Temperature and interspecific competition are fundamental drivers of community structure in natural systems, and can interact to affect distribution, abundance, persistence and many other long-term measures of species performance relating to survival. However, we know little about how interspecific competition affects short-term sensitivity to extreme temperature, information that is critical for evaluating future threats to species from extreme high-temperature events, which are forecast to increase dramatically in frequency, severity, and duration with global warming over the next hundred years.
Using an experimental freshwater community of four rotifers and six ciliates, I conducted a three-factor experiment with two temperature regimes [‘constant’ (room temperature) and ‘increasing’ (temperature increasing incrementally +0.5°C/day)], two community compositions [‘polyculture’ (all species cultured together) and ‘monoculture’ (each species cultured in isolation)], and eight species (Lepadella sp., Lecane sp., Rotaria sp., Paramecium caudatum, Paramecium bursaria, Euplotes daidaleos, Coleps sp., and Halteria sp.). Population density was measured in all communities each day, allowing examination of the effects of interspecific competition on population abundance throughout the term of the experiment. Determination of the lethal temperature in increasing temperature communities allowed examination of the effect of interspecific competition on the temperature-dependent extinction of all eight species.
Results/Conclusions
Densities for many species were ten to forty times greater in ‘monoculture’ than ‘polyculture’, providing strong evidence of interspecific competition. Competition increased sensitivity to extreme temperature in three out of eight species in the community, lowering lethal temperature during an extreme high temperature event by as much as 2.9°C (5.2°F). Surprisingly, effects of interspecific competition on population density and sensitivity to temperature were not correlated (i.e., species with densities more suppressed by competition did not consistently die at lower temperature). This might occur if mechanisms driving reductions in lethal temperature are species specific, such that the effect on lethal temperature depends more on the identity of competitor species (and their respective abilities to modify the shared biological and physical environment in a manner detrimental to lethal temperature) than on the combined effect of all competitors on density. These findings are particularly important for evaluating risk to species living near their critical thermal maxima. They also suggest that differences in community structure in space and time could drive corresponding variability in upper thermal limits, making accurate theoretical and laboratory-based predictions of thermal tolerances more challenging.