Direct and indirect effects of roe deer (Capreolus capreolus) herbivory on an island population of Chequered blue butterfly (Scolitantides orion).

Sammanfattning: Herbivory by ungulates is a known agent of disturbance in many ecological systems around the globe. At high abundances and through a selective foraging behaviour herbivory may inflict significant direct and indirect effects on local plant and animal communities. Direct effects referto ungulates directly interacting with another species by foraging on plants and incidentally on insect eggs, larvae and/or adults. Indirect effects in this context describes ungulate feeding on plants which indirectly alters the availability and quality of food resources utilized by local insect fauna. Ungulates have even been considered as a potential conservation threat topersistence of some insect species. This aim of this thesis was to study the interaction between a common ungulate, the roe deer (Capreolus capreolus), and the rare butterfly Scolitantides orion by sharing the same plant resource, orpine (Hylotelephium telephium).  S. orion is a butterfly species of high conservation concern in Sweden. It is classified as endangered (EN) and has over the last decades experienced significant decrease in population size, mainly as a result of habitat loss and fragmentation. Roe deer has been considered as a potential threat to the population, but the magnitude of the threat has not been studied in detail. The objective of this thesis was to analyse the direct and indirect consequences of roe deer herbivory on S. orion population in the island of Mörtö, Stockholm archipelago. I predicted that roe deer alters the abundance of suitable host plants for oviposition and consumes egg and/or larvae, producing significant differences between protected and unprotected plants. The study was conducted in May-June 2020 in seven selected sites in Mörtö. At the onset of the butterfly season, 47 mesh cages were established sheltering 10.4% of the 1310 plants included in the study. Five inventories were conducted over a five-week time period, where each plant was examined by measuring the height, number of leaves, plant damage and the number of eggs and larvae. These data were used to examine the direct and indirect effect of roe deer foraging.  The results of this study could not confirm that roe deer has a significant direct or indirect effect on S. orion population in Mörtö. Hence, roe deer may not pose as large of a threat as initially expected, at least in this island at this time period. However, this was not the only result from this study. Plant properties such as leaf number and plant height (although not significant) as well as plant quality influenced the host choice for oviposition, with significantly more eggsbeing found on plants with more leaves and less plant damage. Hence, it seems that roe deer and other herbivores indirectly affects the host choice of S. orion by altering plant attractiveness. The reason for this behaviour needs to be studied in detail, but a proposed reason was that females choose plants with less damage to avoid competition and/or predation. Another finding was that a substantial number of eggs were lost over the season, some of which due to incidental feeding by roe deer and other herbivores. However, many eggs were lost without an identified reason, which likely is a result of disease or predators, but this needs to be studiedfurther. Finally, the cage experiment was successful in keeping the roe deer out but not able to cause a difference in egg survival between protected and unprotected plants. This was likely due to low roe deer herbivory in general.  Even though the effect of roe deer on S. orion population was not as significant as expected, it is still important to consider roe deer as a potential threat to the species at its most vulnerable stages as the species is dependent on the host plant for its survival. Hence, it would be interesting to do a similar experiment in a locality with higher deer density to further investigate the effect of roe deer on S. orion populations.