Data from: Physical calculations of resistance to water loss improve predictions of species range models

Species ranges are constrained by the physiological tolerances of organisms to climatic conditions. By incorporating physiological constraints, species distribution models can identify how biotic and abiotic factors constrain a species’ geographic range. Rates of water loss influence species distributions, but characterizing water loss for an individual requires complex calculations. Skin resistance to water loss (ri) is considered to be the most informative metric of water loss rates because it controls for experimental biases. However, calculating ri requires biophysical equations to solve for the resistance of the air that surrounds an organism, termed the boundary layer resistance (rb). Here, we compared theoretical and empirical methods for measuring skin resistance to water loss of a Plethodon salamander collected from nature. For the empirical methods, we measured rb of agar replicas at five body sizes, two temperatures, three vapor pressure deficits, and six flow rates using a flow through system. We also calculated rb using biophysical equations under the same experimental conditions. We then determined the ecological implications of incorporating skin and boundary layer resistance into a species range model that estimated potential activity time and energy balance throughout the geographic range of the study species. We found that empirical methods for calculating rb resulted in negative values of ri, whereas biophysical calculations produced meaningful values of ri. The species range model determined that ignoring realistic boundary layer and skin resistances reduced average estimates of energy balance by as much as 64% and potential activity time by 88% throughout the spatial extent of the model. We conclude that the use of agar replicas is an inadequate technique to characterize skin resistance to water loss, and incorporating boundary layer and skin resistances to water loss improve estimates of activity and energetics for mechanistic species distribution models. More importantly, our study suggests incorporating the physical processes underlying rates of water loss could improve estimates of habitat suitability for many animals.

物种的分布范围受生物体对气候条件的生理耐受限度（physiological tolerances）所制约。通过纳入生理约束条件，物种分布模型（species distribution models）能够解析生物与非生物因子如何限制物种的地理分布范围。水分散失速率会影响物种分布，但对个体水分散失的量化表征需要复杂的计算。生物体表皮的水分散失阻力（skin resistance to water loss, r_i）被认为是表征水分散失速率最具信息量的指标，因其可消除实验偏差的干扰。然而，计算r_i需要借助生物物理方程求解生物体周围空气的阻力，该阻力被称为边界层阻力（boundary layer resistance, r_b）。本研究对比了理论与实测两种方法，用于测定野外采集的无肺螈属（Plethodon）蝾螈的表皮水分散失阻力。在实测方法中，我们借助流式系统（flow through system），设置5个体型梯度、2个温度水平、3个水汽压亏缺值以及6个流速梯度，测定了琼脂复制品（agar replicas）的r_b。我们还在相同实验条件下，通过生物物理方程计算了r_b。随后我们评估了将表皮阻力与边界层阻力纳入物种分布模型后的生态效应——该模型可估算研究物种整个地理分布范围内的潜在活动时长与能量平衡。研究发现，采用实测方法计算r_b会得到负的r_i值，而生物物理计算则可得到具有生物学意义的r_i值。物种分布模型结果显示，若忽略真实的边界层阻力与表皮阻力，模型全域内的能量平衡平均估算值最多会降低64%，潜在活动时长则会减少88%。我们认为，使用琼脂复制品来表征表皮水分散失阻力的方法并不恰当；而将水分散失的边界层阻力与表皮阻力纳入模型，能够提升机制性物种分布模型（mechanistic species distribution models）对物种活动与能量代谢的估算精度。更为重要的是，本研究表明，纳入水分散失速率背后的物理过程，能够改善诸多动物类群的生境适宜性（habitat suitability）估算结果。