EJR01 Foraging decisions underlying restricted space-use: effects of fire and forage maturation on large herbivore nutrient uptake on Konza Prairie

Recent models suggest that herbivores optimize nutrient intake by selecting patches of low to intermediate vegetation biomass. We assessed the application of this hypothesis to plains bison (Bison bison) in an experimental grassland managed with fire by estimating daily rates of nutrient intake in relation to grass biomass and by measuring patch selection in experimental watersheds in which grass biomass was manipulated by prescribed burning. Digestible crude protein content of grass declined linearly with increasing biomass, and the mean digestible protein content relative to grass biomass was greater in burned watersheds than watersheds not burned that spring (intercept; F1,251 = 50.57, P &lt; 0.0001). Linking these values to published functional response parameters, ad libitum protein intake, and protein expenditure parameters, Fryxell's (Am. Nat., 1991, 138, 478) model predicted that the daily rate of protein intake should be highest when bison feed in grasslands with 400 - 600 kg/ha. In burned grassland sites, where bison spend most of their time, availability of grass biomass ranged between 40 and 3650 kg/ha, bison selected foraging areas of roughly 690 kg/ha, close to the value for protein intake maximization predicted by the model. The seasonal net protein intake predicted for large grazers in this study suggest feeding in burned grassland can be more beneficial for nutrient uptake relative to unburned grassland as long as grass regrowth is possible. Foraging site selection for grass patches of low to intermediate biomass help explain patterns of uniform space use reported previously for large grazers in fire-prone systems. This data set was used to test the forage maturation hypothesis in the Konza Prairie bison enclosure from 2012-2013. Our objectives were to quantify foraging site selection of Plains bison in order to determine if bison in a fire-prone grassland selected sites of low-to-intermediate forage biomass as posited by Fryxell’s (1991) forage maturation hypothesis. Additionally, to understand how foraging patterns shifted when grass regrowth was not possible, we quantified the annual diet of four GPS - collared bison via stable isotope analysis of tail hair plucked during roundup.现有研究模型表明，植食动物可通过选择植被生物量处于中低水平的觅食斑块，优化自身营养摄入效率。本研究以受火管理的实验性草原为研究载体，以平原野牛（*Bison bison*）为研究对象，对上述假说进行验证：通过估算与草本生物量相关的每日营养摄入速率，并在通过受控火烧调控草本生物量的实验集水区内测量斑块选择偏好，以此检验该假说在平原野牛中的适用性。研究发现，草本植物的可消化粗蛋白含量随生物量增加呈线性下降趋势；且该春季内，受控火烧集水区内草本生物量对应的平均可消化粗蛋白含量显著高于未火烧集水区（截距项：F₁,₂₅₁=50.57，P<0.0001）。将上述测算结果与已发表的功能响应参数、自由采食蛋白质摄入参数以及蛋白质消耗参数相结合，Fryxell（《美国博物学家》，1991年，第138卷，第478页）提出的模型预测：当野牛在生物量为400~600 kg/ha的草原觅食时，每日蛋白质摄入速率可达峰值。野牛多数活动的火烧草原生境中，草本生物量可获得范围为40~3650 kg/ha，而野牛偏好的觅食区域生物量约为690 kg/ha，与模型预测的蛋白质摄入最大化最优生物量高度吻合。本研究针对大型植食动物预测的季节性净蛋白质摄入结果显示：只要草本能够完成再生，在火烧草原觅食相较于未火烧草原，更有利于动物获取营养。野牛对中低生物量草本斑块的觅食选择偏好，可解释此前在火敏感生态系统中观测到的大型植食动物均匀空间利用模式。本数据集采集自2012-2013年的康扎草原野牛围栏实验，用于验证牧草成熟假说。本研究的核心目标为量化平原野牛的觅食位点选择偏好，以验证火敏感草原中的野牛是否会如Fryxell（1991）的牧草成熟假说所提出的那样，选择中低牧草生物量的觅食区域。此外，为探明草本无法再生时觅食模式的变化情况，本研究通过对围捕过程中采集的野牛尾毛进行稳定同位素分析，量化了4只佩戴GPS项圈的野牛的年度饮食组成。