Livestock grazing is the most prevalent land management practice in western North America and the most widespread cause of habitat degradation in riparian communities. Riparian areas provide high-quality habitat for many species of declining migratory breeding birds. The most prevalent hypotheses to explain declines in migratory birds are habitat loss and related demographic processes on (1) breeding grounds; (2) wintering grounds; and (3) during migration. We tested the hypothesis that passive habitat restoration (removal of livestock) on local breeding grounds would correlate with increasing trends of migratory bird populations in riparian areas in western North America.
We analyzed changes in vegetation and bird abundance at Hart Mountain National Antelope Refuge (HMNAR) in southeastern Oregon over 24 years following removal of livestock grazing that had occurred over 120 years. We quantified long-term changes in overall avian abundance and species richness and, specifically, the abundance of 20 focal species. We then compared the local responses of the focal species to population-scale trends of the same species at three different large spatial scales.
Results/Conclusions
Overall avian abundance increased during the first 12 years after livestock removal and remained consistent from then through year 24. Three times as many species were recruited to the survey sites as were displaced. Most focal riparian woodland-tree or shrub dependent, sagebrush obligate, and grassland or meadow species increased in abundance or remained stable locally, contradicting regional declining or stable trends. In contrast, most riparian woodland-cavity nester species decreased in abundance locally, reflecting disruption of aspen stand dynamics by decades of grazing. Avian nest parasites and exotic competitors of native species declined in abundance locally, matching regional trends. Of the focal species of highest conservation concern, 71% (10 of 14) either increased or remained stable at HMNAR, most contradicting regional trends.
Landscape-scale removal of livestock at HMNAR provided a rare opportunity to quantify, at a large spatial and temporal scale, the impacts of grazing to bird assemblages and to test the effectiveness of passive restoration in riparian habitats. Restoring riparian ecosystems by removing livestock was beneficial to the conservation of declining populations of migratory birds. Thus, exclusion of grazing in riparian areas can be an effective tool in the management of avian species of conservation concern.