Data from: Effects of spatial structure of population size on the population dynamics of barnacles across their elevational range

1. Explanations for why population dynamics vary across the range of a species reflect two contrasting hypotheses: (1) temporal variability of populations is larger in the centre of the range compared to the margins because overcompensatory density dependence destabilizes population dynamics, and (2) population variability is larger near the margins, where populations are more susceptible to environmental fluctuations. In both of these hypotheses, positions within the range are assumed to affect population variability. 2. In contrast, the fact that population variability is often related to mean population size implies that the spatial structure of the population size within the range of a species may also be a useful predictor of the spatial variation of temporal variability of population size over the range of the species. 3. To explore how population temporal variability varies spatially and the underlying processes responsible for the spatial variation, we focused on the intertidal barnacle Chthamalus dalli and examined differences in its population dynamics along the tidal levels it inhabits. Changes in coverage of barnacle populations were monitored for 10.5 years at 25 plots spanning the elevational range of this species. Data were analysed by fitting a population dynamics model to estimate the effects of density-dependent and density-independent processes on population growth. We also examined the temporal mean-variance relationship of population size with parameters estimated from the population dynamics model. 4. We found that the relative variability of populations tended to increase from the centre of the elevational range towards the margins because of an increase in the magnitude of stochastic fluctuations of growth rates. Thus, our results supported hypothesis (2). We also found that spatial variations in temporal population variability were well characterized by Taylor’s power law, the relative population variability being inversely related to the mean population size. 5. Results suggest that understanding the population dynamics of a species over its range may be facilitated by taking the spatial structure of population size into account as well as by considering changes in population processes as a function of position within the range of the species.

1. 物种分布范围内种群动态存在差异的成因，存在两种对立假说：(1) 因过度补偿密度依赖会使种群动态失稳，物种分布中心的种群时间变异性较边缘地带更高；(2) 分布边缘的种群更易受环境波动影响，故其种群变异性更大。两类假说均假定，物种分布范围内的空间位置会对种群变异性产生影响。
2. 与之相对，种群变异性往往与种群平均规模相关，这一现象暗示，物种种群规模在其分布范围内的空间结构，或可作为预测该物种种群规模时间变异性在分布范围内空间差异的有效指标。
3. 为探究种群时间变异性的空间分布规律及其背后的驱动过程，本研究以潮间带藤壶达氏小藤壶(Chthamalus dalli)为研究对象，针对其栖息的潮位梯度开展种群动态差异分析。研究在覆盖该物种海拔分布范围的25个样地中，对藤壶种群的盖度变化开展了长达10.5年的监测。通过拟合种群动态模型，解析密度依赖与密度非依赖过程对种群增长的影响；同时基于种群动态模型估算的参数，检验了种群规模的时间均值-方差关系。
4. 研究发现，由于种群增长率随机波动幅度的增加，种群相对变异性呈现从分布中心向边缘逐步升高的趋势，由此验证了假说(2)。同时，种群时间变异性的空间差异可通过泰勒幂法则(Taylor’s power law)得到良好表征：种群相对变异性与平均种群规模呈负相关关系。
5. 本研究结果表明，若同时考虑物种种群规模的空间结构，以及种群动态过程随分布位置的变化规律，将有助于更深入地理解物种在其整个分布范围内的种群动态特征。