Vegetation characteristics and precipitation jointly influence grassland bird abundance beyond the effects of grazing management

Grassland birds have experienced some of the steepest population declines of any guild of birds in North America. The shortgrass steppe contains some of North America's most-intact grasslands, which makes the region particularly important for these species. Grassland birds differentially respond to variation in vegetation structure generated by spatiotemporally-varying disturbance like grazing management. However, understanding how species respond to characteristics beyond vegetation structure or grazing could better inform management for these species in the shortgrass steppe. We analyzed point count data for 5 grassland bird species breeding on the Central Plains Experimental Range in northeastern Colorado from 2013 – 2017 to examine the predictive capacity of models representing fine-scale (~3-ha) vegetation attributes (vegetation structure and cover type) and topography, combined with interannual precipitation variability (i.e. vegetation-abiotic models). We then compared these models to models based on grazing management treatments (applied to whole pastures, ~130 ha) and edaphic conditions (ecological sites), which represented information more generally available to rangeland managers. Precipitation, vegetation structure, and vegetation cover type influenced all species in a manner consistent with, but more nuanced than, vegetation structure alone. These models also explained more variation in abundance for species that responded to grazing management. Thus, while grazing management can be applied adaptively to improve habitat for these species, our more detailed vegetation-abiotic models identified species-specific habitat components that could be targeted for management. For example, not grazing pastures with extensive, homogenous stands of mid-height grasses (e.g, Hesperostipa comata) for an entire growing season during wet years could be one strategy to enhance Grasshopper Sparrow abundance and stockpile residual forage for future utilization by livestock. Our models provide a better understanding of, and reveal nuances in, the suite of environmental conditions to which grassland birds respond in shortgrass-steppe rangelands.北美各类鸟类生态类群中，草原鸟类的种群下降幅度最为显著。短草草原（shortgrass steppe）拥有北美部分保存最为完好的草原生境，这使得该区域对这类鸟类而言尤为关键。草原鸟类会对放牧管理这类时空动态扰动所塑造的植被结构差异产生差异化响应。然而，若能厘清物种对植被结构或放牧管理以外的环境特征的响应机制，将能更有效地为短草草原内的该类鸟类制定管护策略。 我们分析了2013至2017年间，在科罗拉多州东北部中部平原实验牧场（Central Plains Experimental Range）繁殖的5种草原鸟类的点计数调查（point count）数据，以检验结合了年际降水变异性的细尺度（~3公顷）植被属性（植被结构与覆被类型）及地形模型的预测能力（即植被-非生物模型）。随后，我们将此类模型与基于放牧管理处理（实施于整片放牧地，约130公顷）以及土壤条件（生态立地（ecological sites））的模型进行了对比——后者所涵盖的信息，通常是牧场管护者可便捷获取的。 降水、植被结构与植被覆被类型对所有物种的影响，与仅基于植被结构的模型结果相符，但更为精细复杂。此类模型还能更好地解释那些对放牧管理存在响应的物种的种群丰度变异。因此，尽管放牧管理可通过适应性应用来改善这类鸟类的栖息生境，但我们构建的精细化植被-非生物模型，可识别出可针对性开展管护的物种特异性生境组分。例如，在湿润年份，对连片分布、植被均质的中等高度草本（如贯叶针茅（Hesperostipa comata））占优的放牧地实施全生长季休牧，既可提升蚱蜢雀（Grasshopper Sparrow）的种群丰度，又可留存饲草以供家畜后续取用。 我们的模型能够增进对短草草原牧场中草原鸟类所响应的一系列环境条件的认知，并揭示其中的细微差异。