Thresholds in forest bird richness in response to three types of forest cover in Ontario, Canada

Identification of threshold values in avian responses to habitat loss provides science-based guidelines on how to best manage anthropogenically altered landscapes for wildlife preservation. Our objectives were to test for thresholds in species richness of forest-breeding birds in relation to deciduous, coniferous, mixed, and total forest cover in two landscapes varying in the amount of forest present, and to determine whether thresholds for each forest type were coincident within and among landscapes, and across two spatial scales. We used Ontario Breeding Bird Atlas data to calculate species richness of forest-breeding birds. Percent deciduous, coniferous, mixed, and total forest cover were determined in 250 m radius circles around survey locations (local scale) and for 10 x 10 km squares (landscape scale) in the fragmented, sparsely forested Mixedwood Plains ecozone and in the more continuously forested Ontario Shield ecozone. Best-fit models explaining variation in species richness were selected from candidate sets of models that included 1-6 variables in addition to segmented or linear terms for forest cover. Threshold models had support at the local and landscape scales, and for all forest types. Thresholds ranged from 4 to 45 % cover, being slightly higher in the more continuously forested landscape, and for more prevalent forest types. Thresholds can provide useful targets for conservation efforts, but it is crucial to recognize that habitat levels should be maintained above threshold values to avoid ecological degradation, and to address the habitat requirements of species with occupancy thresholds above those of the assemblage.