Gibson Daniel, Blomberg Erik J, Sedinger James S
Program in Ecology, Evolution and Conservation Biology Department of Natural Resources and Environmental Science University of Nevada Reno Mail Stop 186 Reno Nevada 89557 USA.
Department of Fish and Wildlife Conservation Virginia Polytechnic Institute and State University Blacksburg Virginia 24060 USA.
Ecol Evol. 2016 Apr 24;6(11):3621-3631. doi: 10.1002/ece3.2148. eCollection 2016 Jun.
Plant phenological processes produce temporal variation in the height and cover of vegetation. Key aspects of animal life cycles, such as reproduction, often coincide with the growing season and therefore may inherently covary with plant growth. When evaluating the influence of vegetation variables on demographic rates, the decision about when to measure vegetation relative to the timing of demographic events is important to avoid confounding between the demographic rate of interest and vegetation covariates. Such confounding could bias estimated effect sizes or produce results that are entirely spurious. We investigated how the timing of vegetation sampling affected the modeled relationship between vegetation structure and nest survival of greater sage-grouse (), using both simulated and observational data. We used the height of live grasses surrounding nests as an explanatory covariate, and analyzed its effect on daily nest survival. We compared results between models that included grass height measured at the time of nest fate (hatch or failure) with models where grass height was measured on a standardized date - that of predicted hatch date. Parameters linking grass height to nest survival based on measurements at nest fate produced more competitive models, but slope coefficients of grass height effects were biased high relative to truth in simulated scenarios. In contrast, measurements taken at predicted hatch date accurately predicted the influence of grass height on nest survival. Observational data produced similar results. Our results demonstrate the importance of properly considering confounding between demographic traits and plant phenology. Not doing so can produce results that are plausible, but ultimately inaccurate.
植物物候过程会导致植被高度和覆盖度随时间变化。动物生命周期的关键方面，如繁殖，通常与生长季节重合，因此可能与植物生长存在内在的共变关系。在评估植被变量对种群统计学率的影响时，相对于种群统计学事件的时间来决定何时测量植被，对于避免目标种群统计学率与植被协变量之间的混淆至关重要。这种混淆可能会使估计的效应大小产生偏差，或得出完全虚假的结果。我们使用模拟数据和观测数据，研究了植被采样时间如何影响大艾草松鸡巢穴存活率与植被结构之间的模型关系。我们将巢穴周围活草的高度作为解释性协变量，并分析其对巢穴每日存活率的影响。我们比较了在巢穴命运（孵化或失败）时测量草高的模型与在标准化日期（预测孵化日期）测量草高的模型之间的结果。基于巢穴命运时测量结果将草高与巢穴存活率联系起来的参数产生了更具竞争力的模型，但在模拟场景中，草高效应的斜率系数相对于真实值存在偏高的偏差。相比之下，在预测孵化日期进行的测量准确地预测了草高对巢穴存活率的影响。观测数据也得出了类似的结果。我们的结果表明了正确考虑种群统计学特征与植物物候之间混淆的重要性。不这样做可能会得出看似合理但最终不准确的结果。
