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 isolated Fragments or changes in species abundances among habitats that differ in area, structure, or edge characteristics. We measured the species richness 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 - ranging from unfragmented (169-m(2)) to highly fragmented (1-m(2)) patches - were replicated four times in a Latin square design, and insects were sampled twice during 1995 using 177 pitfall traps. Species richness showed a non-monotonic response to fragmentation, with the fewest species occurring in the slightly fragmented treatment. Responses of rove beetles and ants, the most species-rich and abundant taxa, respectively, were similar to the overall insect community but ants had a stronger and more consistent treatment effect in both sample months. Ordinations of ant and rove-beetle assemblages using nonmetric multidimensional scaling showed that the slightly fragmented treatment differed from other treatments in species occurrence and abundance. The lower species richness in the slightly fragmented treatment was primarily 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 species response to fragmentation was due to the differential effects of habitat edge on species movements across the habitat boundary between unmowed patches and mowed areas. A greater effect due to the amount of habitat edge rather than total patch area, at least among the range of patch sizes studied, suggests that the length of habitat edge may be quite important to the distribution and abundance of ground-dwelling animals in fragmented habitats.