Abstract
Variation in dispersal capacity may influence population genetic variation and relatedness of freshwater animals thus demonstrating how life‐history traits influence patterns and processes that in turn influence biodiversity. The majority of studies have focused on the consequences of dispersal variation in taxa inhabiting riverine systems whose dendritic nature and upstream/downstream gradients facilitate characterizing populations along networks. We undertook extensive, large‐scale investigations of the impacts of hydrological connectivity on population genetic variation in two freshwater bryozoan species whose dispersive propagules (statoblasts) are either attached to surfaces (Fredericella sultana) or are released as buoyant stages (Cristatella mucedo) and that live primarily in either lotic (F. sultana) or lentic environments (C. mucedo). Describing population genetic structure in multiple sites characterized by varying degrees of hydrological connectivity within each of three (or four) UK regions enabled us to test the following hypotheses: (1) genetic diversity and gene flow will be more influenced by hydrological connectivity in populations of C. mucedo (because F. sultana dispersal stages are retained); (2) populations of F. sultana will be characterized by greater genetic divergence than those of C. mucedo (reflecting their relative dispersal capacities); and (3) genetic variation will be greatest in F. sultana (reflecting a propensity for genetic divergence as a result of its low dispersal potential). We found that hydrological connectivity enhanced genetic diversity and gene flow among C. mucedo populations but not in F. sultana while higher overall measures of clonal diversity and greater genetic divergence characterized populations of F. sultana. We suggest that genetic divergence over time within F. sultana populations reflects a general constraint of releasing propagules that might eventually be swept to sea when taxa inhabit running waters. In contrast, taxa that primarily inhabit lakes and ponds may colonize across hydrologically connected regions, establishing genetically related populations. Our study contributes more nuanced views about drivers of population genetic structures in passively dispersing freshwater invertebrates as outlined by the Monopolization Hypothesis (Acta Oecologica, 23, 2002, 121) by highlighting how a range of demographic and evolutionary processes reflect life‐history attributes of benthic colonial invertebrates (bryozoans) and cyclically parthenogenetic zooplankton. In addition, growing evidence that genetic divergence may commonly characterize populations of diverse groups of riverine taxa suggests that organisms inhabiting lotic systems may be particularly challenged by environmental change. Such change may predispose riverine populations to extinction as a result of genetic divergence combined with limited dispersal and gene flow.Open Research Badges This article has earned an Open Data Badge for making publicly available the digitally‐shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.1tm8705.
Highlights
Freshwater environments are heterogeneous in space and time
The production of buoyant dispersal stages by residents of lotic systems may lead to extinction if downstream dis‐ persal greatly exceeds colonization of upstream regions. These is‐ sues led us to test the following hypotheses: 1. Patterns of genetic diversity and gene flow will be more in‐ fluenced by hydrological connectivity in lentic populations of C. mucedo than in lotic populations of F. sultana
There was no evidence that hydrological connectivity influenced other estimates of genetic diversity in both C. mucedo and F. sultana populations (ANOVA tests all with p > .05; Table S4)
Summary
Freshwater environments are heterogeneous in space and time. Natural drivers of such heterogeneity include flooding, drought, landslides, and the gradual infilling of ponds and lakes with sedi‐ ments. How variation in traits that impact dispersal may influence pop‐ ulation genetic variation and biodiversity in lentic environments has received less investigation In part this reflects, the low effec‐ tive dispersal that typifies well‐studied zooplankton taxa and which results from priority effects, local adaptation, and substantial egg banks (De Meester, Gomez, Okamura, & Schwenk, 2002; Mergeay, De Meester, Eggermont, & Verschure, 2011; Okamura & Freeland, 2002). Patterns of genetic diversity and gene flow will be more in‐ fluenced by hydrological connectivity in lentic populations of C. mucedo (a species with relatively high dispersal potential) than in lotic populations of F. sultana (a species with relatively low dispersal potential). C. mucedo undergoes extensive clonal propagation via fission of the gelatinous, caterpillar‐like colonies, and the production of buoyant statoblasts (“floatoblasts”) in large numbers at the end of the summer growing season (Figure 1a). If F. sultana were commonly encountered in relatively large numbers in vertebrate digestive tracts, they are likely to have been noted in the literature
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