Data from: Ecological correlates of the distribution limits of two poeciliid species across a salinity gradient

Identifying the environmental factors responsible for the formation of a species' distribution limit is challenging because organisms interact in complex ways with their environments. However, the use of statistical niche models in combination with the analysis of phenotypic variation along environmental gradients can help to reduce such complexity and identify a subset of candidate factors. In the present study, we used such approaches to describe and identify factors responsible for the parapatric distribution of two closely-related livebearer fish species along a salinity gradient in the lowlands of Trinidad, West Indies. The downstream distribution limits of Poecilia reticulata were strongly correlated with the brackish–freshwater interface. We did not observe significant phenotypic variation in life-history traits for this species when comparing marginal with more central populations, suggesting that abrupt changes in conditions at the brackish–freshwater interface limit its distribution. By contrast, Poecilia picta was present across a wide range of salinities, although it gradually disappeared from upstream freshwater localities. In addition, P. picta populations exhibited an increase in offspring size in localities where they coexist with P. reticulata, suggesting a role for interspecific competition. The parapatric distribution of these two species, suggests that P. reticulata distributions are limited by an abiotic factor (salinity), whereas P. picta is limited by a biotic factor (interspecific competition). Similar parapatric patterns have been previously described in other systems, suggesting they might be a common pattern in nature.

探明驱动物种分布边界形成的环境因子颇具挑战，这是因为生物与其生存环境间存在复杂的交互作用。不过，结合统计生态位模型与沿环境梯度的表型变异分析，可简化此类复杂问题，并筛选出一组候选影响因子。本研究即采用此类方法，对西印度群岛特立尼达低地沿盐度梯度分布的两种近缘胎生鱼类的邻域分布格局进行描述，并解析其驱动因子。

孔雀花鳉（Poecilia reticulata）的下游分布边界与咸淡水交汇区呈显著相关。将该物种的边缘种群与中心种群进行对比后，未观察到其生活史特征存在显著表型变异，这表明咸淡水交汇区的环境突变限制了其分布范围。与之相对，饰纹花鳉（Poecilia picta）可在较宽盐度范围内存活，但逐渐在上游淡水生境中消失。此外，在与P. reticulata共存的生境中，P. picta种群的后代体型有所增大，这表明种间竞争发挥了限制作用。

这两种鱼类的邻域分布格局表明，P. reticulata的分布边界受非生物因子（盐度）限制，而P. picta的分布边界则受生物因子（种间竞争）限制。此前已有其他研究系统报道过类似的邻域分布格局，这表明此类格局在自然界中可能较为普遍。