Cumulative effects of natural and anthropogenic stressors on avian populations

Background/Question/Methods

Evaluating spatial and temporal variation in abundance due to anthropogenic and environmental stressors is a central aspect of ecology.  We conducted an experiment to evaluate the effects of salvage logging on the abundance of 22 avian species and 6 foraging guilds in lodgepole pine (Pinus contorta) forests affected by beetle outbreaks, south-central Oregon, USA, 1996–1998.  Treatments consisted of the removal of lodgepole pine snags only; live trees and ponderosa pine snags were not harvested.  Our hierarchical model requires replicated count data from multiple sample units across several time periods, during which the population is closed, to estimate abundance as a function of harvesting treatment and management district while accounting for imperfect and variable detection probability.  We fit the model to point counts from 12 control and 12 treatment plots with 3 surveys during each breeding season, 1996-1998. 

Results/Conclusions

We found strong evidence that salvage logging had a negative effect on abundance for 1 species, the gray flycatcher (median and 95% credible interval for 1996: -2.1, -4.1 to -0.8; 1997: -1.8, -3.3 to -0.7; 1998: -2.5, -5.7 to -0.8).  The abundance of the 21 remaining species either did not change due to the treatment or displayed broad and inconsistent annual variation.  While 3 foraging guilds increased in abundance, 2 guilds did not change and the response of 1 guild changed direction in different years.  Our results indicate that selective harvesting did not have a negative effect beyond any effect that beetle outbreaks exerted on avian populations.  Other studies have found that avian communities are resilient to moderate levels of management-induced disturbance, including harvesting and prescribed fire.  We suggest that our results arise from the comparatively less severe effects of both the beetle outbreak and the selective logging prescription that we implemented, as opposed to wildfire and clearcut salvage logging, on the habitat structure in the study plots.  We recommend that future research consider different sampling programs for those species with large home ranges (e.g., woodpeckers) that may not be sampled adequately using commonly employed programs (e.g., point count stations).