Patterns in clearcut edge and fragmentation effect studies in northern hardwood-conifer landscapes: Retrospective power analysis and Minnesota results

The effects of edge and fragmentation on avian nesting success are well documented in agricultural landscapes. However, it is unclear whether these effects are common in predominantly forested landscapes. In particular, edge and fragmentation effects caused by clearcutting are poorly understood. To better understand this problem, we examined the relation of nesting success to clearcut edges in north-central Minnesota. We found elevated predation rates near clearcut edges using artificial and natural ground nests. To aid interpretation of studies conducted in the Northern Hardwood-Conifer Forest Region (NHCF) of North America, we estimated statistical power for 26 analyses (subsets of 11 papers and our own analyses) and, where possible, we reanalyzed the data and estimated effect size (with associated confidence intervals). In addition, we examined design issues such as presence of pseudoreplication. Statistical power was low for many of the studies and pseudoreplication was evident in several. Without considering power or design issues, 13 of the analyses found edge effects, 12 showed no effects, and one yielded greater predation rates in unfragmented versus fragmented areas (α=0.05). When we excluded studies with low statistical power (<0.80) and pseudoreplication from analysis, 10 of the remaining studies reported edge effects, 3 showed no effects, and one showed greater predation rates in unfragmented versus fragmented areas. Variability in edge-effect results may be due in part to variability in strength of study design. Previous evaluations have suggested that edge effects are found mainly in agricultural landscapes, but our results suggest that these effects also may be common in extensively forested NHCF landscapes. Within extensive but managed forests of this region, relatively large, contiguous patches of mature forest, unfragmented by clearcutting, may be required to conserve some forest interior bird species.