Ecosystems are connected by flows of material that can couple the dynamics of food webs in complex ways. Flows from one ecosystem that increase the productivity of a recipient in another ecosystem are typically termed subsidies. Theory on subsidies has focused on donor-controlled flows; however, such a strict definition ignores a well-studied class of cross ecosystem material and energy flows empirically considered subsidies – the migrations of taxa with complex life cycles. Diadromous fishes and amphibians exhibit inter- and intra-taxon variability in how much material they move across ecosystems and the form it is released in the recipient ecosystem (i.e., as a consumer or an abiotic resource) with potentially variable effects on ecosystems. Our objective was to investigate how life history strategy and trait variability influence the ecosystem effects of migratory animals with complex life cycles. To address this question, we developed a two-ecosystem model consisting of natal and maturation ecosystems connected by flows of migrating top consumers. Then, we analytically evaluated how trophic stocks and ecosystem fluxes (i.e., production) respond to increasing juvenile and adult migration rates as well as the efficiency with which adults from the maturation ecosystem convert mass directly into feeding juveniles (i.e., subsidy conversion efficiency).
Results/Conclusions
We demonstrate that increasing consumer migration rates always decreases biotic resource stocks in the recipient ecosystem while increasing biotic resource stocks in the donor ecosystem. Thus, increasing the magnitude of the subsidy delivered by these taxa increases the strength of top-down trophic regulation in recipient ecosystems regardless of other ecosystem conditions. In nature, such changes in migration rates can occur with changing growth opportunities that trigger the onset of metamorphosis or maturation. Meanwhile, increasing the subsidy conversion efficiency, which generalizes the gradient from semelparity to iteroparity, exerts top-down regulation of trophic stocks in both ecosystems but has condition dependent effects on production. Overall, our analyses provide three key insights into the ecosystem effects of migrating animals: (i) at equilibrium, trophic stocks have consistent directional responses to changes in migration rates and life history strategy, (ii) iteroparous migratory animals have stronger top-down effects on their resource stocks than semelparous migratory animals, and (iii) the response of ecosystem fluxes to changes in migration rates and life history strategies depends on the relative nutrient recycling efficiency of juvenile mortality and abiotic subsidy pathways in natal ecosystems. The theory we develop will guide future empirical and theoretical work on this common form of subsidy.