Abstract
Abstract Overall impacts of targeted conservation interventions on population growth rate (λ) will depend on within‐year and among‐year variation in exposure of target individuals to interventions, and in intervention efficacy in increasing vital rates of exposed individuals. Juvenile survival is one key vital rate that commonly varies substantially within and among years, and consequently drives variation in λ. However, within‐year, among‐year and overall impacts of targeted interventions on population‐wide survival probabilities of potentially mobile juveniles are rarely quantified, precluding full evaluation and evidence‐based refinement of interventions. We applied multi‐state mark–recapture models to 8 years of ring–resighting data from a threatened red‐billed chough Pyrrhocorax pyrrhocorax population to quantify within‐year and among‐year variation in juvenile exposure to a targeted intervention of supplementary feeding and parasite treatment, and to estimate efficacy in increasing juvenile survival probability. We then combined and up‐scaled these estimated effects to evaluate the impact of the 8‐year intervention on overall population‐wide survival probability and resulting population size. High proportions of surviving juveniles (>70%) were exposed to the intervention across the annual biological cycle in all years. Exposure was associated with higher short‐term survival probabilities through the full annual cycle. Consequently, management increased estimated population‐wide annual juvenile survival by approximately 0.14. However, such effects were only evident in cohorts with low overall annual survival. Population models projected that these impacts on annual juvenile survival substantially reduced population decline, such that population size at the end of the 8‐year intervention was approximately double that without management. Synthesis and applications. Our results show how complex patterns of within‐year and among‐year variation in exposure and efficacy of targeted conservation interventions can arise and scale up to affect population‐level outcomes. We demonstrate overall positive effects of a joint supplementary feeding and parasite treatment intervention on the focal chough population, but also highlight potential routes to improve efficacy, for example, through more precise targeting of interventions and agricultural management actions in the context of among‐year variation in environmental conditions.
Highlights
Effective conservation of threatened populations requires identification and mitigation of key demographic constraints that limit population growth rate (λ), and requires thorough evaluation of management intervention efficacy (Hammers et al, 2015; Plard et al, 2020; Sarno et al, 1999; Sibly & Hone, 2002)
Population models projected that these impacts on annual juvenile survival substantially reduced population decline, such that population size at the end of the 8-year intervention was approximately double that without management
Despite increasing appreciation that vital rates commonly vary within years, and that constraints on λ can be temporally restricted (Flockhart et al, 2015; Guimarães et al, 2020; Rushing et al, 2017; Sergio et al, 2019), individual- and population-level responses to anthropogenic change, including management impacts, are typically evaluated on overall annual timeframes (Marra et al, 2015)
Summary
Effective conservation of threatened populations requires identification and mitigation of key demographic constraints that limit population growth rate (λ), and requires thorough evaluation of management intervention efficacy (Hammers et al, 2015; Plard et al, 2020; Sarno et al, 1999; Sibly & Hone, 2002). To inform decisions to continue or refine this intervention, it is essential to quantify efficacy, and its variation within and among years, and evaluate overall population-level effects Such analyses must account for individuals' movements between managed and unmanaged areas within Islay and resulting temporal variation in management exposure, and account for imperfect detection of individuals across time. We applied multi-state mark–recapture models to 8 years of intensive year-round resighting data from colour-ringed juvenile choughs to quantify variable juvenile exposure to management within and among years, and to estimate management efficacy in increasing within-year and annual juvenile survival We combined these estimates within matrix population projections to estimate overall impacts of the 8-year intervention on population size. We thereby provide the full, quantitative evaluation required to refine ongoing management
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