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Environment and Predation Govern Fish Community Assembly In Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2016) 25, 1194–1205 RESEARCH Environment and predation govern fish PAPER community assembly in temperate streams Xingli Giam* and Julian D. Olden School of Aquatic and Fishery Sciences, ABSTRACT University of Washington, Seattle, Aim The elucidation of patterns and drivers of community assembly remains a WA 98105, USA fundamental issue in ecology. Past studies have focused on a limited number of communities at local or regional scales, thus precluding a comprehensive examination of assembly rules. We addressed this challenge by examining stream fish community assembly within numerous independent watersheds spanning a broad environmental gradient. We aimed to answer the following questions: (1) are fish communities structured non-randomly, and (2) what is the relative importance of environmental filtering, predator–prey interactions and interspecific competition in driving species associations? Location The conterminous USA. Methods We used null models to analyse species associations in streams. Non-random communities were defined as those where the summed number of segregated and aggregated species pairs exceeded the number expected by chance. We used species traits to characterize species dissimilarity in environmental requirements (ENV), identify potential predator–prey interactions (PRED) and estimate likely degree of competition based on species similarity in body size, feeding strategies and phylogeny (COMP). To evaluate the effect of environmental filtering, predation and competition on species associations, we related ENV, PRED and COMP to the degree of species segregation. Results The majority (75–85%) of watersheds had non-random fish communities. Species segregation increased with species dissimilarity in environmental requirements (ENV). An increase in competition strength (COMP) did not appear to increase segregation. Species pairs engaging in predator–prey interactions (PRED) were more segregated than non-predator– prey pairs. ENV was more predictive of the degree of species segregation than PRED. Main conclusions We provide compelling evidence for widespread non- random structure in US stream fish communities. Community assembly is governed largely by environmental filtering, followed by predator–prey interactions, whereas the influence of interspecific competition appears minimal. Applying a traits-based approach to continent-wide datasets provides a powerful approach for examining the existence of assembly rules in nature. *Correspondence: Xingli Giam, School of Keywords Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98105, USA. Assembly rules, co-occurrence, competition, ecological interactions, null E-mail: [email protected] models, North America, rivers, species traits. DOI: 10.1111/geb.12475 1194 VC 2016 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/geb Community assembly in freshwater fishes INTRODUCTION The quest to understand how species communities assemble results of these studies are context specific or represent gen- remains one of the most fundamental, and often controver- eral assembly patterns for each taxonomic group. sial, topics in ecology. Since the pivotal publication of Jared Emerging from the burgeoning literature on species assem- Diamond’s ‘The assembly of species communities’ (Diamond, bly was a meta-analysis indicating that most animal com- 1975), intense investigation has centred on the operation of munities had fewer species co-occurrence than expected by environmental filtering, the definition of assembly rules, the chance (Gotelli & McCabe, 2002). Notably, that study importance of null models and the role of species neutrality reported that negative species co-occurrences were more (Hubbell 2001; Leibold et al., 2004). Although their relative common in warm-blooded than cold-blooded animals, and roles are debated, key processes involved in community that among cold-blooded taxa, fish communities were prob- assembly include biotic interactions in the form of interspe- ably randomly structured. Despite representing a significant cific competition and predation (M’Closkey, 1978; Connor & advance in the field, the approach used by Gotelli & McCabe Simberloff, 1979), environmental filtering (Heino, 2013; Kraft (2002) was complicated by the fact that C-scores (which et al., 2015) and historical effects such as dispersal limitation quantify the degree of segregation or aggregation between a owing to physical barriers (Dias et al., 2014). These processes pair of species) were averaged over all species pairs. Gotelli & can shape co-occurrence patterns among species pairs Ulrich (2012) suggested that this approach might miss poten- (Gotelli & McCabe, 2002; Veech, 2014) and in whole meta- tially important pairwise associations between particular pairs communities (Leibold & Mikkelson, 2002; Almeido-Neto of species. Thus, the particular processes contributing to et al., 2008; Presley et al., 2010) as well as produce patterns community structure require further examination. in phylogenetic or trait dispersion within local communities Here, we examined the patterns and drivers of fish com- (Webb et al., 2011; Liu et al., 2013). munity assembly across diverse taxonomies (500 species) and Ecological theory and empirical evidence suggest that com- geographies (c. 8000 stream locations) in the conterminous petition and predation can limit co-occurrences of interacting USA. Freshwater fishes are a good model for community species (i.e. negative species associations) (Diamond, 1975; assembly analyses because watersheds represent naturally Englund et al., 2009). By contrast, environmental filtering and bounded, independent regions within which species disperse historical processes can either: (1) increase species co- and interact (Leprieur et al., 2011). This facilitated a occurrences when two or more species are adapted to similar robust test of the assembly rule concept using numerous environments, have similar niche requirements or have similar independent sets of interacting communities across a broad biogeographical histories, or (2) limit co-occurrences when environmental gradient. By combining pairwise species different species are adapted to different environments, have co-occurrence analyses with trait-based inference of species different niche requirements or disperse from different histori- interactions (McGill et al., 2006; Frimpong & Angermeier, cal pools (Heino, 2013; Dias et al., 2014). 2010), we aimed to answer the following questions: (1) are Null models are commonly used to test whether an freshwater fish communities structured non-randomly within observed pattern of species co-occurrence is likely to be real watersheds, and (2) what processes (i.e. environmental filter- or the result of random processes (Gotelli & Graves, 1996). ing, predator–prey interactions, interspecific competition) In freshwater ecosystems, for instance, Matthews (1982) drive species associations? By doing this we hope to advance found the number of negative associations among stream the current understanding of the nature of assembly rules in fishes to be no more than that derived from random com- freshwater fish communities. munity assembly. By contrast, Winston (1995) found mor- phologically similar fish species to co-occur less often than METHODS random (inferring the importance of interspecific competi- Species community dataset tion), whereas Peres-Neto (2004) demonstrated that environ- mental filtering shaped fish communities in Brazilian We compiled a database of species occurrence for 7846 sites streams. Divergent mechanisms influencing fish communities (i.e. fish communities) across 1502 watersheds (i.e. HUC8 are also evident in lakes, where studies support both environ- hydrological units as defined by the United States Geological mentally mediated patterns (Jackson et al., 1992) and assem- Survey) in the conterminous USA (Fig. 1). The sites were bly rules resulting from biological interactions (Englund surveyed between 1990 and 2012 by US federal government et al., 2009). Regardless of taxonomy, the mechanisms (or agencies [e.g. the EPA and Regional Environmental Monitor- lack thereof) governing how communities are assembled vary ing and Assessment Program (EMAP and REMAP), the EPA in both time and space (Lockwood et al., 1997). However, National Rivers and Streams Assessment (NRSA), the USGS most existing studies have investigated species co-occurrence National Water Quality Assessment Program (NAWQA)], and community assembly rules in a single region or interact- state natural resource and environmental agencies and uni- ing metacommunity (e.g. Connor & Simberloff, 1979; Mat- versity researchers (see Appendix S1 in Supporting Informa- thews, 1982; Jackson et al., 1992; Winston, 1995; Peres-Neto, tion for full list). All surveys were designed to characterize 2004; Englund et al., 2009). It remains unclear whether the the entire fish community, which includes both native and Global Ecology and Biogeography, 25, 1194–1205, VC 2016 John Wiley & Sons Ltd 1195 X. Giam and J. D. Olden Figure 1 (a) Map of 7846 candidate sites/fish communities located within 1502 watersheds. We selected only those watersheds with at least 10 sites and 10 species (224 watersheds containing 3670 communities) for our null model analysis because of statistical power considerations. Abiotic and biotic interactions that could structure fish communities include: (b) environmental filtering – many species such as central stoneroller
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