Biodiversity of coastal ecosystems: exploring spatial and temporal patterns in intertidal macroinvertebrate communities of sandy beaches
Biodiversity plays a vital role in the function, stability, and resilience of ecosystems. Concern about biodiversity loss and the alteration of ecological communities has driven major efforts to catalog and monitor species in ecosystems across the globe and to quantify relationships between biodiversity and ecosystem function. Evaluating responses of biodiversity and community structure to abiotic and biotic drivers, such as climate, disturbance, and species interactions requires comparisons across relevant spatial and temporal scales.
Located at the boundary between land and sea, coastal ecosystems and the biodiversity they support are subject to increasing threats from escalating anthropogenic and climatic impacts. Sandy beach ecosystems, which support unique macroinvertebrate communities and provide irreplaceable ecosystem functions (e.g. nutrient recycling and wildlife support), experience heavy human use and manipulations making them particularly vulnerable to urbanization and environmental change. For temperate sandy beaches the degree of vulnerability is expected to vary with major trophic guilds due to differences in functional, trophic, and life history traits, with greater impacts expected for wrack-associated than lower beach species. To gain insights on mechanisms influencing ecological communities and functioning, I explored questions concerning responses of biodiversity, community structure, and composition of sandy beach ecosystems to environmental and anthropogenic factors operating over a range of temporal and spatial scales for my dissertation research. To evaluate these questions, we conducted 60 intertidal macroinvertebrate surveys at 41 study beaches spanning >1800 km of California coastline (~9 degrees of latitude) from 2009-2015. These were the first quantitative surveys ever conducted at many of these beaches.
In my first chapter, I addressed the important issue of comparing species richness across surveys conducted using different sampling methods by developing a calibration approach to adjust species richness for sampling effort. In my second chapter, I assessed the direction and magnitude of change in intertidal biodiversity over time by comparing values of species richness from surveys collected >30 years apart (1970s and 2009-11) using the calibration from my first chapter. In my third chapter, I evaluated ecological impacts of coastal urbanization by comparing intertidal macroinvertebrate communities between urban beaches with intense maintenance regimes and reference beaches lacking such maintenance. In my final chapter, I evaluated spatial patterns in biodiversity, structure, and composition and the role of environmental factors in structuring intertidal macroinvertebrate communities of sandy beaches at different spatial scales.
Overall, findings from my dissertation suggest sandy beaches are more diverse and biogeographically complex than generally acknowledged and that central and southern California beaches may represent a global biodiversity hotspot for intertidal macroinvertebrates. The peak in intertidal species richness of sandy beach macroinvertebrates in temperate latitudes is not consistent with the well-known pattern of a negative trend in species richness with latitude reported for a wide range of ecosystems and taxa. My results highlight the importance of connectivity to donor ecosystems and the strong influence of subsidies on intertidal macroinvertebrate communities of sandy beaches, particularly that of drift macroalgae on wrack-associated species. The functional, trophic, and life history traits of wrack-associated macroinvertebrates, a major component of intertidal diversity (>40% of species), make this guild disproportionately vulnerable to human and climatic impacts compared with lower beach species. Human impacts associated with urbanization (e.g. mechanized grooming, sediment fills) strongly degraded intertidal macroinvertebrate biodiversity, structure, and function and appeared to influence sandy beach communities more at local than regional spatial scales. Though local scale processes masked the influence regional and global scale processes at most study beaches, we observed biodiversity declines consistent with habitat loss from sea-level rise.