Written by: Alex Blomfield
Butterflies are sensitive environmental indicators, making them vulnerable to habitat loss and environmental change. The ecological requirements of lepidoptera are relatively well documented, but in an increasingly fragmented landscape, species’ persistence is influenced not only by the provision of adult and larval resources, but by the spatial configuration of habitat patches.
The pearl-bordered fritillary, Boloria euphrosyne, a specialist butterfly of early successional habitat, has faced significant declines in the UK. The Morecambe Bay Limestones, located in the north-west of England, are of national importance for biodiversity, and have a reputation as the national stronghold for this species. Nevertheless, populations in this region are in decline and several colonies have been lost over the last decade.
The pearl-bordered fritillary (Boloria euphrosyne)
My PhD research aimed to assess population persistence and dispersal in the pearl-bordered fritillary, both by looking at abundance trends in English populations and by focusing on the Morecambe Bay region. Site abundance indices from the United Kingdom Butterfly Monitoring Scheme (UKBMS) were used to investigate population abundance and extinction risk at sites across England, with the influence of climatic, habitat and spatial site attributes assessed. Several factors were shown to be important regulators of abundance and extinction risk, including site connectivity, climate and nitrogen deposition. Notably, the results suggested that the impacts of nitrogen deposition and elevated rainfall may be dependent on habitat structure, with woodland populations likely to be most negatively impacted. Negative impacts from nitrogen deposition through mechanisms including microclimate cooling are plausible in this species and require further investigation.
Population size, structure and mobility was also characterised at a core site network in the Morecambe Bay region by mark-release-recapture. A mark-release-recapture involves marking butterflies with a unique ID code, before releasing them unharmed. The subsequent recapture of marked individuals enables a picture of population size and mobility to be built up and can be helpful in identifying areas of suitable breeding habitat or barriers to movement.
The marking code used in the mark-release-recapture study (left) and a marked butterfly marked as number 8 (right)
In addition, morphological data was obtained, using non-lethal methods, and tested as a proxy for mobility. Morphological comparisons between historical specimens and butterflies from current populations in the Morecambe Bay region were used to assess changes in dispersal capacity over time, associated with increased isolation of habitat patches. Morphological comparisons highlighted changes including declines in thorax size and wing loading between historical and current populations. Morphological changes may suggest that current populations have reduced capacity for powered flight and are instead well suited for energy-efficient gliding flight.
Overall, the research has shown that while population declines in the pearl-bordered fritillary are likely multifactorial, site isolation has important consequences for population persistence and dispersal, including potential effects on extinction risk and flight capacity.
Mating pearl-bordered fritillary butterflies