Evaluating the efficacy of independent versus simultaneous management strategies to address ecological and genetic threats to population viability

Abstract

Small, declining populations can face simultaneous, interacting, ecological and genetic threats to viability. Conservation management strategies designed to tackle such threats independently may then prove ineffective. Population viability analyses that evaluate the efficacy of management strategies implemented independently versus simultaneously are then essential to the design of effective management plans, yet such quantitative evaluations are typically lacking. We used stochastic individual‐based models, parameterised with high‐quality multi‐year demographic and genetic data, to evaluate the efficacy of independent or simultaneous ecological (supplementary feeding) and genetic (translocations to alleviate inbreeding) management strategies for a red‐billed chough (Pyrrhocorax pyrrhocorax) population of major conservation concern. This population is experiencing ecological threats from food limitation and genetic threats from escalating inbreeding. Conservation managers therefore face a dilemma: supplementary feeding may be ineffective if inbreeding is limiting stochastic population growth rate (λs), while translocations may be ineffective if food is limiting. Model simulations suggested that the focal population will decline to extinction relatively rapidly with no conservation management (mean λs ≈ 0.86) and with genetic management alone (λs ≈ 0.90). Ecological management alone reduced, but did not halt the population decline (λs ≈ 0.93). However, simultaneous genetic and ecological management yielded population stability (λs ≈ 1), with genetic rescue lasting ~25 years. These outcomes arose because the capacity for translocations to alleviate inbreeding depression is limited by food availability, while supplementary feeding cannot achieve population viability in the presence of accumulating inbreeding. However, supplementary feeding improved environmental quality enough to allow expression of variance in fitness and thus inbreeding depression, meaning that reductions in inbreeding following translocations can increase λs. Synthesis and applications. Our analyses suggest that simultaneous management of ecological and genetic threats will be critical to ensuring viability of Scotland's chough population; neither strategy independently is likely to achieve population persistence and may consequently waste conservation resources. Managers of other resource‐limited, inbred populations should consider that the efficacy of strategies designed to alleviate ecological and genetic threats may be interdependent, such that holistic management is essential to ensure population viability.

Collateral benefits of targeted supplementary feeding on demography and growth rate of a threatened population

“…Finding long-term strategies to improve habitat quality on Islay, particularly of sand dune systems known to be important for both sub-adult and adult choughs, while also addressing known genetic threats (Trask et al, 2019), is imperative to ensure long-term population viability.…”

“…Therefore, our results, and the observation that population size has been approximately stable since 2014, imply that supplementary feeding has been an effective intervention to prevent a rapid population decline. Stochastic population viability analyses, which also considered genetic effects, consequently estimated substantial positive effects on population viability (Trask et al., 2019).…”

“…Improvements would ideally be achieved through agri‐environmental schemes, including livestock and grassland management initiatives that promote spatial and temporal diversity, abundance and availability of invertebrates. Finding long‐term strategies to improve habitat quality on Islay, particularly of sand dune systems known to be important for both sub‐adult and adult choughs, while also addressing known genetic threats (Trask et al., 2019), is imperative to ensure long‐term population viability.…”

“…Yet, since λ is not equally sensitive to all vital rates, the degree to which demographic responses to interventions affect λ should be explicitly evaluated through population models (e.g. Oro et al., 2008; Trask et al., 2019).…”

The ghost of connections past: A role for mainland vicariance in the isolation of an insular population of the red‐billed chough (Aves: Corvidae)

“…Generation time of a population, defined as the average age of mothers of newborn individuals in a population with a stable age distribution, is T = α + s/(λ -s), where α is the age in years at first breeding, λ is the annual geometric growth rate of the population and s is the annual probability of survival(Lande et al 2002). Using long-term field data from Iberia and La Palma (GB, unpublished data) and demographic data from a well-studied chough population(Trask et al, 2019), we assumed the population to be in demographic equilibrium (λ = 1), an average age at first breeding of 3 years, and an approximate value of s of 0.75, to estimate a generation time in choughs of approximately 6 years.…”