We examined the effects of isolation and site size on the abundance, density, emigration and immigration of the butterfly Parnassius smintheus in a series of sub-alpine meadows. Site size was measured either as habitat area or as number of host plants within each meadow. This distinction allowed us to test the hypothesis that population density measured over area (a generalized individuals-area relationship) should tend to decrease with increasing area due to inclusion of more ‘non-habitat’ in larger areas. In general, area measured either as meadow area or host plant abundance explained little deviance in butterfly abundance or movement. Immigration increased with the connectivity (the inverse of isolation) of meadows. We found that individuals-area relationships defined over meadow area showed patterns of decreasing butterfly density with increasing meadow area. This pattern was partially generated by a trend for host plant density to decrease with increasing meadow area. However, this trend was not universal as some small meadows had similar host plant densities to large meadows. Decreasing density with increasing meadow area primarily arose due to decreasing butterfly density with increasing host plant abundance, indicating that multiple mechanisms can produce decreasing population density with increasing area.