Spatiotemporal, multiscale gradient analysis of longitudinal community structure and habitat relationships for plants, birds, and small mammals in two shredded riparian ecosystems, Central Valley, California.

摘要

This dissertation is organized into three chapters that investigate the role of habitat shredding on biodiversity. Chapter One introduces the concept of habitat shredding and makes predictions about its effects on biodiversity, based on wildlife responses to other forms of fragmentation.; Chapter Two uses multivariate gradient analysis to investigate longitudinal and local variation for habitat structure, plants, small mammals, and birds along two shredded riparian ecosystems in California's Central Valley. Variation was strongest for small mammals, followed by habitat structure, plants, and birds. For all groups, obligate and facultative riparian species were more abundant in the upper reaches, while edge-associated species were more abundant in the lower reaches. Shrub cover was strongly correlated with many species response variables. The two riparian ecosystems, though sharing similar extrinsic environmental influences, differed significantly in their response profiles, possibly attributable to differences in edaphic, hydrological and management regimes. Longitudinal gradients appeared to be driven largely by disturbance gradients that increased in magnitude downstream, but that could be further modified by local factors.; The final chapter involved a spatiotemporal gradient analysis of avian species and functional groups to (1) determine the strength of longitudinal gradients, (2) identify spatial and temporal patterns of distribution and abundance, (3) correlate species response variables with habitat factors, and (4) investigate whether climatic events and changes in stream flow management affected patterns of avian distribution and abundance across years. Spatiotemporal variation was greatest for disturbance-sensitive species, interior habitat specialists, and understory-nesting species. Neotropical passage migrants---birds that used the riparian corridor as a stopover site during migration---were more abundant in sites adjacent to major migratory routes, while neotropical migratory summer breeders were more abundant in sites of higher habitat quality. Abundance of interior habitat specialists and understory-nesting species declined in El NiNo years, but increased in the following years, suggesting that these climatic events may have short-term detrimental effects, but long-term positive effects on these functional groups.