Simple rules describe bottom-up and top-down control in food webs with alternative energy pathways

Background/Question/Methods

Many human impacts have their largest direct effects either at the top or at the bottom of a food web through, e.g., harvesting and nutrient fertilization. To predict their ecosystem-wide consequences, we must understand how these effects propagate. We suggest that many communities can be approximated by simplified webs consisting of two major, alternative energy pathways (= food chains) originating from distinct types of primary producers (differing in, e.g., microhabitat or in defendedness against herbivores). We asked how structural properties of such food webs (i.e. lengths of the two constituent chains and presence/absence of a generalist apex predator connecting the chains) affect their responses (trophic level biomass; energy flow along each chain) to fertilization and harvesting of top consumers. We answered these questions by analyzing how the equilibrium solutions of webs with different topologies respond to changes in (i) the total amount of nutrient in the system and (ii) the mortality rate of top consumers.

Results/Conclusions

Analyses of model food webs with linear interaction coefficients reveal that effects of fertilization and top consumer harvesting are entirely determined by only two topological properties: (i) whether the two constituent chains have separate top consumers or a generalist apex predator; (ii) whether one or both chains are of odd or of even length. While the responses of odd-length chains to fertilization and top consumer harvesting depend on the length of the other chain, fertilization always increases, and enhanced top consumer harvesting always decreases, energy flow along even-length chains. Presence of a generalist apex predator creates apparent competition between chains that may drive members of odd-length chains extinct with fertilization. With separate top consumers in each chain, increasing the harvesting of the top consumer of an odd chain always decreases the density of the other top consumer. Both fertilization and harvesting can thus have negative effects on non-targeted populations that often deviate from the predictions of classical food chain theory. Our modelling framework provides new theoretical insights into how bottom-up and top-down forces propagate through whole ecosystems and may serve as a useful alternative benchmark to food chain theory.