Data from: Phenotype-dependent selection underlies patterns of sorting across habitats: the case of stream-fishes

Spatial and temporal heterogeneity within landscapes influences the distribution and phenotypic diversity of individuals both within and across populations. Phenotype-habitat correlations arise either through phenotypes within an environment altering through the process of natural selection or plasticity, or phenotypes remaining constant but individuals altering their distribution across environments. The mechanisms of non-random movement and phenotype-dependent habitat choice may account for associations within highly heterogeneous systems, such as streams, where local adaptation may be negated, plasticity too costly and movement is particularly important. Despite growing attention, however, few empirical tests have yet to be conducted. Here we provide a test of phenotype-dependent habitat choice and ask: 1) if individuals collected from a single habitat type continue to select original habitat; 2) if decisions are phenotype-dependent and functionally related to habitat requirements; and 3) if phenotypic-sorting continues despite increasing population density. To do so we both conducted experimental trials manipulating the density of four stream-fish species collected from either a single riffle or pool and developed a game-theoretical model exploring the influence of individuals’ growth rate, sampling and competitive abilities as well as interference on distribution across two habitats as a function of density. Our experimental trials show individuals selecting original versus alternative habitats differed in their morphologies, that morphologies were functionally related to habitat-type swimming demands, and that phenotypic-sorting remained significant (although decreased) as density increased. According to our model this only occurs when phenotypes have contrasting habitat preferences and only one phenotype disperses (i.e. selects alternatives) in response to density pressures. This supports our explanation that empirical habitat selection was due to a combination of collecting a fraction of mobile individuals with different habitat preferences and the exclusion of individuals via scramble competition at increased densities. Phenotype-dependent habitat choice can thereby account for observed patterns of natural stream-fish distribution.

景观内的时空异质性（spatial and temporal heterogeneity）会影响种群内部及跨种群个体的分布与表型多样性。表型-生境关联（phenotype-habitat correlations）的形成途径主要有二：一是环境内的表型通过自然选择（natural selection）或表型可塑性（plasticity）发生改变；二是表型本身保持稳定，但个体改变其在不同生境间的分布格局。非随机扩散（non-random movement）与依赖表型的生境选择（phenotype-dependent habitat choice）或可解释高度异质系统（如溪流（streams））内的关联模式：在这类系统中，局部适应（local adaptation）可能被抵消，表型可塑性的成本过高，而扩散作用尤为关键。尽管相关研究日益受到关注，但目前开展的实证检验（empirical tests）仍寥寥无几。本研究针对依赖表型的生境选择开展实证检验，并提出以下三个研究问题：1）从单一生境类型中采集的个体是否仍会选择原生生境；2）生境选择决策是否依赖表型，且与生境需求存在功能关联；3）随着种群密度升高，表型分选（phenotypic-sorting）是否仍会持续。为此，我们开展了两项工作：一是通过操控从单一急滩（riffle）或静潭（pool）采集的4种溪流鱼类（stream-fish）的种群密度进行室内实验；二是构建博弈论模型（game-theoretical model），探究个体生长速率（growth rate）、取样能力、竞争能力（competitive abilities）以及干扰作用（interference）对两种生境间分布格局的影响，且该影响随种群密度变化而改变。实验结果显示：选择原生生境与替代生境的个体在形态特征上存在显著差异，且形态特征与生境类型对应的游泳需求存在功能关联；随着种群密度升高，表型分选效应虽有所减弱，但仍显著存在。根据模型推演，该现象仅在满足以下条件时出现：不同表型具有相反的生境偏好，且仅有一种表型会响应密度压力发生扩散（即选择替代生境）。该结果佐证了我们的解释：实证观测到的生境选择行为，源于两方面的共同作用——一是采集样本中包含了部分具有不同生境偏好的扩散个体，二是当种群密度升高时，个体通过掠夺性竞争（scramble competition）被排除出原生生境。综上，依赖表型的生境选择可合理解释自然溪流鱼类的分布格局。