| Background and Aim
Many European butterflies are declining due to habitat loss and fragmentation. One example is the marsh fritillary (Euphydryas aurinia) which is classified as vulnerable (VU) in Sweden. The species is still relatively common on Gotland, even though it declined considerably after the drought in 2018. The aim of this project is to assess the habitat requirements (based on larvae autumn nests, see below), population densities (from capture-mark-release), and colonization-extinction dynamics (from yearly patch occupancy) of the marsh fritillary. Understanding the habitat requirements and population dynamics is key to give more accurate management recommendations.
Methods
Isabel will work with a large metapopulation near Slite on northwestern Gotland, where patch occupancy has been followed since 2017 (in 256 habitat patches). During the summer of 2021 she will collect new data on patch occupancy and analyze the colonization-extinction dynamics over time in relation to patch area, quality (e.g. grazing, ground moisture) and connectivity using logistic regression. Moreover, Isabel will conduct capture-mark-release studies in two subsets of the patch network (differing in connectivity) to estimate population densities based on Jolly-Seber models for open populations. In one of these subsets (the more isolated one) the species went completely extinct after the drought and had not yet recovered in 2020. During the autumn Isabel will also conduct a survey of larvae colonies (the larvae spin silk nests around the host plant, Succisa pratensis) and host plants in transects, representing hectare grid cells, throughout the landscape. The number of nests will be analyzed in relation to host plant densities, connectivity, grazing pressure, and ground moisture using generalized linear models. One part of this will also be to evaluate newly restored areas.
Hypothesis:
1. The local extinction probability decreases with increasing patch area and the colonization probability increase with increasing connectivity (in accordance with metapopulation theory)
2. Population densities are higher in the well-connected subset compared to more isolated one
3. The marsh fritillary population has not yet recovered from the 2018 drought
4. The number of larvae autumn nests are higher in areas with high host plant densities and high connectivity, but lower in grazed areas
The grant will be used for field equipment and travel-expenses such as car rental and the ferry to Gotland (she will go two times to Gotland one in May and one in September). Housing in Slite is financed by Calluna AB. |