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.探明物种分布极限形成的关键环境因子极具挑战，因为生物与其生存环境之间存在着复杂的交互作用。不过，将统计生态位模型（statistical niche models）与沿环境梯度的表型变异分析相结合的研究方法，有助于简化这类复杂交互作用，并筛选出候选关键因子子集。本研究即采用此类方法，对西印度群岛特立尼达岛低海拔区域内，沿盐度梯度分布的两种近缘胎生鳉鱼的邻域分布格局展开描述，并解析其分布限制因子。孔雀花鳉（Poecilia reticulata）的下游分布极限与咸淡水交汇带呈现极强的相关性。对比该物种的边缘种群与核心种群时，未发现其生活史特征存在显著表型变异，这表明咸淡水交汇带环境条件的突变限制了孔雀花鳉的分布范围。与之形成对比的是，饰斑花鳉（Poecilia picta）可栖息于宽泛的盐度区间内，但逐渐在上游淡水生境中消失。此外，在与孔雀花鳉同域共存的生境中，饰斑花鳉种群的后代体型呈现增大趋势，这提示种间竞争可能参与限制其分布。综上两种花鳉的邻域分布格局表明，孔雀花鳉的分布受非生物因子（盐度）限制，而饰斑花鳉的分布则受生物因子（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，这提示这类分布模式可能在自然界中广泛存在。