Trophic cascades and diversity maintenance mechanisms

Background/Question/Methods Our recent work on species coexistence mechanisms has focused attention on multi-species density-dependent feed back loops as fundamental to both coexistence and exclusion. These density-dependent feed back loops can result from competition for resources or from density-dependent predation, also known as “apparent competition.” Coexistence occurs when intraspecific density dependence, of whatever form, is stronger than interspecific density dependence. Exclusion occurs otherwise. Thus, coexistence and exclusion mechanisms can be competition-based, if the key density dependence arises from competition, or predation-based, if predation is the source of that density dependence. Moreover, when both are present, competition and density-dependent predation can undermine each others effects, reinforce one another or interact to create new mechanisms, depending on the specific circumstances. This understanding implies that the whole foodweb potentially has effects on species coexistence of any guild of species in any trophic level. Hence, trophic cascades should have important effects on coexistence of species in ecological guilds. We investigated this idea in Lotka-Volterra foodweb models using a combination of analytical and simulation techniques. Results/Conclusions In models with three trophic levels, competition and predation have equal roles in diversity maintenance in focal guilds in the middle (second) trophic level. Adding a fourth trophic level has profound effects. In the simplest case where fourth-trophic species set average densities of third-trophic species, density-dependent predation on the second trophic level is lost, and competition alone sets up coexistence and exclusion mechanisms. Predation on second-trophic species retains a nondensity-dependent role, but cannot stabilize coexistence. However, this case requires fourth-trophic species to be limited by third-trophic species alone, and for fourth-trophic species to be as diverse as third-trophic species. Relaxing these assumptions by adding a fifth trophic level, reducing the diversity of the fourth trophic level, or adding non-trophic limitation in the fourth trophic level, lessens the control of the fourth trophic level on the third, permitting varying degrees of density-dependent predation on second-trophic species. However, many different outcomes are possible depending on the nature of the constraints that the fourth trophic level places on the third trophic level. These results emphasize the potential for far-reaching trophic effects in ecosystems, and in particular that trophic cascades potentially have a major role in diversity maintenance mechanisms.