Stress tolerance and population stability of rock pool Daphnia in relation to local conditions and population isolation

Small fragmented populations can lose genetic variability, which reduces population viability through inbreeding and loss of adaptability. Current and previous environmental conditions can also alter the viability of populations, by creating local adaptations that determine responses to stress. Yet, most studies on stress tolerance usually consider either the effect of genetic diversity or the local environment, missing a more holistic perspective of the factors contributing to stress tolerance among natural populations. Here, we studied how salinity stress affects population growth of Daphnia longispina, Daphnia magna, and Daphnia pulex from rock pools with varying degrees of population isolation and salinity conditions. Standing variation of in situ rock pool salinity conditions explained more variation in salt tolerance than the standing variation of population isolation or genetic diversity, in both a pulse and a press disturbance experiment. This indicates that the level of stress, which these natural populations experience, influences their response to that stress, which may have important consequences for the conservation of fragmented populations. However, long-term population stability in the field decreased with population isolation, indicating that natural populations experience a variety of stresses; thus, population isolation and genetic diversity may stabilize population dynamics over larger spatiotemporal scales.