The combined impacts of climate change and land use are projected to drive unprecedented rates of environmental change and biodiversity loss this century. Given the rapidly growing human populations in mediterranean-climate systems, land use may pose a more immediate threat to biodiversity than climate change, however, the relative future impact of each driver remains largely unaddressed. Focusing on California sage scrub (CSS), a plant association of considerable diversity, endemism, and threatened status in the mediterranean-climate California Floristic Province, I investigate the future threats of projected land use and climate change on CSS distribution and diversity. California sage scrub is highly reduced and fragmented in distribution and contains a large number of threatened and endangered species that may be particularly vulnerable to further habitat loss from future land use and climate change.
My objectives are to (1) assess the role of climate in shaping current patterns of CSS species and floristic group distributions at a regional scale, (2) predict changes in the distribution of CSS species and growth forms, testing whether niche, geographic, or bioclimatic factors explain the magnitude of species response to projected climate change and identifying potential future impacts on CSS community structure and diversity, and (3) compare the relative threats of projected land use and climate change on CSS in California to better understand how threats may vary spatially and temporally. Using a species distribution modeling approach, I modeled the contemporary climate relationship and future (mid- and late-century) geographic distributions of 33 dominant CSS shrub species assuming two possible climate change trajectories (warmer wetter and warmer drier) and two dispersal scenarios (unlimited dispersal and no dispersal). Habitats loss from future land use was calculated from projected land use overlays (objective 3).
Current models reveal climate is a strong predictor of individual CSS species and floristic group distributions at regional scales. Modeling at a floristic group level provides important information about the differences in current climatic niches within CSS, highlighting the potential for community-level modeling approaches to investigate plant distribution patterns. Under projected climate change, I predict two overall patterns in CSS habitat change consistent across climate change trajectories: southern habitat contraction and northern habitat expansion. By late-century (2080s) species habitat losses will range from moderate (unlimited dispersal) to severe (no dispersal), with succulent species showing minimal habitat loss and overall net habitat gains (unlimited dispersal). Individual shifts in the distribution of CSS species translate to considerable community restructuring and diversity shifts at northern and southern extents of CSS, with implications for future CSS management and conservation.
I find the degree of threat posed by climate change relative to land use depends upon dispersal scenario, with land use and climate change posing similar future threats under no dispersal scenarios and land use posing a greater future threat under unlimited dispersal scenarios. Impacts will also vary spatially between Central Coast and South Coast California Ecoregions, with high CSS habitat and diversity losses from both land use and climate change predicted in the South Coast even under best-case unlimited dispersal scenarios, but considerable habitat gains and increased diversity predicted in the Central Coast. Furthermore, I find that regions of the South Coast that are currently intact but projected to undergo future anthropogenic conversion will have disproportionately high losses in CSS species richness driven by climate change. These findings highlight the potential for land use and climate change to have compounding negative impacts on CSS and emphasize the necessity to include analyses of both drivers in conservation and resource management planning.