Extinction Risk and Lack of Evolutionary Rescue under Resource Depletion or Area Reduction

Evolutionary adaptations following environmental deterioration can sometimes rescue populations from extinction. Here we provide a scenario in which such evolutionary rescue will be difficult. Using a rather general model for fluctuating r- and K-selection in a density-dependent population, we show that reduction of available resources will not necessarily induce evolution of adaptations to counteract such changes provided that density regulation acts through available resources per individual. In large populations, resource depletion may induce a change in stationary distribution of population size while the optimal phenotype remains unchanged. Under a period of continuous reduction in available resources, increased strength of K-selection will occur in the sense that individuals are able to live and reproduce under less favorable conditions. Smaller growth rates as a consequence of K-selection and trade-offs between intrinsic growth rate r and carrying capacity K may then have a considerable negative effect on the persistence of the population even after the reduction of available resources is stopped. This negative effect comes in addition to the purely ecological effect of reduced time to extinction because of a reduction in K and increased demographic stochasticity. Continuous reduction in the available area or in available resources per individual may result in long-run maladaptation even if demographic noise increases and, finally but too late, induces r-selection.