Assisted colonisation prospects for the black-veined white butterfly in England

The black-veined white butterfly, Aporia crataegi, reached the north-western edge of its European geographic distribution in the British Isles in the 19th century, but became extinct in the early 20th century, following several cold decades. Substantial areas of potential breeding habitat in southern Britain are currently available to this species, which requires scattered hawthorn (Crataegus monogyna) and suckering blackthorn scrub (Prunus spinosa), including infrequently cut hedgerows. These habitats are needed at a relatively large scale as the butterfly occurs as networks of colonies (metapopulations), ranging over large tracts of connected landscape. A number of events have increased habitat availability over the past 70 years (myxomatosis reduced rabbit populations, which permitted host plant scrub regeneration; hedgerow management policies reduced cutting frequencies; rewilding and landscape connectivity initiatives are resulting in additional scrub). However, while A. crataegi males occasionally disperse several kilometres, it is unlikely that A. crataegi females will cross the English Channel in sufficient numbers to establish populations in southern England, without assistance. Here, we (i) provide a review of the literature on species interactions, the habitat requirements, distribution and dispersal of A. crataegi and (ii) provide evidence that southern, eastern and central England are likely to be climatically suitable for reintroductions of A. crataegi. Substantial areas of England are already expected to be amongst the climatically most suitable parts of Europe for this insect. We identify a landscape of at least 100 km2 containing multiple patches of suitable habitats, and highlight co-benefits for other species that inhabit scrubland and successional mosaics. We use a climate-matching approach to assess climatically-similar locations to obtain source material most likely to establish in Britain. Areas of northern France and mid elevations in the Iberian Peninsula, including in the Pyrenees, provide potential suitable source locations due to close climatic matching and a large number of species records. We recommend reestablishment from more than one source, providing genetic diversity in the reintroduced population, enabling subsequent local adaptation to British conditions. We highlight the opportunity for monitored releases to be undertaken within the landscape highlighted here, so as to evaluate population growth, host plant use, and rates of colonisation away from release sites that differ in their management, habitat, host plant characteristics, and proximity to other sites. This approach would aim to develop knowledge as a living lab to inform future best practice releases. In conclusion, the black-veined white, A. crataegi, has potential to become a model species for assisted colonisation projects where natural and human-created barriers have prevented range expansion into regions where the 21st century climate is suitable for a species.