Birds and butterflies along an urban gradient: Surrogate taxa for assessing biodiversity?

This study examines whether birds and butterflies may be used as surrogates for one another in assessing biodiversity at the community level. To do this, I compared the distribution and abundance of bird and butterfly species across an urban gradient by surveying six sites near Palo Alto, California, USA (all former oak woodlands) to see if these taxa have responded similarly to urbanization. The sites represent a gradient of urban land use ranging from relatively undisturbed to highly developed and include nature preserves, recreational areas, golf courses, residential neighborhoods, office parks, and business districts. At the community level, the two taxa displayed similar patterns across the gradient: species richness and Shannon diversity peak at intermediate levels of development, and the oak-woodland species gradually drop out at more developed sites. These measures are highly correlated between the two groups. The two taxa differed in their patterns of total abundance, however. Butterfly abundance was highest at the preserve and decreased as the sites became more urbanized, while bird abundance peaked at a site of intermediate development. These results suggest that, on spatial scales from 1 to 10 km, the two taxa display similar patterns with regard to urbanization and that one group can be used to infer the response of the other in assessing biodiversity with these measures at the community level.