Ecology, 101(2), 2020, e02940 © 2019 by the Ecological Society of America Local environmental factors influence beta-diversity patterns of tropical fish assemblages more than spatial factors 1,2,3 1 2 EDWIN O. LOPEZ -DELGADO , KIRK O. WINEMILLER, AND FRANCISCO A. VILLA-NAVARRO 1Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843 USA 2Grupo de Investigacion en Zoologıa, Facultad de Ciencias, Universidad del Tolima, Tolima, Colombia Citation: Lopez-Delgado, E. O., K. O. Winemiller, and F. A. Villa-Navarro. 2020. Local environmental fac- tors influence beta-diversity patterns of tropical fish assemblages more than spatial factors. Ecology 101(2):e02940. 10.1002/ecy.2940 Abstract. A major goal in ecology is to understand mechanisms that influence patterns of biodiversity and community assembly at various spatial and temporal scales. Understanding how community composition is created and maintained also is critical for natural resource management and biological conservation. In this study, we investigated environmental and spatial factors influencing beta diversity of local fish assemblages along the longitudinal gradi- ent of a nearly pristine Neotropical river in the Colombian Llanos. Standardized surveys were conducted during the low-water season at 34 sites within the Bita River Basin. Physical, chemi- cal, and landscape parameters were recorded at each site, and asymmetric eigenvector maps were used as spatial variables. To examine the relative influence of dispersal and environmental variables on beta diversity and its components, distance-based redundancy analysis (db-RDA) and variation partitioning analysis were conducted. We proposed that spatial scale of analysis and position within the river network would constrain patterns of beta diversity in different ways. However, results indicated that in this system, high beta diversity was consistent among species assemblages no matter the scale of analysis or position within the river network. Spe- cies replacement (turnover) dominated beta diversity, an indication of the importance of spe- cies sorting. These findings suggested that conservation of fish diversity in tropical rivers requires maintenance of both habitat heterogeneity (spatial variation in habitat conditions) and connectivity at the scale of entire river basins. Key words: Beta-diversity partition; fluvial gradient; Llanos; metacommunity; river network; species sorting; species turnover. metacommunities (Ricklefs and Schluter 1993, Leibold INTRODUCTION et al. 2004, Dray et al. 2012). Biodiversity is a broad concept that encompasses Theoretical and empirical research has given rise to more than just the number of species found in a certain four conceptual models of metacommunity dynamics, place and includes multiple levels of organization from each carrying different assumptions about processes genes to ecosystems as well as ecological and evolution- structuring local communities. The patch dynamics ary processes that maintain various components (Mou- model focuses on patch colonization-extinction pro- chet et al. 2010). The measurement of biodiversity has cesses that are affected by a trade-off between species been a major challenge in community ecology (Villeger colonization and competitive abilities, whereas the spe- et al. 2013), and multiple concepts and approaches have cies-sorting model emphasizes organisms’ ability to been adopted (Clements 1916, Gleason 1926, Whittaker select and occupy areas with suitable environmental con- 1960, Cody et al. 1975, Simberloff 1983, Hubbell 2001). ditions. The mass effects model focuses on source–sink Traditionally, ecologists have focused on factors that patch dynamics driven by high dispersal rates, habitat affect communities locally. However, during the last heterogeneity and life history differences among species, 25 yr a large number of studies have shown how local whereas the neutral model assumes interspecific niche communities exchange organisms to form metacommu- difference is unimportant and patches have stochastic nities, and how the processes of speciation, extinction, colonization-extinction dynamics (Leibold and Mikkel- dispersal, and environmental filtering interact at various son 2002, Leibold et al. 2004, Leibold and Chase 2017). spatial and temporal scales to structure In riverine systems, the concept of patch dynamics has been recognized as being generally applicable to benthic algae and aquatic macroinvertebrate metacommunities, Manuscript received 1 April 2019; revised 20 September the mass effect paradigm to intertidal invertebrates and 2019; accepted 18 October 2019. Corresponding Editor: F. Stephen Dobson. coral reef fishes, and the species-sorting paradigm 3 E-mail: [email protected] to numerous taxa, including algae, meiofauna, Article e02940; page 1 Article e02940; page 2 EDWIN O. LOPEZ-DELGADO ET AL. Ecology, Vol. 101, No. 2 macroinvertebrates, and fishes (Winemiller et al. 2010). Species replacement (i.e., turnover) and richness dif- In an African river floodplain, Jackson et al. (2013) ference can be calculated from dissimilarity coefficients found that mass effect, patch dynamics, and species-sort- and then used as measures of variation in assemblage ing concepts all were consistent with fish assemblage composition (b diversity; Legendre 2014). Species structure, and Lopez-Delgado et al. (2019) inferred that replacement describes the simultaneous gain and loss of species sorting had a strong influence on spatial varia- species along spatial or temporal gradients that poten- tion of fish assemblages in a Neotropical river system. tially are caused by environmental filtering, biotic inter- Recently, Heino et al. (2015b) and Brown et al. (2017) actions or historical factors (Podani and Schmera 2011, criticized research aimed at confirming alternative meta- Legendre and De Caceres 2013). Conversely, a species community paradigms and proposed the existence of richness difference occurs when one community includes structures beyond the four proposed by Leibold et al. a larger set of species than other, including cases in (2004). which one community is a nested subset of species that Advances in metacommunity ecology have been comprise the other (nestedness sensu Baselga 2012). accompanied by new conceptual and statistical methods Richness differences reflect contrasts in niche diversity that facilitate better understanding of how community across spatial or temporal scales and may involve barri- composition varies in space and time (Podani and Sch- ers to dispersal, competitive exclusion, or other mecha- mera 2011, Baselga et al. 2012, Podani et al. 2013, nisms affecting colonization and extinction (Ricklefs Legendre 2014). Spatial variation in species composition 2006, Schmera and Podani 2011, Baselga 2012, Legendre was originally defined as beta diversity by Whittaker 2014, Ricklefs and He 2016). (1960). Historically, spatial variation in assemblage com- The dendritic structure of fluvial networks has been position has been interpreted as species-specific shown to affect b-diversity patterns of microbial, plant, responses to environmental gradients, with locations amphibian, and fish assemblages in various types of having similar conditions predicted to have similar aquatic systems (Heino et al. 2015b, Tonkin et al. 2018). assemblages (Whittaker 1960). However, some studies For example, Vitorino Junior et al. (2016) inferred that have shown that geographic distance explains patterns environmental filtering and dispersal constraints of assemblage similarity better than local environmental affected patterns of species replacement in headwater factors, which suggests strong influence from dispersal tributaries of a tropical river drainage. In contrast, spe- dynamics (Baselga and Jimenez-Valverde 2007). Because cies turnover was lower in the river mainstem, likely beta diversity evaluates more than just species richness, because of greater habitat connectivity and dispersal and assemblage composition contains additional infor- allowing for a strong influence of the mass effect on mation reflecting ecological and evolutionary processes metapopulation dynamics. Datry et al. (2016) proposed (Baselga et al. 2012, Legendre and De Caceres 2013), that changes in b diversity along longitudinal fluvial gra- beta diversity has attracted great attention from commu- dients are related to the unidirectional flow of water and nity ecologists during the past 25 yr. Analysis of beta passive dispersal of organisms “downstream” in river diversity allows ecologists to test hypotheses about networks, generating an increase in species b diversity mechanisms that generate and maintain biodiversity from upstream to downstream. Consequently, spatial (Legendre and De Caceres 2013), which is crucial both variation in aquatic community structure is strongly for understanding how biodiversity is related to ecosys- influenced by dispersal (mass effect) and habitat selec- tem functioning and for biodiversity conservation. tion (species sorting). Headwater reaches of tributaries Over the last decade, several indices have been devel- are relatively isolated and, together with the unidirec- oped to decompose beta diversity into species replace- tional flow of water, have less dispersal, which should ment, richness difference, and nestedness components result in communities that more strongly reflect species (Baselga and Jimenez-Valverde 2007, Podani and Sch- sorting. Conversely, high habitat connectivity within the mera 2011, Schmera