How density dependence, genetic erosion and the extinction vortex impact evolutionary rescue

Abstract

Following severe environmental change that reduces mean population fitness below replacement, populations must adapt to avoid eventual extinction, a process called evolutionary rescue. Models of evolutionary rescue demonstrate that initial size, genetic variation and degree of maladaptation influence population fates. However, many models feature populations that grow without negative density dependence or with constant genetic diversity despite precipitous population decline, assumptions likely to be violated in conservation settings. We examined the simultaneous influences of density-dependent growth and erosion of genetic diversity on populations adapting to novel environmental change using stochastic, individual-based simulations. Density dependence decreased the probability of rescue and increased the probability of extinction, especially in large and initially well-adapted populations that previously have been predicted to be at low risk. Increased extinction occurred shortly following environmental change, as populations under density dependence experienced more rapid decline and reached smaller sizes. Populations that experienced evolutionary rescue lost genetic diversity through drift and adaptation, particularly under density dependence. Populations that declined to extinction entered an extinction vortex, where small size increased drift, loss of genetic diversity and the fixation of maladaptive alleles, hindered adaptation and kept populations at small densities where they were vulnerable to extinction via demographic stochasticity.

How bottlenecks shape adaptive potential: from theory and microbiology to conservation biology

“…Specifically, bottlenecked populations can fall into an "extinction vortex", often characterized by a complex interplay between genetic drift, demographic stochasticity, and environmental fluctuations (Soulé, 1986). A population bottleneck reduces fitness directly through increased genetic load and indirectly through erosion of genetic variation, leading to population decline, exacerbating the effects of genetic drift, demographic stochasticity, and environmental fluctuations until extinction (Nordstrom et al, 2023). Theoretical models have highlighted a critical level of gene flow that allows a metapopulation to survive over long timescales, even if it is often ultimately driven to extinction (Hanski and Ovaskainen, 2003;Gyllenberg and Hanski, 1992;Lande et al, 1998).…”

“…For instance, Nordstrom et al (2023) showed through stochastic individual-based simulations that considering population growth with negative density dependence (i.e., intraspecific competition) or density independence leads to different outcomes of evolutionary rescue. This prediction regarding the impact of density dependence vs. independence was empirically tested and confirmed with flour beetles (Olazcuaga et al in prep.).…”

Immigration delays but does not prevent adaptation following environmental change: experimental evidence

“…Each experimental population received one of three immigration treatments (zero, one, or five individuals every generation), and we observed the effects of immigration on population persistence and growth. In habitats like these that have limited space and degraded resources, population recovery may be constrained by density-dependent processes like competition for food and space (Osmond & de Mazancourt, 2013; Nordstrom et al ., 2023). Thus, we evaluated both density-dependent and density-independent population growth through time to tease apart the impacts of negative density dependence and adaptation.…”