Testing the Water–Energy Theory on American Palms (Arecaceae) Using Geographically Weighted Regression

Water and energy have emerged as the best contemporary environmental correlates of broad-scale species richness patterns. A corollary hypothesis of water–energy dynamics theory is that the influence of water decreases and the influence of energy increases with absolute latitude. We report the first use of geographically weighted regression for testing this hypothesis on a continuous species richness gradient that is entirely located within the tropics and subtropics. The dataset was divided into northern and southern hemispheric portions to test whether predictor shifts are more pronounced in the less oceanic northern hemisphere. American palms (Arecaceae, n = 547 spp.), whose species richness and distributions are known to respond strongly to water and energy, were used as a model group. The ability of water and energy to explain palm species richness was quantified locally at different spatial scales and regressed on latitude. Clear latitudinal trends in agreement with water–energy dynamics theory were found, but the results did not differ qualitatively between hemispheres. Strong inherent spatial autocorrelation in local modeling results and collinearity of water and energy variables were identified as important methodological challenges. We overcame these problems by using simultaneous autoregressive models and variation partitioning. Our results show that the ability of water and energy to explain species richness changes not only across large climatic gradients spanning tropical to temperate or arctic zones but also within megathermal climates, at least for strictly tropical taxa such as palms. This finding suggests that the predictor shifts are related to gradual latitudinal changes in ambient energy (related to solar flux input) rather than to abrupt transitions at specific latitudes, such as the occurrence of frost.水资源与能量现已成为当前阐释大尺度物种丰富度格局的最优环境关联因子。水热动态理论的一项推论假说指出，随着绝对纬度升高，水资源的影响会逐渐减弱，而能量的影响则会逐步增强。本研究首次采用地理加权回归（Geographically Weighted Regression, GWR），针对完全处于热带与亚热带范围内的连续型物种丰富度梯度验证该假说。本数据集被划分为北半球与南半球两部分，以检验预测变量的影响偏移在海洋性更弱的北半球是否更为显著。本研究以美洲棕榈科植物（Arecaceae，共547种）作为模式类群，该类群的物种丰富度与分布格局已被证实对水资源与能量具有强烈响应。研究对不同空间尺度下，水资源与能量解释棕榈物种丰富度的能力进行了局域量化，并将其与纬度进行回归分析。研究发现了与水热动态理论相符的显著纬度梯度趋势，但南北半球的结果并未表现出质性差异。局域建模结果中存在较强的固有空间自相关，且水资源与能量变量间存在共线性，这些均被视为重要的方法学挑战。本研究通过采用同时自回归模型（Simultaneous Autoregressive Models, SAR）与变异分区分析，解决了上述问题。本研究结果表明，水资源与能量解释物种丰富度的能力，不仅会在从热带到温带乃至寒带的大尺度气候梯度中发生变化，在恒热气候范围内同样会出现变化——至少对于棕榈这类严格的热带类群而言是如此。该发现表明，预测变量的影响偏移与环境能量（与太阳辐射输入相关）随纬度的渐变有关，而非与特定纬度处的突变过渡（如霜冻的出现）相关。