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 analyzed the 40-year trajectory 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. Habitat closure was quantified via the Normalized Difference Vegetation Index (NDVI), 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 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.