Curry J. Cunningham

Evolutionary rescue in a changing world.

Evolutionary rescue occurs when adaptive evolutionary change restores positive growth to declining populations and prevents extinction. Here we outline the diagnostic features of evolutionary rescue and distinguish this phenomenon from demographic and genetic rescue. We then synthesize the rapidly accumulating theoretical and experimental studies of evolutionary rescue, highlighting the demographic, genetic, and extrinsic factors that affect the probability of rescue. By doing so, we clarify the factors to target through management and conservation. Additionally, we identify several putative cases of evolutionary rescue in nature, but conclude that compelling evidence remains elusive. We conclude with a horizon scan of where the field might develop, highlighting areas of potential application, and suggest areas where experimental evaluation will help to evaluate theoretical predictions.

Size Selectivity of Predation by Brown Bears Depends on the Density of Their Sockeye Salmon Prey

Can variation in prey density drive changes in the intensity or direction of selective predation in natural systems? Despite ample evidence of density-dependent selection, the influence of prey density on predatory selection patterns has seldom been investigated empirically. We used 20 years of field data on brown bears (Ursus arctos) foraging on sockeye salmon (Oncorhynchus nerka) in Alaska, to test the hypothesis that salmon density affects the strength of size-selective predation. Measurements from 41,240 individual salmon were used to calculate variance-standardized selection differentials describing the direction and magnitude of selection. Across the time series, the intensity of predatory selection was inversely correlated with salmon density; greater selection for smaller salmon occurred at low salmon densities as bears’ tendency to kill larger-than-average salmon was magnified. This novel connection between density dependence and selective predation runs contrary to some aspects of optimal foraging theory and differs from many observations of density-dependent selection because (1) the direction of selection remains constant while its magnitude changes as a function of density and (2) stronger selection is observed at low abundance. These findings indicate that sockeye salmon may be subject to fishery-induced size selection from both direct mechanisms and latent effects of altered predatory selection patterns on the spawning grounds, resulting from reduced salmon abundance.

Selecting for the phenotypic optimum: size‐related trade‐offs between mortality risk and reproductive output in female sockeye salmon

Summary 1. Selection drives evolutionary changes but is often difficult to quantify and tightly link to phenotypic trait distributions in wild populations. 2. Sampling a single population of sockeye salmon (Oncorhynchus nerka) for over a decade, we calculated the expected spawning success for females resulting from: (i) increased fecundity with body length, (ii) the mortality cost of prolonged marine residency necessary to achieve large size, (iii) size-selective natural mortality on the spawning grounds from biotic (bear and gull predation) and abiotic (stranding) processes and (iv) exploitation by a size-selective commercial fishery. We quantified the size-specific probability of different modes of death and the resultant potential for successful spawning, and then modelled the theoretical relationship between female length and fitness (spawning success) in the population. 3. This optimal distribution closely matched the observed length distribution of the focal population, when removals by the size-selective commercial fishery were included. We then used a likelihood-based approach to compare competing model predictions to length distributions from other populations in the watershed with different levels of size-selective freshwater mortality, as determined by the physical characteristics of the spawning grounds. 4. This study provides a quantitative framework for assessing female spawning success in wild populations, as represented by the expected number of eggs deposited per spawning female. These results advance previous analyses of natural selection in that predictions for phenotypic distributions were generated and then compared to those observed in situ, rather than assuming the adaptive nature of observed distributions.

Alaskan brown bears (Ursus arctos) aggregate and display fidelity to foraging neighborhoods while preying on Pacific salmon along small streams

Abstract The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer‐mediated nutrient transport. Yet, much of the site‐specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio‐telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012–2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed‐capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6‐week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.