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Environmental Change and Extended Phenotypes: Does Eutrophication Influence Nest Building in Sticklebacks?



Ulla Tuomainen, Department of Biosciences, University of Helsinki, P.O. Box 65, FI-00014 Helsinki, Finland.



Many species use extended phenotypes, such as purpose-built nests, to increase their reproductive success. These traits have to be adjusted to local environmental conditions to maximize fitness. An important question is whether species are able to adjust their extended phenotypes to human-induced rapid environmental changes. We investigated whether populations of threespine stickleback, Gasterosteus aculeatus, exposed to different degrees of human-induced eutrophication during the last decades, have differentiated phenotypically in their nest-building behaviour. Stickleback males build nests that they use both in mate attraction and for offspring protection and whose characteristics vary with environmental conditions. We allowed males from parallel pairs of mildly and severely eutrophied habitats to build nests under standardized conditions in the laboratory. We recorded the time it took the males to build a nest, the size and neatness of the completed nest and the use of nest ornaments. We found eutrophication at the site of capture to influence nest-building time – males from eutrophied habitats built faster. However, eutrophication did not alter nest structure or the use of nest ornaments. This is probably because the nests are concealed in vegetation or under stones and females cannot evaluate them before they have followed the male to the nest, and predators cannot detect them before close to the nest. Thus, reduced long-range visibility does not influence the use of nests as mate choice cues or for offspring protection. This contrasts with a recorded effect of eutrophication on courtship behaviour, whose efficiency depends on long-range visibility. This suggests that traits are adjusted to eutrophication depending on the influence of eutrophication on the function of the traits.