Reciprocal feedbacks between spatial subsidies and reserve networks in meta-ecosystems

Background/Question/Methods

The role of local top-down processes such as predation and herbivory in initiating trophic cascades that control the structure of ecosystems has long been recognized. However, theory is only beginning to describe how spatial subsidies such as nutrient flows and larval dispersal between interconnected meta-ecosystems can act as important drivers of community organization by modifying local top-down processes. Here, we develop a mathematical meta-ecosystem model of coral reefs to understand how the reciprocal feedbacks between spatial subsidies and reserve networks modulate the importance of herbivory across scales in a simple herbivore-macroalgae-coral system. Although current non-spatial models predict that increasing herbivore density via no-take reserves will always promote coral abundance by reducing its main macroalgal competitor, our meta-ecosystem model allows to test the role of regional connectivity in mediating the importance of local top-down control of coral abundance by herbivores.

Results/Conclusions

Reducing connectivity by increasing the distance between reserves causes coral abundance to increase and macroalgal abundance to decrease despite a reduction in herbivore abundance within reserves. Hence, for corals, the regionally beneficial effects of limited connectivity in the form of competitive release from macroalgae trump the local costs associated with reduced grazing of macroalgae by herbivores. In addition, we reveal a trade-off between local and global conservation objectives when designing reserve networks: small and aggregated reserves based on the scale of dispersal maximize the abundance of corals and herbivores globally, whereas large and isolated reserves always maximize the abundance of corals within reserves, regardless of the scale of dispersal. The existence of such "conservation traps", which inadvertently maximize population-level objectives at local scales at the cost of community-level objectives at global scales, suggests the importance of adopting a more holistic perspective when implementing management strategies. Finally, our model demonstrates that all reserve networks, regardless of their spatial configuration, increase the mean nutrient concentration available in the system by almost an order of magnitude. This unexpected feedback between spatial management and ecosystem functioning emphasizes the benefits of adopting a meta-ecosystem perspective to fully appreciate the effects of reserve networks in complex and interconnected ecosystems.