Phenotype‐dependent selection underlies patterns of sorting across habitats: the case of stream‐fishes

Abstract

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.

Matching habitat choice and plasticity contribute to phenotype–environment covariation in a stream salamander

“…, Jacobson et al. ), but this is the first evidence in a stream amphibian. These observations suggest that our results may be useful as a foundation for future research at the intersection of community ecology and ecomorphology in plethodontid salamanders.…”

“…Stream plethodontids are an important model system in community ecology (Hairston 1987, Rissler et al 2004, Bruce 2011), but the ecological effects of phenotypic plasticity have been understudied, particularly relative to other amphibians (Relyea 2001, Van Buskirk 2009). Likewise, the phenotypes of stream fishes are known to differ among flow-delineated habitats (Langerhans 2008, Senay et al 2015, Jacobson et al 2017, but this is the first evidence in a stream amphibian. These observations suggest that our results may be useful as a foundation for future research at the intersection of community ecology and ecomorphology in plethodontid salamanders.…”

“…, Jacobson et al. ). Therefore, it is possible that other abiotic or biotic conditions influence morphological divergence, including substrate size (Montgomery and Buffington ), prey distribution (Brown and Brussock ), or dissolved oxygen (Matthews and Berg ).…”

“…, Jacobson et al. ) suggest that streams may be useful study systems for testing mechanisms that produce phenotype–environment covariation.…”

“…The spatial dimensions of flow-delineated habitats are highly variable, but generally scale with the size of streams (e.g., as a function of discharge, bank-full width, stream order) such that habitats may differ between meters of channel length in headwaters, and at larger scales (10-100 m) in mainstem streams and rivers (Frissell et al 1986). This environmental heterogeneity and associated patterns of phenotypic variation in stream organisms (Langerhans 2008, Senay et al 2015, Jacobson et al 2017 suggest that streams may be useful study systems for testing mechanisms that produce phenotype-environment covariation.…”