# Habitat Loss and Other Threats to the Survival of _Parnasius apollo_ ([PERSON], 1758) in Serbia
## 1 Introduction
### Ecological Pressures: Habitat Loss, Succession, and Land-Use Change  
_P. apollo_ is among the most visually distinctive and ecologically significant species of diurnal _Lepidoptera_ in Europe. A stenophagous relict of glacial fauna, it is strictly associated with subalpine and montane ecosystems where host plants of the genus _Sedum_--particularly _S. album_ and _S. teleplium_--persist in sun-exposed, sparsely vegetated habitats crucial for larval development [55; 56]. Yet across its range, including in Serbia, _P. apollo_ has experienced precipitous declines over the past several decades. The principal drivers of this decline are habitat loss and fragmentation, largely resulting from urban expansion, infrastructure development, intensified agriculture, and the widespread abandonment of traditional pastoral systems. These pressures are compounded by climate change, the spread of invasive vegetation, and direct anthropogenic threats such as overcollection. Notably, the cessation of seasonal grazing--a cornerstone of traditional land use across much of _P. apollo_'s range--has emerged as a critical but underappreciated cause of habitat degradation. For centuries, extensive livestock grazing shaped open, thermophilic microhabitats, maintaining the low vegetation cover required for _Sedum_ proliferation and the butterfly's reproductive success. Following grazing abandonment, several ecologically detrimental changes occur [52; 54; 55; 56]:  
* Vegetation succession leads to the encroachment of tall grasses, shrubs, and invasive species, which block sunlight at ground level and displace sun-loving succulents like _S. album_. * Loss of habitat heterogeneity erodes the mosaic of rocky surfaces, bare patches, and low grasses, disrupting the availability of microhabitats required for larval and adult stages. * Decline in spatial connectivity restricts dispersal and gene flow, isolating populations and increasing vulnerability to local extinction. These transitions typically unfold within one to two decades and, while the resulting vegetation may appear verdant, such environments become functionally depauperate and unsuitable for stenophagous taxa such as _P. apollo_. Beyond its aesthetic appeal, _P. apollo_ functions as an ecological sentinel of high-altitude grasslands. The species displays a trophic dichotomy: \"albophagous\" populations depend on _Sedum album_, while \"telehiophagous\" populations utilize _S. telebihum_. This divergence is not only trophic but also historical and latitudinal, reflecting evolutionary dynamics driven by glacial-interglacial cycles [18; 57]. With more than 200 described subspecies radiating from a Neogene origin [18], _P. apollo_ populations have nevertheless exhibited steady declines since the 19 th century--driven by a convergence of climatic shifts, anthropogenic habitat transformation, and intra-populational vulnerabilities such as genetic erosion and maladaptive behaviors [57]. While host plant availability has traditionally been viewed as a key determinant of larval presence, recent long-term studies in Finland challenge this assumption. In the southern Finnish archipelago, _Hyloteleplium telebitium_ remained abundant, yet larval presence declined dramatically--from 75% to 20% of surveyed islands over two decades [55]. The decline was not due to host plant scarcity but rather to habitat succession, the loss of structural heterogeneity, and the abandonment of management practices like rotational grazing and shrub clearance. These processes led to the closure of formerly suitable open habitats. Additional stressors--most notably intensified climate change--further threaten _P. apollo_ metapopulations by inducing extreme weather events, shifting phenological cues, and modifying species interactions [55; 56]. A recent comparative habitat analysis in southwestern Finland revealed a strong positive correlation between butterfly presence and rocky terrain rich in larval host plants and nearby nectar sources, while tree and shrub cover (a proxy for succession) correlated negatively [56].