Within-year and among-year variation in impacts of targeted conservation management on juvenile survival in a threatened population

1. 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. 2. We applied multi-state mark-recapture
models to eight 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 eight-year
intervention on overall population-wide survival probability and resulting
population size. 3. 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. 4. Population models projected that
these impacts on annual juvenile survival substantially reduced population
decline, such that population size at the end of the eight-year
intervention was approximately double that without management. 5.
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 positive effects of a major
intervention, but also highlight potential routes to improve efficacy, for
example through more precise targeting of agricultural management actions
in the context of among-year variation in environmental conditions.