Embracing inherent and imposed sources of heterogeneity in rangeland bird management

Abstract

AbstractRangeland simplification is a global threat to biodiversity. Historically, rangelands exhibited varied vegetation structure, which provided a diverse wildlife habitat. Vegetation heterogeneity resulted from interactions between topoedaphic (inherent heterogeneity) and disturbance factors (imposed heterogeneity). Until recently, these interactive sources of heterogeneity have been ignored in rangeland management and study. Under an emerging paradigm that embraces heterogeneity, contributions of disturbance and topoedaphic conditions to rangeland biodiversity remain largely unknown. Using the avian community, we assessed the effects of wetland area, ecological site, and topographic roughness (inherent heterogeneity) and four grazing strategies (imposed heterogeneity; patch‐burn grazing with one or two seasons of fire, season‐long grazing, and modified twice‐over rotational grazing with variably stocked paddocks) on vegetation structure, avian diversity, avian community composition, and avian densities. The interaction between inherent and imposed heterogeneity influenced avian community composition but not species‐specific densities. The effects of management on vegetation structural heterogeneity varied over years, though patch‐burn management generated heterogeneity and temporal stability in vegetation structure compared to season‐long grazing and modified twice‐over rotational grazing. Patch‐burn and season‐long grazing pastures had higher diversity than modified twice‐over rotational grazing. Community composition was sensitive to ecological site, topographic roughness, and wetland area, suggesting that both the fire‐grazing interaction and inherent heterogeneity maintain biodiversity. Out of our six grassland obligate focal bird species, two responded to inherent heterogeneity, while three were most sensitive to imposed heterogeneity. One species, western meadowlark (Sturnella neglecta), did not respond to either source of heterogeneity, potentially indicating responsiveness to factors outside our study, such as annual precipitation or heterogeneity at fine scales. Our results indicate that both inherent and imposed heterogeneity are important in shaping grassland bird abundance, diversity, and community composition and that conserving rangeland biodiversity in the future will require managing for greater imposed heterogeneity while embracing existing landscape variability.