Abstract
Change in body size can be driven by social (density) and non-social (environmental and spatial variation) factors. In expanding metapopulations, spatial sorting by means of dispersal on the expansion front can further drive the evolution of body size. However, human intervention can dramatically affect these founder effects. Using long-term monitoring of the colonization of the remote Kerguelen islands by brown trout, a facultative anadromous salmonid, we analyse body size variation in 32 naturally founded and 10 human-introduced populations over 57 years. In naturally founded populations, we find that spatial sorting promotes slow positive changes in body size on the expansion front, then that body size decreases as populations get older and local density increases. This pattern is, however, completely different in human-introduced populations, where body size remains constant or even increases as populations get older. The present findings confirm that changes in body size can be affected by metapopulation expansion, but that human influence, even in very remote environments, can fully alter this process.
Highlights
Body size is a fundamental trait known to be linked with metabolic rate, physiology, life history and fitness [1]
Benefiting from a 57-years long-term monitoring (1962–2019), we investigate how juvenile body size is influenced by phenotypic sorting during natural metapopulation expansion, how juvenile size changes as the population grows and how human influence changes in juvenile 2 body size pattern
Looking at changes in juvenile body size over a half-century (10–15 generations), our analysis aimed at revealing large scale patterns driven by metapopulation expansion
Summary
Body size is a fundamental trait known to be linked with metabolic rate, physiology, life history (reproduction, competition, survival, dispersal) and fitness [1]. If dispersers represent a non-random sample of the origin population in terms of body size wise [7,9], and provided that body size at age is inheritable and they manage to reproduce locally, they may have an effect on the structure of the recipient population Both processes within and between populations driving body size evolution are likely to be strongly impacted by the current pace and magnitude of global change [10,11]. Humans transported and introduced species intentionally or not in new environments where they might first settle, eventually invade [12] These situations present an adequate context to study the pace and direction of body size changes, since they enable the study of sharp density gradients from the core to the moving edge of the distribution area. This pattern implies the colonization of new habitat patches to found new local populations, expanding further the metapopulation, with dispersal events between patches [15]
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