Abstract
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.
Highlights
Natural populations across the world are experiencing large-scale changes in both biotic and abiotic conditions due to human activity
We investigated habitat use of two stickleback morphotypes in coastal Baltic Sea populations in order to determine the extent of intraspecific niche partitioning
Comparison of morphotype relative abundance in deeper central waters of the bays confirmed our hypothesis and earlier findings (Eriksson et al, 2021) that the proportion of completely plated stickleback morphotypes increases with increasing abundances of fish predators
Summary
Natural populations across the world are experiencing large-scale changes in both biotic and abiotic conditions due to human activity. When predation is relaxed as a consequence of top predator loss, the resulting increased intraspecific competition between phenotypically similar individuals can promote divergence in resource exploitation (Zandonà et al, 2017) Such niche segregation may be followed by reduced gene flow between groups, reproductive isolation, and, if persisting. Incompletely plated individuals make up a greater proportion of the stickleback population in areas of lower piscivorous fish biomass and greater benthic production, suggesting a trade-off between predator defense and resource utilization (Eriksson et al, 2021). Stickleback are more abundant in macroalgal beds of the habitat-forming brown algae Fucus vesiculousus (hereafter referred to as “Fucus”) compared with meadows of rooted, submerged vegetation and bare substrate (Gagnon et al, 2019) In those Fucus patches, the high density of stickleback may increase intraspecific competition for food or space, possibly leading to selection for competitive ability (foraging or territory defense), rather than antipredator traits. We hypothesized that completely plated stickleback (a) make up a greater proportion of the stickleback community in bays where piscivorous fish are more abundant and (b) are less abundant in vegetated patches along the shoreline
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