Spatial population synchrony—correlated temporal fluctuations of populations in different locations—is a fundamental feature of population dynamics. Despite the ubiquity and ecological importance of spatial population synchrony, there are large aspects of this phenomenon that are not well understood. We identified three major gaps in knowledge about the patterns and drivers of spatial population synchrony: (1) Spatial synchrony may occur over different timescales (e.g., annual vs. decadal) or change over time, but traditional approaches obscure timescale-specific patterns and secular trends. (2) Spatial synchrony may not occur in the same way in different regions, but most studies overlook the geography of spatial synchrony. (3) Multiple processes can cause spatial synchrony, such as correlated environmental fluctuations (i.e., the Moran effect) and dispersal, but the relative impacts of such drivers are usually unknown. To help resolve these gaps, we applied several novel statistical techniques to a 33-year time series of a giant kelp (Macrocystis pyrifera) metapopulation distributed across nearly 1,000 km of coastline in California, USA, as well as several spatially- and temporally-explicit oceanographic environmental covariates.
Results/Conclusions
Using a clustering algorithm to identify areas of the California coastline with relatively high within-group and low among-group synchrony, we discovered that giant kelp populations exhibit two major patterns of spatial synchrony. Using wavelet and wavelet phasor mean fields, we characterized the time- and timescale-dependence of synchrony within the clusters, finding that kelp populations in northern California (north of Point Conception) are highly synchronous on short timescales (predominantly annual). By contrast, kelp populations in southern California show little spatial synchrony on short timescales but stronger spatial synchrony on longer timescales (2–3 years) and in response to irregular annual events. Preliminary analysis using wavelet linear models to determine the drivers of spatial synchrony within each cluster at different timescales suggests that (1) sub-annual synchrony is driven by large waves that destroy kelp, (2) annual synchrony is driven by high ocean temperatures that correspond with nutrient (surface nitrate) limitation, and (3) multi-year synchrony is driven by decadal-scale climate variability (the North Pacific Gyre Oscillation, NPGO). Together, these result provide evidence for regional-scale geographic variation in the strength, timescales, and drivers of spatial synchrony in California giant kelp metapopulations.