Ecological drivers for Parnassius apollo conservation on Gotland: behaviour and habitat use

This thesis is led by Hedda Brunbäck Larsen.

Central Research Questions

How does habitat quality affect the behavior and population density of P. apollo? What does the time budgets of larvae and adult P. apollo look like? What factors influence the oviposition site selection of female P. apollo? How does the distribution of host plant and nectar sources relate to its density and distribution?

Background and Aims

The Apollo butterfly is red-listed as near threatened (NT), with a declining population in Europe. On Gotland, one of the remaining strongholds, the species inhabit alvar grasslands as well as areas of limestone coniferous semi-open forests. The loss of habitat and habitat fragmentation increases the risk of extinction, and the increasingly dry summers seem to negatively affect the host plant and nectar sources of the Apollo. The unusually dry spring of 2025 may further stress these populations, potentially affecting larval development and host plant availability. While previous work from Franzén et al. (2024) documented substantial populations across large areas on Gotland that are ideal for comprehensive study, critical gaps remain in our knowledge of how habitat quality influences these populations. My project aims to fill these gaps by providing ecological data through investigating how variations in habitat quality affects P. apollo behaviour, population densities, and egg-laying preferences to identify key factors for effective conservation on Gotland.

Methods

This project will be done on northern Gotland close to Slite, and the fieldwork will start in mid March and end in the end of August 2025. The fieldwork will involve multiple methods across areas of different habitat quality. I will observe individual P. apollo to look into time budget analysis for both adult and larvae, where I will use focal animal sampling to record the time spent on specific behaviours such as feeding, resting, moving, sunbasking and mating. Density estimations will also be done to compare adults and larvae. For this I will walk transects across the habitat for both larvae and adults. This will be done in 9 hectare grids cells in 3 different areas (i.e. 27 in total). The area will be ranked from low-medium-high quality areas for the species.

To perform a habitat assessment, encounters during behavioural analysis will be marked with a flag and its coordinates will be put into Fieldmaps to be able to revisit these areas. We will then look at the abundance and distribution of host plants (Sedum) and nectar sources within these areas, as well as temperature measurements. Coordinates will also be put into Fieldmaps when oviposition sites are observed. On these sites we will look at the distance to and abundance of  host plant, substrate and tree/bush cover within the defined areas (>10m) and temperatures. To assess habitat quality in the transects I will distribute vegetation plots along the transect where host and nectar plant abundance will be measured as well as vegetation height.