Just how important are structural elements as habitat components? Indications from a declining lycaenid butterfly with priority conservation status

Abstract

Recently, it has been suggested that habitats for insect herbivores have been too narrowly defined, often on the basis of larval hostplants; in particular, non-consumable resources (called utilities; structural elements) have been ignored. Here, the importance of utility resources for roosting and mate location has been examined in the silver-studded blue butterfly Plebejus argus (L.) (Lycaenidae) on the Great Ormes Head, North Wales, UK. The methods included using dedicated surveys and correspondence analysis applied to behavioural observations in relation to vegetation structure on a transect through a key patch for this metapopulation model species. A substantial and significant bias in roosting (97%) and mating (75%) is found to occur outside hostplant areas on shrubs and rank bunched grasses and forbs. Population density is higher in shrubby areas and shrubs are increasingly occupied during the afternoons and night, during the late flight season and in cloudy, cool and windy weather. These findings suggest that shrubs are a valuable habitat component for this butterfly, at least at this coastal location, and important for their conservation. As scrub growth is inimical to calcicolous grassland, population status will depend on a fine balance between shrub and hostplant cover dependent on grazing and browsing by the indigenous goat, sheep and rabbit populations as well as on controlled cutting and burning. Opportunities exist for increasing population size and distribution on the headland but this will need to be managed carefully. There are also implications for metapopulation dynamics studies; the status of shrubs neighbouring host plant areas switches from that of barriers to resources and refuges.