PARENT SESSION
Contributed Oral Session 70: Avian Ecology: Habitat; Human Impact
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 524 A, Level 5, Palais des congrès de Montréal
The case for ecological thresholds: Birds in managed forest landscapes as a focal system.
Guénette, Jean1, Robichaud, Isabelle*,1, Villard, Marc1, 1 Chaire de recherche du Canada en conservation des paysages, Moncton, NB, Canada
ABSTRACT- Thresholds in ecological responses to environmental gradients have theoretical and practical appeal. Nonetheless, empirical evidence for such thresholds remains elusive. We define an ecological threshold as a sharp change in a system property over a narrow range in an environmental gradient. To assess the generality of threshold responses, we used a short-term, spatially extensive survey of forest birds as a case study. We identified thresholds in significant bird responses to habitat characteristics using logistic regression and receiver-operating characteristic (ROC) analysis. A simulation showed that the shape of the logistic curve was very sensitive to deviations from a perfect model, whereas a ROC-derived parameter of model performance and the associated threshold value were robust to such deviations. Analyses were restricted to 42 species present in at least 5% of stations and vocal enough to be surveyed with point counts. Of the 42 species, 14 exhibited significant threshold responses to mature forest cover at the landscape scale (1-km radius). For the 9 species responding positively, thresholds ranged from 56% and 67%, the ovenbird being the most sensitive species and dark-eyed Junco the least sensitive. Thresholds obtained could be classified into two main types: nearly perfect prediction of species absence but imperfect prediction of presence and vice versa. Each threshold type reflects different processes and has contrasting implications for conservation. Results further indicate that, for a large set of species, landscape context is an important predictor of their distribution, and blocks of up to 200 ha of mature forest have to be maintained uncut or under selection cut systems to ensure high levels of habitat occupancy. Should threshold values or ranges prove to be consistent spatially and temporally, they will provide critical insight for informed conservation planning.
Key words: Ecological thresholds, Forest landscapes, Forest birds, ROC analysis