Morphological clines in dendritic landscapes

Abstract

Summary1. In complex landscapes such as river networks, organisms usually face spatio‐temporal heterogeneity and gradients in geomorphological, water, ecological or landscape characteristics are often observed at the catchment scale. These environmental variables determine developmental conditions for larval stages of freshwater insects and influence adult phenotypic characteristics. Environmental clines are therefore expected to generate morphological clines. Such a process has the potential to drive gradual geographical change in morphology‐dependent life history traits, such as dispersal.2. We studied the influence of aquatic and terrestrial environmental factors on morphological variations in Calopteryx splendens across the Loire drainage. To investigate these effects we took explicitly into account the hierarchical structure of the river network.3. We analysed eight morphological traits. Results showed significant body size variation between tributaries and the presence of a morphological cline at the drainage scale. We observed an effect of pH and water temperature on body size. Individuals in downstream sites were larger than individuals in upstream sites, and adults whose larval stages were exposed to alkaline pH and high temperatures during summer were larger.4. Body size affects flight abilities in insects. Thus, our results suggest that morphological clines may generate an asymmetric dispersal pattern along the downstream–upstream axis, downstream populations dispersing farther than upstream ones. Such a process is expected to influence population genetic structure at the drainage scale if larval drift and floods do not balance an asymmetrical dispersal pattern of adults along the downstream–upstream gradient. To assess the influence of environmental gradients on the variation of life history traits it is important to understand the population biology of freshwater insects, and more generally of riverine organisms. It is also essential to integrate such data in conservation or restoration programmes.