Abstract
Generalist species commonly have a fundamental role in ecosystems as they can integrate spatially distinct habitats and food-web compartments, as well as control the composition, abundance and behavior of organisms at different trophic levels. Generalist populations typically consist of specialized individuals, but the potential for and hence degree of individual niche variation can be largely determined by habitat complexity. We compared individual niche variation within three generalist fishes between two comparable lakes in the Czech Republic differing in macrophyte cover, i.e. macrophyte-rich Milada and macrophyte-poor Most. We tested the hypothesis that large individual niche variation among generalist fishes is facilitated by the presence of macrophytes, which provides niches and predation shelter for fish and their prey items. Based on results from stable nitrogen (δ15N) and carbon (δ13C) isotopic mixing models, perch (Perca fluviatilis L.) and rudd (Scardinius erythrophthalmus (L.)) showed larger individual variation (i.e., variance) in trophic position in Milada as compared to Most, whereas no significant between-lake differences were observed for roach (Rutilus rutilus (L.)). Contrary to our hypothesis, all the three species showed significantly lower individual variation in the relative reliance on littoral food resources in Milada than in Most. Rudd relied significantly more whereas perch and roach relied less on littoral food resources in Milada than in Most, likely due to prevalent herbivory by rudd and prevalent zooplanktivory by perch and roach in the macrophyte-rich Milada as compared to macrophyte-poor Most. Our study demonstrates how the succession of macrophyte vegetation, via its effects on the physical and biological complexity of the littoral zone and on the availability of small prey fish and zooplankton, can strongly influence individual niche variation among generalist fishes with different ontogenetic trajectories, and hence the overall food-web structures in lake ecosystems.
Highlights
Generalist species that feed on multiple trophic levels commonly have a fundamental role in ecosystems
Generalist populations with wide trophic niche typically consist of highly specialized individuals that only use a subset of the entire population niche [5]
The occurrence and degree of individual niche variation, and potential for speciation and habitat coupling, within generalist populations is commonly controlled by niche availability and ecological opportunities [9,10,11]
Summary
Generalist species that feed on multiple trophic levels (cf. [1]) commonly have a fundamental role in ecosystems. Generalist populations with wide trophic niche typically consist of highly specialized individuals that only use a subset of the entire population niche [5] Such individual specialization can, in turn, have complex population-, community- and ecosystem-level effects, including speciation processes, intra- and inter-specific interactions as well as nutrient cycles and energy flow ([6,7,8] and references therein). There is limited empirical evidence of how macrophyte vegetation affects individual niche variation and the overall population-level niche width of generalist fishes. We use a novel opportunity to compare how different succession stages (i.e., abundance) of macrophyte beds influence trophic niche width of three generalist fishes with contrasting foraging strategies. The species can be expected to show markedly different responses to the presence or absence of macrophytes, both in terms of the trophic position occupied by the populations and individuals within, as well as in terms of their use of littoral and pelagic food resources
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