Spotted owls, forest fragmentation, and forest heterogeneity

The Spotted Owl (Strix occidentalis) has been a focal species in the United States in terms of loss and fragmentation of old coniferous forests. Past research has shown a strong association between Spotted Owls and old coniferous forests. Thus, these vegetation types are considered synonymous with Spotted Owl habitat. Past fragmentation of old coniferous forests in the Pacific Northwest, the Sierra Nevada, southern California, and the Southwest has resulted from natural disturbance (e.g., fire), edaphic conditions, and timber harvesting. These processes have occurred at different rates and levels. We reviewed the existing literature on the effects of forest fragmentation and heterogeneity on Spotted Owls at three different scales: a range-wide scale where once-connected populations have been isolated from each other, a population scale where populations with different fragmentation regimes have different demographics, and a territory scale where individuals occupying territories with different fragmentation regimes have different fitness. Studies at the range-wide scale have concentrated on processes, such as juvenile dispersal. There are no published studies on the effects of fragmentation or heterogeneity at the population scale, although the potential exists for examining those effects with current studies. Lack of empirical data on the effects of fragmentation on Spotted Owls led to the development of spatially-explicit simulation models as an aid to reserve design for this species. In addition, some populations of Spotted Owls are naturally disjunct at the range-wide scale. Most empirical studies have concentrated on the territory scale, and most of those studies have examined the effects of fragmentation and heterogeneity on occupancy. We attempted a simple meta-analysis using effect sizes estimated from these studies. However, this analysis was hampered by lack of replicated studies among subspecies and among provinces within subspecies. In addition, studies did not use similar metrics to describe fragmentation and heterogeneity. Thus, empirical studies following simulation models are equivocal in their conclusions. Many questions remain unanswered concerning the effects of forest fragmentation and heterogeneity on Spotted Owls. We provide a set of key questions that need to be addressed to better understand the effects of fragmentation and heterogeneity on Spotted Owls. We also suggest that future research concentrate on understanding natural disturbance regimes and the extent to which timber harvesting is compensatory or additive to natural disturbance regimes. Research on the effects of fragmentation on Spotted Owls should also include alternative hypotheses that some levels of fragmentation and/or heterogeneity may benefit Spotted Owl populations.