Background/Question/Methods
Temperature affects metabolic and developmental rates, thus influencing a species' fitness in a given environment. In communities with multiple trophic levels, the response of individual species to temperature changes can have significant impacts on both community diversity and community stability. These effects are investigated in a a simple food web model with one host and two parasitoids that engage in intraguild (IG) predation, where species experience a seasonally varying climate. For each species the temperature sensitivity is described by an Arrhenius-Boltzmann relationship that scales the vital rates with temperature. We use the model to quantify the long-term response to variations in the mean temperature and the amplitude of the seasonal cycle. We also explore qualitative differences between the bifurcation structure of simpler versus more complex models (including life history and nonlinear functional responses), and systems in which host, IG prey and IG predator have different temperature sensitivities.
Results/Conclusions
The combined effects of competition and predation drive community responses to warming. Competition between two parasitoids (no IG predation) is unaffected by temperature change if parasitism and mortality rates scale similarly with temperature. A linear food chain (host-parasitoid-hyperparasitoid) cannot explain changes in the intermediate species. In the full system, however, warming can cause a collapse of the IG prey or IG predator population, depending on their relative temperature sensitivity. This can lead to an abrupt increase in host population abundance in systems with alternative stable equilibria, a case that may be particularly relevant to biological control systems in which multiple parasitoids control a pest insect. A particularly interesting series of bifurcations occurs when the host species is more sensitive than both parasitoids and when all species exhibit Type-2 functional responses; warming then leads successively to catastrophic collapse of the IG prey population and gradual re-assembly of the three-species food web. A decrease in the amplitude of the seasonal cycle (due for example to differential warming of summers and winters) can interact positively or negatively with the mean temperature increase, though this effect tends to be small.