Small-scale habitat preferences
I found that host plant and lichen cover best explained the occurrence of Apollo larvae. Since Apollo larvae are monophagous on their host plant (Sedum album) and the probability of the caterpillars finding a host plant likely depends on the density of the host plant surrounding the egg-laying place, the importance of higher host plant densities in the habitat is clear and explains higher larval presence in plots with more host plant cover.
The importance of lichens for Apollo larvae has never been demonstrated, but lichens have been shown to be important for several other lepidopterans (especially moths) as a source of food, shelter and good places to sunbath and could have similar uses for Apollo larvae on Gotland.
Based on the host plant cover and the cover of lichens the area with the best quality for larvae at a small-scale is the well-established habitat, which has the highest overall cover of host plants and lichens. This is followed by the general habitat and targeted restoration, and lastly the non-targeted restoration, where further improvements are needed.
Large-scale habitat preferences
I found that open land and nectar abundance best explained maximum numbers of imago. The availability of host plants and nectar resources is closely tied to open land cover. The habitat of the Apollo butterfly is consequently restricted to open areas. Forests have also shown to act as a barrier for the Apollo and restrict the species’ dispersal.
Based on open land cover and nectar availability the area with the highest quality for the Apollo is the well-established habitat, followed by the general habitat and targeted restoration. In contrast, the non-targeted restoration area had the lowest nectar plant abundance, possibly due to its lower ground moisture content, and needs further improvement to ensure good habitat quality for the Apollo.
This study also shows that the Apollo feeds on a narrow range of nectar plants, as the number of observed forage plants was limited.The results of the study further show that while there are some nectar plants that the Apollo feeds on regularly at the study sites, the heterogeneity of plant species may play a crucial role in feeding preferences, since they appear to shift over time. The heterogeneity of nectar plants in the habitat may also be an important factor in the foraging behaviour of the Apollo, as males and females may have different preferences for flower types.
Assessment of restored areas
Comparing restored areas with reference areas, the targeted restoration successfully created suitable habitat for the Apollo butterfly, evident in increased utilization by both larvae and adult butterflies. Imago population exhibited lower declines and higher increases, indicating potential for continued improvement.
In contrast, the non-targeted restoration did not meet habitat requirements, with no larvae present and low imago densities that did not improve. These findings emphasize the importance of understanding species’ habitat preferences and implementing targeted restoration measures.