Armstrong-McGehee mechanism revisited: competitive exclusion and coexistence of nonlinear consumers

A number of mechanisms have been proposed to explain the coexistence of species engaging in exploitative competition. The Armstrong-McGehee mechanism relies on different levels of nonlinearity in functional response between competing consumers and their ability to avoid competitive exclusion through temporal resource partitioning during endogenously generated fluctuations. While previous studies have mainly focused on cases where one consumer has nonlinear functional response and the other consumer has linear functional response, our study assessed coexistence and competitive exclusion under a more realistic scenario with two nonlinear consumers. Using analytical and numerical methods we found that the potential of coexistence of the two consumers decreases with increasing nonlinearity in the more linear species; increasing nonlinearity in the more nonlinear species, however, resulted in non-monotonic changes in the parameter space allowing coexistence. When coexistence potential is quantified under the presupposition that each consumer must be able to persist with the resource by itself, coexistence becomes consistently less likely with increasing similarity of the functional responses of the two consumers. Our results suggest that the Armstrong-McGehee mechanism is unlikely to operate as the sole coexistence-promoting mechanism in communities with generally nonlinear consumer-resource interactions. However, its role as a module in more complex systems and in synergy with other factors remains to be established.