Multi‐year occupancy and habitat use in a threatened butterfly reintroduced onto a restored peatland
Andrew W. Osborne, Stuart Marsden, Sarah M. Griffiths, Simon J. M. Caporn, Emma CoulthardAbstract
Introduction
Over half of insect species reintroductions are unsuccessful, frequently because of insufficient habitat resources and a knowledge‐gap regarding the target species habitat requirements. Multi‐season occupancy models, seldom used with invertebrates, offer the opportunity to disentangle the effects of habitat suitability and confounding factors such as weather and flight period on the success of reintroduction schemes.
Objectives
The aim was to model initial occupancy, colonization, and extinction patterns, and habitat use through the first 6 years of a reintroduction program for the threatened Large heath butterfly ( Coenonympha tullia ) onto a newly restored lowland bog habitat.
Methods
Butterflies were repeatedly surveyed each year within three hundred and eighty 9‐m 2 compartments (“sites”). Occupancy was related to local habitat measurements such as the cover of Erica tetralix and Eriophorum vaginatum , and covariates affecting detectability such as temperature and flight period metrics.
Results
Occupancy declined steadily during the 5 years post‐release, although C. tullia is still present. Detection probability was positively correlated with higher air temperature. Sites initially occupied or colonized by the butterfly tended to have high percentages of E. tetralix or E. vaginatum , while sites where the species disappeared had low habitat resource provision.
Conclusions
Our analysis indicates that the reintroduced population is failing to thrive, with consistently reducing occupancy between 2021 and 2025, probably not helped by a series of extreme weather events during the winter, spring and early summer of 2024. Most areas of the site are inhospitable in terms of habitat, and increasing the provision of habitat resource plant species is a priority.