Habitat resources, remnant vegetation condition and area determine distribution patterns and abundance of butterflies and day-flying moths in a fragmented urban landscape, south-west Western Australia

The creation of cities, towns and farms following European settlement of Australia has fragmented the original vegetation. Many native species that were previously widespread are now found only within isolated remnants of their original habitat. These relictual populations are at increased risk of decline and local extinction, so identifying the factors that determine their persistence is important for ongoing management and conservation. I compared the effects of site area, connectivity, vegetation condition and habitat resources on the presence, abundance and total number of species of butterflies and day-flying moths within 46 urban fragments of remnant vegetation in south-west Western Australia. Site area and vegetation condition were the dominant determinants of species presence: large sites with more undisturbed vegetation favoured 16 of 20 native species and only one (Geitoneura minyas) benefited from disturbance. Another nine species that were not sufficiently widespread or abundant to enable individual analysis were collectively more prevalent in larger sites. Resource quality and quantity dominated the patterns of site occupancy, consistent with habitat resources, not metapopulation effects, determining current distribution patterns. The total number of species at each site reflected the collective responses of the individual species: increasing with area and declining with vegetation disturbance. The effects of area and vegetation condition were not simply additive: disturbance had a far greater impact on small remnants. Restoration or maintenance of high vegetation condition will be essential to maintain regional species diversity and to prevent local extinctions of butterflies and day-flying moths, especially in small remnants.