Experimental effects of habitat fragmentation on rove beetles and ants: patch area or edge?

The effects of habitat fragmentation may include the loss of species from i solated Fragments or changes in species abundances among habitats that diff er in area, structure, or edge characteristics. We measured the species ric hness and abundance of ground-dwelling insects in a 1.14-ha old field that was mowed to produce patches of unmowed vegetation which differed in size, degree of isolation, and the amount of habitat edge. Four treatments - rang ing from unfragmented (169-m(2)) to highly fragmented (1-m(2)) patches - we re replicated four times in a Latin square design, and insects were sampled twice during 1995 using 177 pitfall traps. Species richness showed a non-m onotonic response to fragmentation, with the fewest species occurring in th e slightly fragmented treatment. Responses of rove beetles and ants, the mo st species-rich and abundant taxa, respectively, were similar to the overal l insect community but ants had a stronger and more consistent treatment ef fect in both sample months. Ordinations of ant and rove-beetle assemblages using nonmetric multidimensional scaling showed that the slightly fragmente d treatment differed from other treatments in species occurrence and abunda nce. The lower species richness in the slightly fragmented treatment was pr imarily due to a subset of ant and rove beetle species that showed a lower abundance than in other treatments, possibly because this treatment had the greatest amount of habitat edge. We hypothesize that the non-monotonic spe cies response to fragmentation was due to the differential effects of habit at edge on species movements across the habitat boundary between unmowed pa tches and mowed areas. A greater effect due to the amount of habitat edge r ather than total patch area, at least among the range of patch sizes studie d, suggests that the length of habitat edge may be quite important to the d istribution and abundance of ground-dwelling animals in fragmented habitats .