Abstract
Simple SummaryNorth American grassland birds evolved with American bison (Bison bison), until overhunting drove bison to near-extinction > 150 years ago. Bison have now been reintroduced to many areas that provide important nesting habitat for grassland birds, which are now among the most rapidly declining birds in North America. However, little is known about bison interactions with birds such as Bobolinks (Dolichonyx oryzivorus), obligate grassland nesting songbirds of conservation concern. Using data collected over an 18-year period, we assessed the effects of bison reintroduction, together with other land management and climate factors, on Bobolinks in a private conservation area comprising 24 km2 of native grasslands in the North American Great Plains. In grasslands where bison were reintroduced, Bobolink abundance (adult numbers) declined by 62%, and productivity (juvenile numbers) declined by 84%. By contrast, Bobolink populations remained stable over the same time period in adjacent grasslands where bison were not reintroduced. Bobolink abundance and productivity increased in years following warmer and wetter winters, but nevertheless declined over time in grasslands where the bison population doubled. Where bison are reintroduced and confined in high densities, overgrazing, trampling, and related impacts may drive severe declines in Bobolinks and other grassland birds of conservation concern.Among the most rapidly declining birds in continental North America, grassland birds evolved with American bison (Bison bison) until bison nearly became extinct due to overhunting. Bison populations have subsequently rebounded due to reintroductions on conservation lands, but the impacts of bison on grassland nesting birds remain largely unknown. We investigated how bison reintroduction, together with other land management and climate factors, affected breeding populations of a grassland bird species of conservation concern, the Bobolink (Dolichonyx oryzivorus). We quantified population changes in Bobolinks over an 18-year period in conservation grasslands where bison were reintroduced, compared with adjacent grasslands grazed by cattle and where hay was harvested after the bird breeding season. Four years after bison reintroduction, the bison population in the study area had doubled, while Bobolink abundance declined 62% and productivity declined 84%. Our findings suggest that bison reintroduction as a conservation strategy may be counterproductive in grassland fragments where overgrazing, trampling, and other negative impacts drive declines in grassland breeding birds. Where bird conservation is an objective, small grassland reserves may therefore be inappropriate sites for bison reintroduction. To maximize conservation benefits to birds, land managers should prioritize protecting grassland birds from disturbance during the bird breeding season.
Highlights
Grassland and farmland bird populations worldwide have declined steeply for decades [1,2,3], warranting urgent conservation action
Bobolink declines intensify in response to increased grazing intensities [27,92], suggesting that overgrazing and associated negative impacts contributed to the Bobolink declines we found after bison reintroduction
Our finding that Bobolinks declined following bison reintroduction on private conservation grasslands adds to previous research that identified declines in other grassland bird species and population parameters following bison reintroductions [10,36]
Summary
Grassland and farmland bird populations worldwide have declined steeply for decades [1,2,3], warranting urgent conservation action. In North America, grassland birds evolved alongside American bison (Bison bison; hereafter, bison), until overhunting drove bison to near extinction in the late 19th century. The reintroduction of bison to many areas in North. America is celebrated as a conservation success story, as bison numbers have rebounded from fewer than one thousand to hundreds of thousands of individuals today [6]. Reintroduction efforts highlight the ecological importance of bison and evidence that popular interest in bison encourages public support for conservation [8,9]. Grassland bird responses to bison grazing have rarely been studied due to the absence of bison from most
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