Implementing an emergency headstarting program for a critically endangered day gecko using demographic insights

Conservation interventions are often implemented urgently, with limited empirical feedback to inform their design. Among these, headstarting programs (captive-rearing of young) may be promising for populations suffering from low recruitment and when effective in-situ actions are not yet identified, but may fail without robust demographic knowledge. Knowledge gaps hinder the implementation of effective conservation interventions for many threatened species, including insular reptiles like geckos, whose demography remains poorly understood. Here, we investigated the demography of the Critically Endangered Manapany day gecko ( Phelsuma inexpectata ) to identify drivers of population decline and guide the implementation of an emergency headstarting program, applied for the first time to a gecko species. Using five years of capture-mark-recapture data (2015–2019) from two populations, we estimated demographic parameters and conducted population viability analyses (PVA), including headstarting scenarios with different rearing durations and delays. Despite stable survival of older individuals, abundance halved after 2016 due to near-zero young survival, causing a 90% decline in recruitment. Low annual adult female survival (0.59) required high adult turnover to sustain population size, increasing vulnerability to stochastic events. We observed male-biased recruitment, which, combined with lower female survival, led to a male-skewed adult sex-ratio (3:1 by 2019). Consequently, predicted extinction probabilities exceeded 38% over 20 years. PVA revealed that immediate and continuous headstarting could reduce extinction risk below 2% over 20 years. Our comprehensive demographic study identifies low young survival as the key driver of population decline, supporting targeted headstarting as an emergency measure for a Critically Endangered gecko. • Low young survival drove sharp recruitment decline, pushing populations to extinction • Female survival remained low, causing highly dynamic population turnover • Male-biased recruitment and survival caused unstable sex structure of populations • Headstarting may rapidly increase adult recruitment in lizard populations • Headstarting may prevent imminent extinction when in-situ solutions are lacking