Worldwide cross-ecosystem carbon subsidies and their contribution to ecosystem functioning
Ecosystems are widely inter-connected by spatial flows of resources1,2, yet primarily studied in a local context. Meta-ecosystem models suggest that cross-ecosystem subsidies can play an essential role in ecosystem functioning, notably by controlling local availability of resources for biological communities3-6. The general contribution of these resource connections to ecosystem functioning, however, remains unclear in natural systems, due to the heterogeneity and dispersion of data across the ecological literature. Here we provide the first quantitative synthesis on spatial flows of carbon connecting ecosystems worldwide. These cross-ecosystem subsidies range over eight orders of magnitude, between 10-3 and 105 gC m-2 yr-1, and are highly diverse in their provenance. We found that spatial carbon flows and local carbon fluxes are of the same order of magnitudes in freshwater and benthic ecosystems, suggesting an underlying dependency of these systems on resources provided by connected terrestrial and pelagic ecosystems respectively. By contrast, in terrestrial systems, cross-ecosystem subsidies were two to three orders of magnitude lower than local production (grasslands and forests), indicating a weaker quantitative influence on functioning. Those subsidies may still be qualitatively important, however, as some have high nutrient content7,8. We also find important gaps in carbon flow quantification, notably of cross-ecosystem subsidies driven by animal movements, which likely leads to general underestimations of the magnitude and direction of cross-ecosystem linkages9. Overall, we demonstrate strong ecosystem couplings, suggesting that ecosystems can be vulnerable to alterations of these flows and pointing to an urgent need to re-think ecosystem functioning in a spatial perspective.