# Habitat Loss and Other Threats to the Survival of _Parnasius apollo_ ([PERSON], 1758) in Serbia
## 4 Discussion  
The results of this study provide conclusive evidence that the decline and local extinction of _Parnassius apollo_ in Serbia are primarily driven by land-use change, particularly the abandonment of traditional extensive grazing. This shift has initiated ecological succession, resulting in the overgrowth of formerly open, sunlit pastures, loss of larval host plants (_Sedum album_), and fragmentation of critical microhabitats. These findings align with previous European studies that identified cessation of pastoralism as a central factor in butterfly population decline [53; 57]. Vegetation trend analysis from 2015 to 2024 revealed clear structural differences between sites with and without _P. apollo_ populations. Persisting populations are associated with habitats exhibiting stable vegetation composition--low exposed soil coverage, moderate grass density, and consistently high shrub and tree presence. In contrast, extinct populations correspond with unstable vegetation dynamics, dominated by increased grass proliferation and declining shrub density, indicative of degraded successional stages unsuitable for butterfly persistence. While the drivers of decline are consistent across degraded sites, the persistence of _Parnassius apollo_ in Raska and Rastiste suggests the existence of localized refugia--areas where favorable microclimatic conditions, vegetation structure, and land-use legacy have buffered against broader landscape degradation. These sites exhibit consistently high shrub and tree coverage, moderate and stable grass density, and low levels of exposed soil--features that collectively sustain optimal thermal and hydric regimes for larval development and adult activity. Furthermore, MODIS-based productivity metrics indicate that Raska and Rastiste maintain higher gross primary productivity (GPP) under stress conditions, likely due to deeper soils, denser canopies, or reduced exposure. These locations thus serve as functional ecological strongholds, offering critical insight into the conditions required for species persistence amid widespread decline. Recognizing and preserving such refugial habitats is essential for developing targeted conservation strategies, especially under accelerating climate change scenarios that may further amplify habitat fragmentation and resource instability. These vegetation changes directly impair the availability of _Sedum album_, the butterfly's monophagous larval host. Although tolerant of poor substrates, _S. album_ requires high light exposure and is quickly suppressed by taller vegetation in ungrazed environments [19]. Prior research has shown that host plant presence alone is insufficient for larval development; habitat structure and microclimatic conditions are equally critical [18; 57]. Field observations confirm the severe decline or complete absence of _S. album_ at formerly occupied sites, consistent with this understanding. Remote sensing datasets (MODIS and Sentinel) reinforce these conclusions. Declines in Leaf Area Index (LAI) and Fraction of Absorbed Photosynthetically Active Radiation (FPAR) at sites such as Brzece and Bucje indicate canopy thinning and reduced photosynthetic efficiency. Vegetation productivity, as measured by gross primary productivity (GPP), also shows a statistically significant downward trend, consistent with elevated land surface temperatures (LST) [103], increased potential evapotranspiration (PET), and reduced Temperature Condition Index (TCI)--a proxy for thermal stress [75; 84]. Climatic stressors, particularly rising winter temperatures, shortened snow cover duration, and extreme summer heat, further reduce larval survival and disrupt adult emergence [53; 57]. These effects are magnified in fragmented habitats, where dispersal and recolonization are limited [68]. Our data confirm that thermally variable sites with low vegetation productivity exhibit greater vulnerability to climate-induced local extinction.