The quality and isolation of habitat patches both determine where butterflies persist in fragmented landscapes

(H)abitat quality and metapopulation effects are the main hypotheses that c urrently explain the disproportionate decline of insects in cultivated Hola rctic landscapes. The former assumes a degradation in habitat quality for i nsects within surviving ecosystems, the latter that too few, small or isola ted islands of ecosystem remain in landscapes for populations to persist. T hese hypotheses are often treated as alternatives, and this can lead to ser ious conflict in the interpretations of conservationists. We present the fi rst empirical demonstration that habitat quality and site isolation are bot h important determinants of where populations persist in modern landscapes. We described the precise habitat requirements of Melitaea cinxia, Polyomma tus bellargus and Thymelicus acteon, and quantified the variation in carryi ng capacity within each butterfly's niche. We then made detailed surveys to compare the distribution and density of every population of each species w ith the size, distance apart and quality of their specific habitats in all their potential habitat patches in three UK landscapes. In each case, withi n-site variation in habitat quality explained which patches supported a spe cies' population two to three times better than site isolation. Site area a nd occupancy were not correlated in any species. Instead of representing al ternative paradigms, habitat quality and spatial effects operate at differe nt hierarchical levels within the same process: habitat quality is the miss ing third parameter in metapopulation dynamics, contributing more to specie s persistence, on the basis of these results, than site area or isolation. A reorientation in conservation priorities is recommended.