The temporal scale of environmental forcing in intertidal communities along the West Coast of the United States

The relative importance of intrinsic and extrinsic factors in explaining population dynamics has long been a contentious issue. Most studies relate long-term trends in environmental conditions to population properties such as cover and recruitment. However, the putative underlying processes responsible for these relationships—annual growth and larval colonization—respond to variation in environmental conditions at multiple temporal and spatial scales. This apparent disconnect between the temporal scale of biotic processes and that of abiotic conditions is explored for intertidal filter-feeder populations located along an 1800 km stretch of the West Coast of the United States. Using linear regression and regression tree methods, we show the relative importance of chlorophyll-a, sea surface temperature and upwelling index in predicting species cover across temporal scales. Results reveal a significant relationship between annual chlorophyll-a concentration and the annual cover of several filter-feeding species. We further highlight how this relationship involves time lags and varies among species and among years. We finally show how recent theories involving nonlinear and spatially-explicit processes predict fluctuations in the covariance between such processes and patterns that are compatible with data. This work highlights the importance of matching the temporal scale of the underlying processes with that of the prevailing environmental conditions. Doing so suggests that intertidal populations intrinsically oscillate in and out of environmentally forced regimes.