From success to uncertainty: the endangered Apollo butterfly, Parnassius apollo, in Štramberk four decades after reintroduction

Reintroductions are vital for conserving endangered butterflies, yet their long-term sustainability is rarely evaluated over multiple decades. We analysed the 40-year dataset of the Apollo butterfly (Parnassius apollo) in Štramberk (Czech Republic), the longest-monitored reintroduced population of this species globally. Using ordinal logistic regression and 30 years of environmental data, we evaluated how habitat succession and climatic variables affect population abundance, which was estimated through standardized field surveys and expert counts. Habitat closure was quantified via the satellite-based Normalized Difference Vegetation Index (NDVI) derived from Landsat imagery, while climatic predictors included temperature, precipitation, and humidity indices. Following its 1986 reintroduction, the population peaked at over 2,000 individuals in the late 1990s but has since declined to critically low levels. This decline is primarily driven by vegetation succession, with NDVI being the strongest predictor of population collapse. Each 0.1 unit increase in NDVI (a change corresponding to a typical decadal successional shift) was associated with an 86% reduction in the odds of the population reaching higher abundance categories. While climatic factors modulate annual performance, they do not explain the downward trend. Our results suggest that habitat degradation is the primary threat, implying that appropriate management remains a powerful tool for ensuring population persistence. Effective conservation of P. apollo in low-elevation habitats requires active management that maintains open, sun-exposed rocky surfaces to ensure high solar radiation and cold, dry winter microhabitats. Targeted removal of woody encroachment on south-facing slopes is essential to maintain vegetation-free patches that allow for winter freezing and desiccation. Such habitats provide the necessary resilience against both successional closure and detrimental climatic variability during the overwintering period.