Predicting extinctions: interspecific competition,predation and population variability in experimental Daphnia populations

We examine how interspecific competition and two types of size-selective predation affect population density, variability and persistence in laboratory cultures of two species of Daphnia, D. magna and D. longispina. When both species were analysed together, and for D. longispina alone, there were weak negative relationships between mean population density and population variability. Interspecific competition resulted in lower population densities and higher population variability. Extinct populations had lower densities and were also more variable than persisting ones. There was still an effect of population variability on extinction probability after the effect of density on population variability had been accounted for. Hence, the effects of population density and variability on population persistence were partly independent of each other. The effects of size-selective predation on population persistence were more species-specific and not directly related to density or variability. Since the effects of species interactions on persistence were large, we suggest that it is likely that population vulnerability analyses not incorporating effects of interspecific interactions are often misleading.