Effects of habitat patch size and temperature on the distribution and abundance of ground beetles (Coleoptera: Carabidae) in an old field

The effects of habitat size, patch isolation, and temperature on the distribution and abundance of ground beetles (Coleoptera: Carabidae) were studied within an old field in southwestern Ohio. A grass field was subdivided by mowing to create 16 plots, each containing 9 unmowed habitat patches that differed in size among 4 treatments (1 m(2), 4 m(2), 9 m(2), and coni iguous). A total of 177 pitfall traps was placed in unmowed patches and in mowed areas, and 444 beetles were captured from midsummer to fall of 1995. More than 75% of the Harpalus pensylvanicus (DeGeer) captures were in 1-m(2) patches during midsummer, but 92 and 87% were in the mowed area during late summer and fall, respectively. Peak captures of Calathus opaculus LeConte changed from 72% in patches >4 m(2) during late summer to 81% in 1-m(2) patches and mowed areas during fall. Pterostichus atratus (Newman) was abundant in late summer when 76% of the captures were in patches >9 m(2) patches. Thus, large vegetation patches (greater than or equal to 4 m(2)) as well as open areas were important to the distribution and abundance of carabid beetles in an uncultivated habitat. Seasonal changes in captures of H. pensylvanicus and C. opaculus among habitats were consistent with their preferred temperatures. Despite habitat and seasonal differences, all 3 species showed the same positive relationship between distribution and abundance across treatments and months. We suggest that this general distribution-abundance pattern can be explained by species responses to the thermal environment as it varies among habitats and season.