Effects of intra‐ and interspecific interactions on species responses to environmental change

Abstract

The extent of directional environmental change expected in the next century underscores the need to understand density‐dependent population regulation. Direct density dependence generated by intraspecific competition and/or predator–prey interactions should buffer environmentally produced changes in density‐independent growth rates ( r ). Density dependence generated by interspecific competition should magnify sensitivity to changes in r . We tested these predictions by assembling protist communities in laboratory microcosms and subjecting the communities to directional environmental change. Two experiments used pairs of competing protists ( Colpidium striatum Stein and Tetrahymena thermophila Nanney & McCoy, and Paramecium tetraurelia Sonneborn and Blepharisma americanum Suzuki) along with single‐species controls. Two experiments used predator–prey pairs ( Didinium nasutum Müller or Euplotes patella Ehrenberg preying on Colpidium ) and controls containing only prey. We grew replicates of each species combination in two temperature regimes (constant or slowly increasing temperature). Independent of these experiments, we quantified the temperature dependence of r (intrinsic rate of increase) for each species. Comparison of effects of temperature on r to effects on mean density revealed that intraspecific competition buffered species densities against temperature changes that increased r by over 200%. Interspecific interactions did not affect species responses to environmental change. Temperature change had weak effects on species densities, whether or not other species were present. The results suggest that natural populations, regulated by direct density dependence, may be buffered against directional environmental change. Environmental change will have large effects when populations experience little density dependence, and when environmental change has different effects on the vital rates of different species.