophic structure at Hastings Natural History Reserve. Chapter 3 investigates the impacts of grazing and invasive plants on the arthropod community in a manipulative plant provenance x grazing experiment that had been in place for two years at the Sierra Foothills Research and Extension Center. Few long-term studies have explored the interactions between vertebrate herbivore guilds and the arthropod community. We studied the long-term impact on arthropod community structure and composition of removing vertebrate herbivores from a California oak savannah. We used paired plots to sample arthropods and plant communities inside and outside 11 long-term vertebrate exclosures. We found that arthropods respond to vertebrate access mainly via plant compositional shifts, but that different groups of vertebrates cause changes via different pathways. Large herbivores, including deer, caused a shift from native perennial to exotic annual plants which increased plant nutritional content and supported more herbivore and detritivore arthropods. Burrowing vertebrates such as gophers reduced the predatory arthropod community by reducing vegetation structure, releasing other arthropod groups and resulting in an overall increase in arthropod abundance. These results highlight that alterations to different parts of the vertebrate community can have qualitatively different effects on arthropod community structure. The invasion and spread of non-native plants and the prevalence of livestock grazing are two human-induced disturbances that can have widespread ecosystem impacts. We used a manipulative study to examine how the replacement of native perennial grasses by exotic annual grasses and the introduction of heavy cattle grazing impact arthropod community structure. Arthropod richness, diversity, and abundance did not vary predictably between native perennial and exotic annual grass communities, though arthropods responded to plant community characteristics. In the presence of grazing, arthropod biovolume increased in annual grass treatments, which could trigger a positive-feedback cycle with vector-transmitted plant pathogens to further increase the prevalence of non-native annual grasses. Arthropod community was strongly impacted by cattle grazing, which decreased the biovolume of predatory arthropods, causing a top-down increase in herbivorous arthropod biovolume. Grazing thus impacts plant composition both through the direct removal of biomass and increased plant species evenness, and through arthropod-mediated impacts. Together, these studies show that human alterations of different vertebrate guilds can impact the trophic structure of the arthropod community. Because arthropods mediate a number of ecosystem services, disturbances that alter arthropod assemblages can have far-reaching ecosystem impacts. By exploring the mechanisms by which manipulations of the vertebrate guild alter arthropod community, this thesis demonstrates the importance of horizontal and vertical diversity in multi-trophic interactions.
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