# Habitat Loss and Other Threats to the Survival of _Parnasius apollo_ ([PERSON], 1758) in Serbia
## 1 Introduction
### Host Plant: Heavy Metals, Habitat, and Climatic Conditions  
_Sedum album_ serves as the primary larval host plant for many central and southern European populations of _Parnassius apollo_ (Lepidoptera: Papilionidae), a threatened butterfly species undergoing a steep population decline. Experimental studies on the _S. album-P. apollo_ interaction have revealed that elevated concentrations of secondary metabolites in the host plant can increase larval detoxification costs and impair nutrient assimilation, ultimately resulting in higher larval mortality [18; 19]. _Sedum album_ L. is a perennial succulent in the Crassulaceae family, distributed widely across temperate Europe--from North Africa to Scandinavia, and from the Iberian Peninsula to the Caucas [20]. As a chamaephyte, it is one of the most frequently encountered _Sedum_ species on the Balkan Peninsula, thriving in dry grasslands, rocky outcrops, and forest edges, particularly on limestone and serpentine soils [21]. Several congeners of _S. album_ have been documented as efficient accumulators of heavy metals. These include _S. plumbizincicola_ a Cd and Zn hyperaccumulator; [22], _S. alfredii_ (Cd hyperaccumulator; [23]), _S. spectablie_ (Cd accumulator; [24]), and _S. sediforme_ Cu-tolerant; [25]. However, the metal accumulation potential of _S. album_ under natural field conditions remains largely unexplored. Globally, anthropogenic activities such as mining have elevated heavy metal concentrations in ecosystems, translocating metals from deep lithospheric sources into previously uncontaminated biomes [6]. Acidic precipitation enhances this impact by solubilizing metal ions and increasing their bioavailability [6; 26; 27]. Mining operations in proximity to Mount Stol and Mount Kopaonik have been associated with increased atmospheric deposition of metal-laden dust--an insidious yet impactful mode of metal pollution. Notably, the recent recolonization and range expansion of _P. apollo_ in certain areas of Western Europe have been correlated with a regional decline in airborne metal fallout [6]. Metal content analyses of larval and host plant samples from recolonized islands in the Finnish archipelago (Kemio-Hankoniemi) suggest that the decline in heavy metal deposition in recent years may have enabled partial recovery, while elevated metal loads in the past likely contributed to historical population collapses _P. apollo_ populations [6]. A broad array of laboratory and field studies has documented the deleterious effects of toxic metals on arthropods, including elevated mortality and reduced reproductive success across numerous taxa [6; 28; 29; 30; 31; 32; 33; 34; 35; 36]. The highest sensitivity is generally observed in detritivores, fungivores, and parasitoids, whereas herbivores--including butterfly larvae--often exhibit comparatively lower direct mortality [37; 38; 39; 40; 41]. This relative tolerance may arise from indirect ecological effects: host plants compromised by pollution may become more susceptible to herbivory, while declines in parasitoid populations reduce mortality pressures on larval insects [6; 29; 37; 38; 39; 40]. Despite multiple conservation efforts, _P. apollo_ remains endangered. A notable example is the reintroduction program in Poland's Pieniny National Park, which began in the late 20 th century with a founding population of only 20-30 individuals [51]. Although initial results were encouraging, the program encountered unforeseen developmental anomalies in released butterfly individuals, including wing malformations and abnormal behavior. Subsequent analyses detected the presence of pathogenic bacteria, including _Yersinia pseudotuberculosis_ ([PERSON] and _Serratia_ spp., in both normal and malformed individuals--potentially compromising long-term population viability [51]. In addition to biotic stressors, climatic conditions critically influence habitat suitability for _P. apollo_.