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 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 influences of density-dependent growth and erosion of genetic diversity simultaneously 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.

Longevity hinders evolutionary rescue through slower growth but not necessarily slower adaptation

“…However, on longer timescales (such as those required for adaptation to very novel conditions), low recruitment per reproductive bout associated with longer-lived species will slow the time it takes for these populations to escape from low densities (Pimm et al (1988)) where populations are susceptible to genetic or demographic stochasticity. Although not explicitly incorporated into our model, this increased time spent at low density can have long-term genetic consequences by forcing populations through a bottleneck (Whitlock (2000)), possibly slowing rates of phenotypic change (Frankham et al (1999)) and making populations vulnerable to entering an extinction vortex (Gilpin & Soulé (1986), Nordstrom et al (2023)). Although prior rescue literature has predicted longevity would adversely affect rescue through reduced rates of adaptation, it does not appear to address this important demographic consequence of slower paces-of-life.…”

“…Survival of each adult is determined by a Bernoulli draw, where for individual i the probability of survival was s i exp (− αN t −1 ), where N t −1 is the population size in the previous time step (according to the census taken at the end of time step t − 1, immediately preceding selection on survival). The Ricker term causes negative density dependence (Ricker (1954)), which is important for accurately assessing population growth and extinction risk under intraspecific competition (Scholl et al (2022), Nordstrom et al (2023)) and also reduces computational load of simulations. Because the Ricker term is applied to survival (i.e., all individuals are subject to identical strength of density dependence in each time step), and reproduction is density independent, density dependence does not perturb populations from their stable age distribution.…”

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

“…To do this, we used red flour beetle ( Tribolium castaneum ) populations without a history of inbreeding or bottlenecks to model a situation where large, diverse populations experience sudden habitat change, causing them to become both small and fragmented. 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). We exposed these populations to a new, challenging environment, and then implemented three different rates of immigration: 0, 1, or 5 individuals each generation from a large population not adapted to the new environment.…”

“…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.…”

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

“…Data accessibility. All data (simulation output), simulation code, and analysis code are provided in Dryad Digital Repository and at https://github.com/melbourne-lab/evo_rescue_ndd_erosion [54]. Additional model details, analysis methods, and simulation results are provided in the electronic supplementary material [55].…”