Ecological systems are governed by a combination of density-dependent (i.e. intrinsic regulation) and density-independent factors (i.e. extrinsic forcing). However, resolving their relative importance remains a contentious issue in ecology. One potential solution to this quandary lies in disentangling the effects of density-dependent and density-independent factors according to their respective spatial scales. This approach contends that patterns and their causal processes share a common spatial scale. In marine systems, it has lead to the hypothesis that density-dependent processes (e.g. competition, predation) regulate abundance at local scales (< 1 km) whereas environmental heterogeneity governs the regional distribution (>10 km) of abundance. However, recent theory has shown that localized processes can interact with limited dispersal to generate patterns across scales and affect the regional distribution of abundance. Here, we perform spatial synchrony analysis on metapopulation models and survey data to assess the occurrence and implications of such cross-scale interactions in intertidal populations along the West coast of the United States.
Results/Conclusions

Our analysis reveals that intertidal populations along the West coast of the United States exhibit a nonlinear and regional (1800 km) spatial synchrony pattern that is inconsistent with the quasi-linear pattern of the environment (i.e. chlorophyll-a, sea surface temperature and upwelling index). Using metapopulation models, we show that the occurrence of this nonlinear spatial synchrony pattern is caused by the interaction between local density-dependent processes and limited dispersal. This cross-scale interaction occurs because limited dispersal couples local populations, thus allowing processes that are circumscribed spatially to scale-up and affect patterns at the regional scale. Dispersal can thus modulate the relative importance of intrinsic regulation and extrinsic forcing in ecological systems: limited dispersal promotes intrinsic regulation through cross-scale interactions whereas regional dispersal leads to extrinsic forcing of spatial synchrony by the environment. These cross-scale interactions are a general feature of metapopulations undergoing local fluctuations and limited dispersal, and their occurrence and prominence has important implications for our ability to both predict and conserve patterns in natural populations.