The effects of island area, isolation and source population size on the presence of the grayling butterfly Hipparchia semele (L.) (Lepidoptera: Satyrinae) on British and Irish offshore islands

Records of Hipparchia semele on British and Irish islands have been modelled against island area, isolation (sea and land distance) and the size of the nearest potential source populations. All three variables have been found to contribute significantly to the presence or absence of H. semele on the islands. Isolation is a more significant predictor than island area. This result differs from the multiple species case where area was found to be a more important influence than isolation. Records on islands are also shown to depend on the size of populations at the nearest sources; this underpins the relationships identified for the multiple species case, first, between the number of species on islands and at nearest sources and, second, between the incidence of species on islands and at nearest sources. There are clear indications that smaller islands may become increasingly marginalized for H.semele; with ongoing habitat loss, because isolation increases and source populations become sparser, the probability of H. semele recolonizing islands also decreases.