A new mechanistic approach for the further development of a population with established size bimodality

RESEARCH ARTICLE A new mechanistic approach for the further development of a population with established size bimodality Lisa Heermann1*, Donald L. DeAngelis2, Jost Borcherding1 1 Institute for Zoology of the University of Cologne, Department of General Ecology, Cologne, Germany, 2 US Geological Survey, Wetlands and Aquatic Research Center, Gainesville, FL, United States of America a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Heermann L, DeAngelis DL, Borcherding J (2017) A new mechanistic approach for the further development of a population with established size bimodality. PLoS ONE 12(6): e0179339. https://doi.org/10.1371/journal. pone.0179339 Editor: Elena Gorokhova, Stockholm University, SWEDEN Received: January 11, 2017 * Abstract Usually, the origin of a within-cohort bimodal size distribution is assumed to be caused by initial size differences or by one discrete period of accelerated growth for one part of the population. The aim of this study was to determine if more continuous pathways exist allowing shifts from the small to the large fraction within a bimodal age-cohort. Therefore, a Eurasian perch population, which had already developed a bimodal size-distribution and had differential resource use of the two size-cohorts, was examined. Results revealed that formation of a bimodal size-distribution can be a continuous process. Perch from the small size-cohort were able to grow into the large size-cohort by feeding on macroinvertebrates not used by their conspecifics. The diet shifts were accompanied by morphological shape changes. Intra-specific competition seemed to trigger the development towards an increasing number of large individuals. A stage-structured matrix model confirmed these assumptions. The fact that bimodality can be a continuous process is important to consider for the understanding of ecological processes and links within ecosystems. Accepted: May 26, 2017 Published: June 26, 2017 Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability Statement: Data can be found within Dryad at doi:10.5061/dryad.7h766. Funding: The study was financially supported by the German Research Foundation (http://www.dfg. de) to JB (DFG BO 1507/5-1, 5-2) and by the U.S. Geological Survey’s Greater Everglades Priority Ecosystems Science program (http://www.usgs. gov <http://www.usgs.gov>) to DLD (KA30CTF). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Introduction The way a population’s size distribution changes with time depends on biotic and abiotic factors influencing growth and development of individuals. Bimodality is a specific case of sizestructure that has often been studied (e.g., [1,2]) and that can be created and modified by changing impacts of these factors over time [2]. Besides size-selective mortality, or a broad initial size-distribution, evolving into a bimodal size distribution [3], differential growth is an important mechanism leading to bimodality [4]. Growth rates of individuals can be variously influenced, for instance, by size-dependent factors such as foraging ability or risk avoidance [4], or by individuality in use of food resources [5–7]. Extrinsic factors like environmental heterogeneity can equally well account for development of bimodality via different growth rates, and may be further adjusted through inter-and intraspecific competition and predation (for a review see [4]). While individuals are growing, these interactions of competitive and predatory processes may change distinctly and are an important component of ontogenetic niche shifts [8]. PLOS ONE | https://doi.org/10.1371/journal.pone.0179339 June 26, 2017 1 / 18 Development of an established size-bimodality Competing interests: The authors have declared that no competing interests exist. Ontogenetic diet shifts are common in a wide range of taxa; e.g., invertebrates, reptiles and fish (for a review see [9]). In particular, gape-limited predators, such as most piscivorous fish, undergo discrete changes in diet [10], whereby the size-dependence of these diet shifts leads to inter-individual variation in growth, dramatically influencing the size structure of the population [11], potentially enabling bimodality to emerge. Many studies on the development of bimodality either identified an initial size difference [12] or a single discrete accelerated growth period of one part of the population [1,13] as the origin of within-cohort size variation. However, in a population with a bimodal size distribution, there are often also some individuals with intermediate size. Can these individuals be assigned to the upper edge of the fraction of small individuals or to the lower edge of the large ones? Such a static view may, however, neglect dynamic processes that allow some individuals within the cohort to shift from the smaller to the larger fraction, beyond initial size differences or one-time growth accelerations. One consequence of a broad size-distribution (like bimodality) within an age-cohort can be mainly attributed to differential resource use and the consequential reduction of intra-cohort competition [13,14,15]. One can hypothesize the existence of medium-sized individuals that consume alternative food resources that are not used by the bulk of small or large individuals (e.g., feeding on macroinvertebrates if the majority of the conspecifics prey zooplankton). To test this hypothesis, a system in which a bimodal size distribution with differential usage of food resources by two size-classes already existed was examined to answer (1) if medium-sized individuals differ in food consumption compared to their small and large conspecifics of the size distribution, (2) if this alternative usage of food resources may allow a shift from the fraction of small-sized fish to the fraction of large-sized fish within a bimodal size-distribution, and (3) if abandonment of alternative food consumption (when entering the larger-sized fraction and starting to use their food resources) would then allow more individuals of the small-sized fraction to use this alternative pathway. Such a scenario would provide evidence that pathways exist for a continuous shift from the small to the large fraction within a bimodal age-cohort. A stage-structured matrix model was analysed numerically for the small size-cohort to support findings of the field study and further understand the mechanism behind a shift from the small to the large fraction within a bimodal agecohort. Material and methods Study site Field experiments were conducted in two ponds; pond 1 (P1) 0.6 ha and one pond 2 (P2) 0.7 ha in size, at a fish farm in Lohmar, Germany (50˚49’33 (...truncated)