Interactions between ecological disturbances: burning and grazing and their effects on songbird communities in northern mixed-grass prairies

摘要

Historically, North American prairies were strongly influenced by two natural disturbances, fire and grazing, and their interaction. However, the frequency and size of fires has been greatly altered over time, while native ungulates have been replaced by livestock; this may have had strong ecological influences on modern prairies. The feedback hypothesis proposes that grazing by ungulates will increase the duration of fire effects because ungulates will be attracted to burned patches. We conducted point-count surveys in burned-grazed, burned-ungrazed, unburned-grazed, and unburned-ungrazed sites over a 5-year period following fires that occurred naturally in 2006 in southern Saskatchewan, Canada, to test predictions related to avian community composition related to the feedback hypothesis. Generalized linear mixed models were used to analyze interactions among burning, grazing, and time since burning. Six of the nine avian species we studied responded positively or negatively to burning or grazing, although there were few statistically significant effects on the vegetation structure variables we measured. We found mixed evidence that grazing increased the duration of effects of burning, cumulatively providing little evidence for the feedback hypothesis. Nonetheless, effects of burning and grazing differed from and interacted with one another; for example, short-term effects of burning on Sprague’s Pipits (Anthus spragueii) and Baird’s Sparrows (Ammodramus bairdii) were greater than effects of grazing, and effects of grazing and burning in combination were frequently greater than effects of a single disturbance. Therefore, both should be integrated into management for the conservation of grassland songbirds.