Habitat specificity, restricted range and metapopulation persistence of the slender scotch burnet moth Zygaena loti in western Scotland

1. Many species have highly localized distributions in the British Isles and have persisted for considerable periods of time in isolation from their main ranges on the European continent. Many of these have survived in warm man-made refugia in southern England. 2. The slender scotch burnet moth Z. loti is one of many insects that are confined in the British Isles to small areas of western Scotland. Its ecology was studied to identify its conservation requirements and factors explaining its persistence and restricted distribution. 3. Most populations were small and the majority occurred in herb-rich grassland on vegetated talus slopes below south-facing Tertiary basalt cliffs. Mobility was limited, but nevertheless the colonies form a metapopulation. Female oviposition and larval behaviour were concentrated around patches of bare soil and small foodplants, amongst short, dark vegetation suggesting that a specialized microclimate is important. The requirements of Z. loti are similar to butterflies at the north-west limits of their European ranges in southern England. 4. The vegetation of the thin soils was early successional and exhibited signs of frequent disturbance. Soils were more developed, and densities of moths were lower, on colonies that were more heavily vegetated. Suitable areas are created by rockfalls or slope slippage and maintained by small-scale disturbance or by grazing. The thin soils and high grazing levels suggest that vegetation colonization is slow, so although site occupancy is limited ultimately by vegetation succession, suitable conditions may persist for long periods if local disturbance continues. 5. Colonies have persisted in isolation for so long and on such small sites because of the low turnover of sites caused by slow succession, the instability of the basalt slopes and regular grazing. A recent reduction in grazing pressure has led to a contraction in the range of the species to the most unstable sites. 6. The enhanced microclimate associated with habitat aspect, local shelter and short vegetation, as well as soils influenced by basalt, help explain the restriction of Z. loti and related species to this part of the western seaboard of Scotland. But the unusually oceanic climate of this region probably explains the presence of these and many other species that are separated from their main ranges elsewhere in Britain and Europe.