The diversity and abundance of North American butterflies vary with habitat disturbance and geography

Aim We used data from the annual Fourth of July Butterfly Count for the yea rs 1989-97 to examine patterns of species richness and total butterfly abun dance across North America and within topographically diverse and disturbed landscapes. Location We analysed counts from 514 different locations in North America. The counts represent all areas of the USA and southern Canada, with a few M exican sites as well, although most counts were in the eastern USA. Methods First, we standardized published count data according to the effort expended per count (total party-hours). Using regression analysis and anal ysis of variance, we then examined the impact of latitude, longitude, topog raphical relief, habitat disturbance and different climatic measures on the species richness and total abundance of butterflies per count. We also exa mined the abundance of exotic species in disturbed landscapes. Results Our analyses suggest that: (1) species richness is highest at low l atitudes and near Rocky Mountain longitudes; (2) the total abundance of ind ividuals is highest in northern US latitudes and Great Plains longitudes; ( 3) species richness but not total abundance increases with greater topograp hical relief; (4) species richness and diversity indices are lower in more disturbed habitats; and (5) the abundance of the introduced Pieris rapae (L .) is greater in more disturbed habitats. Main conclusions Different factors control the abundance and species richne ss of North American butterflies. Along with geographical location, habitat disturbance and topographical variability affect species richness. Our ana lysis also shows the value of broad-based monitoring regimes, such as the N orth American Fourth of July Butterfly Count.