Spatial synchrony cascades across ecosystem boundaries and up food webs via resource subsidies

Cross-ecosystem subsidies are critical to ecosystem structure and function, especially in recipient ecosystems where they are the primary source of organic matter to the food web. Subsidies are indicative of processes connecting ecosystems and can couple ecological dynamics across system boundaries. However, the degree to which such flows can induce cross-ecosystem cascades of spatial synchrony, the tendency for system fluctuations to be correlated across locations, is not well understood. Synchrony has destabilizing effects on ecosystems, adding to the importance of understanding spatiotemporal patterns of synchrony transmission. In order to understand whether and how spatial synchrony cascades across the marine-terrestrial boundary via resource subsidies, we studied the relationship between giant kelp forests on rocky nearshore reefs and sandy beach ecosystems that receive resource subsidies in the form of kelp wrack (detritus). We found that synchrony cascades from rocky reefs to sandy beaches, with spatiotemporal patterns mediated by fluctuations in live kelp biomass, wave action, and beach width. Moreover, wrack deposition synchronized local abundances of shorebirds that move among beaches seeking to forage on wrack-associated invertebrates, demonstrating that synchrony due to subsidies propagates across trophic levels in the recipient ecosystem. Synchronizing resource subsidies likely play an underappreciated role in the spatiotemporal structure, functioning, and stability of ecosystems. Significance Many ecosystems depend on resource inputs, called subsidies, from other ecosystems, influencing their structure and function. Using a model system, we demonstrate how resource subsidies can synchronize the dynamics of recipient ecosystems across space: Synchronous offshore kelp supply, mediated by wave action and beach width, yielded synchronous deposition of kelp wrack (detritus) on open coast sandy beaches, which then cascaded through the trophic levels of the terrestrial recipient system, synchronizing local abundances of shorebirds that move among beaches to forage on invertebrate wrack consumers. Cross-ecosystem synchronization via subsidies likely plays a major but previously unrecognized role in the spatiotemporal dynamics and stability of recipient ecosystems.