Ecology across boundaries: Reciprocal subsidies between aquatic and terrestrial ecosystems structure consumer-resource dynamics

Background/Question/Methods

Cross-ecosystem movements of material and energy occur across all ecosystems but particularly reciprocal subsidy fluxes across the freshwater-land interface have received major scientific attention. Freshwater ecosystems have been suggested to receive higher amounts of subsidies than terrestrial ecosystems, potentially leading to higher secondary production in freshwaters. In addition, major food web effects of subsidies are expected to be indirect resulting from increased numbers or modified resource preferences of the recipient consumers. Therefore, subsidies might promote systematic differences between freshwater and terrestrial ecosystems. Here we use a meta-analytic approach to investigate the magnitude and direction of subsidy fluxes across the freshwater-land interface and subsequent responses in recipient consumers.

Results/Conclusions

We find that terrestrial and freshwater ecosystems differ both in the magnitude of subsidies they receive, and in the recipient trophic levels. Terrestrial ecosystems generally receive low subsidy amounts at one trophic level (i.e. predator level), whereas aquatic ecosystems receive high subsidy amounts at various trophic levels (i.e. from consumer to predator level). Despite the large discrepancy in magnitude, the direct contribution of these subsidies to consumer diet and biomass is surprisingly similar in freshwater and terrestrial ecosystems. We propose a framework conceptualizing potential consequences for food web dynamics resulting from the input of subsidies. We suggest that subsidy inputs can cause bottom-up and top down forces in aquatic ecosystems, whereas in terrestrial ecosystems subsidy inputs dominantly trigger top-down effects. However, we provide evidence that the strength of subsidy-mediated top-down control might depend on resource specialization and foraging mode of the recipient predator in both ecosystems. Moreover, we discuss the role of different consumers in nutrient-translocation and spatial dynamics as potentially important explanations for systematic variation in the effects of subsidies between terrestrial and freshwater ecosystems. To fully understand the consequences of subsidies on food web dynamics in recipient ecosystems, it is crucial that future research explicitly incorporates their indirect effects.