The paradox of enrichment in metaecosystems

Background/Question/Methods

At the convergence of community, ecosystem and spatial ecology, the concept of metaecosystems considers a spatially structured environment in which species as well as inorganic nutrients and detritus can diffuse between localities. This new framework promises to be a powerful tool to address questions at large spatial scales for which feedbacks between nutrient dynamics and species communities are crucial for ecosystem functioning. Here we revisited the renowned paradox of enrichment within the metaecosystem framework. This paradox states that ecosystem enrichment could destabilize consumer-resource interactions instead of benefiting their demography. Species dispersal has been proved to dampen this enrichment destabilization in metacommunities. However, spatial flows of nutrient, detritus and local recycling could change local enrichment and potentially modify this outcome. We studied the effect of either nutrient, detritus, producer or consumer spatial flows, combined with changes of regional enrichment, on the stability of a two-patch metaecosystem model.

Results/Conclusions

We found first that spatial flows of nutrient and detritus are destabilizing whereas spatial flows of producers and consumers are either neutral or stabilizing. These opposite effects on stability are linked to opposite effects on spatial synchrony. We also found unexpected additional stabilizing effects of consumer spatial flows at intermediate diffusion rates, for higher enrichment. Furthermore, we found that consumer spatial flows could lead to different equilibriums in strongly enriched metaecosystems. Then, initial small changes in densities could result in radically divergent distributions of biomasses.

Our metaecosystems' study reveals that nutrient spatial dynamics can produce complex interactions with species dynamics. Source-sink dynamics do not inevitably produce stabilizing compensatory effects, for instance when only nutrients can diffuse. However, the paradox of enrichment is unlikely to occur in well connected localities because it can be dampen by a global dispersal, which redistributes regionally the local excess of fertility. Stabilize the effects of a more global enrichment (regional scale) requires consumer spatial flows at intermediate dispersal rates. This points out the specific importance of consumers' dispersal to neutralize this paradox in nature.