of. 2021. Habitat segregation of plate phenotypes in a rapidly expanding population of three-spined stickleback. Ecosphere 12(6):e03561.

Intraspecific niche differentiation can contribute to population persistence in changing environments. Following declines in large predatory fish, eutrophication, and climate change, there has been a major increase in the abundance of threespine stickleback (Gasterosteus aculeatus) in the Baltic Sea. Two morphotype groups with different levels of body armor—completely plated and incompletely plated—are common in coastal Baltic Sea habitats. The morphotypes are similar in shape, size, and other morphological characteristics and live as one apparently intermixed population. Variation in resource use between the groups could indicate a degree of niche segregation that could aid population persistence in the face of further environmental change. To assess whether morphotypes exhibit niche segregation associated with resource and/or habitat exploitation and predator avoidance, we conducted a field survey of stickleback morphotypes, and biotic and abiotic ecosystem structure, in two habitat types within shallow coastal bays in the Baltic Sea: deeper central waters and shallow near‐shore waters. In the deeper waters, the proportion of completely plated stickleback was greater in habitats with greater biomass of two piscivorous fish: perch (Perca fluviatilis) and pike (Esox lucius). In the shallow waters, the proportion of completely plated stickleback was greater in habitats with greater coverage of habitat‐forming vegetation. Our results suggest niche segregation between morphotypes, which may contribute to the continued success of stickleback in coastal Baltic Sea habitats.

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Declines of large predatory fish due to overexploitation are restructuring food webs 1 across the globe. It is now becoming evident that restoring these altered food webs requires 2 addressing not only ecological processes, but evolutionary ones as well, because human-induced 3 rapid evolution may in turn affect ecological dynamics. In the central Baltic Sea, abundances of 4 the mesopredatory fish, the three-spined stickleback (Gasterosteus aculeatus), have increased 5 dramatically during the past decades. Time-series data covering 22 years show that this increase 6 coincides with a decline in the number of juvenile perch (Perca fluviatilis), the most abundant 7 predator of stickleback along the coast. We studied the interaction between evolutionary and 8 ecological effects of this mesopredator take-over, by surveying the armour plate morphology of 9 stickleback and the structure of the associated food web. First, we investigated the distribution of 10 different stickleback phenotypes depending on predator abundances and benthic production; and 11 described the stomach content of the stickleback phenotypes using metabarcoding. Second, we 12 explored differences in the relation between different trophic levels and benthic production, 13 between bays where the relative abundance of fish was dominated by stickleback or not; and 14 compared this to previous cage-experiments to support causality of detected correlations. We 15 found two distinct lateral armour plate phenotypes of stickleback, incompletely and completely 16 plated. The proportion of incompletely plated individuals increased with increasing benthic 17 production and decreasing abundances of adult perch. Stomach content analyses showed that the 18 completely plated individuals had a stronger preference for invertebrate herbivores (amphipods) 19 than the incompletely plated ones. In addition, predator dominance interacted with ecosystem 20 production to determine food web structure and the propagation of a trophic cascade: with 21 increasing production, biomass accumulated on the first (macroalgae) and third (stickleback) 22 trophic levels in stickleback-dominated bays, but on the second trophic level (invertebrate 23 The rise of the stickleback; Britas Klemens Eriksson et al. 2020 3 herbivores) in perch-dominated bays. Since armour plates are defence structures favoured by 1 natural selection in the presence of fish predators, the phenotype distribution suggest that a novel 2 low-predation regime favours sticklebacks with less armour. Our results indicate that an 3 interaction between evolutionary and ecological effects of the stickleback take-over has the 4 potential to affect food web dynamics. 5 6

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