Trophic dynamics of meta-ecosystems: Connectedness alters interaction outcome

Background/Question/Methods

Ecosystems are open entities that receive energy and nutrients from adjacent systems, have mobile predators that feed across boundaries, and include organisms with complex life stages that develop as larvae in one system and emerge to be adults in another. This connectedness may integrate adjacent food webs into one meta-ecosystem, potentially altering the trophic dynamics in both systems. The connections between meta-ecosystem components are likely to change with the productivity or biomass of donor and recipient habitats, shifting the strength of interactions within constituent food webs. We evaluated meta-ecosystem structure between coupled terrestrial and aquatic habitats across a gradient of grassland to forested landscapes and investigated the consequences of meta-ecosystem structural changes, testing whether varying terrestrial detritial subsidies affected trophic cascade strength in aquatic systems. Semi-quantitative food webs that included reciprocal subsidy fluxes were compiled through surveys and existing databases to assess meta-ecosystem structure. Trophic cascade strength was tested with an experimental manipulation of detrital subsidies (5 levels) in a freshwater lake benthic community. 

Results/Conclusions

Terrestrial and aquatic systems became more connected along the tussock to forest gradient, with an increase in the ratio of terrestrial to aquatic subsidies in more forested landscapes. Detrital subsidies had unexpected effects on trophic cascade strength. Low detrital levels led to positive indirect effects of predators on primary production (a classic trophic cascade), but high levels of detritus led to a negative indirect effect on primary production. Our work suggests subsidy variation along meta-ecosystem gradients may affect direct and indirect trophic interactions within both systems influencing trophic dynamics of the meta-ecosystem as a whole. We are now examining the interactions between subsidies entering at different trophic levels and their effect on individual ecosystem and meta-ecosystem trophic dyamics.