Hydrological connectivity and environment characteristics explain spatial variation in fish assemblages in a wet–dry tropical river

“Diverse habitats across catchments support diverse fish assemblages, and this diversity is important for community resilience which supports the provision of ecosystem services (Hooper et al., 2005; Oliver et al., 2015).”

“Alterations to the natural flow regime that disrupt this annual ‘pulse’ are therefore likely to have significant impacts on lotic fish communities (Poff et al., 1997; Douglas et al., 2019).”

“To understand how the variance explained in these two GDMs was partitioned between hydrological connectivity and environmental variables we then performed a variance partitioned GDM using connectivity and environmental variables as predictors (Borcard et al., 1992; Anderson & Crib-ble, 1998).”

“Freshwater ecosystems are already experiencing greater rates of biodiversity loss than terrestrial systems (Tickner et al., 2020) and face many current and future anthropogenic pressures globally, including water resource development and climate change (Reid et al., 2019).”

“Expanding human populations create increased demand for new water resource projects, which have the potential to substantially impact fresh-water fish assemblages (Nilsson et al., 2005).”

“…and position along the river network, changes to flow may alter the environment through sediment erosion and deposition, changes to depth, bank shape, and vegetation via scouring, and changes to aquatic vegetation cover (Bunn & Arthington, 2002; Schleiss et al., 2016; Spurgeon et al., 2019).”

“To explore whether variation in fish species presence/absence across the catchment was related to their feeding and reproductive trait classifications, we again used dbRDA (Legendre & Anderson, 1999).”

“We calculated fish species turnover (Simpson’s pair-wise dissimilarity matrix using the ‘betapart’ package in R Baselga & Orme, 2012; R Core Team, 2021)) from the presence and absence of species across sites.”

“Biodiversity plays a critical role in the long-term resilience of ecosystems, allowing them to maintain the enhanced state through which the production of ecosystem services occurs, including social and economic functions (Elmqvist et al., 2003; Douglas et al., 2019).”

“…traits, along with biological interactions and biogeographic features (e.g. natural rock barriers) in the environment, lead to spatial variation in species composition filling the variety of ecological niches that occur across environmental gradients (Chase & Myers, 2011; Zeni & Casatti, 2014).”

“Equally, finer-scale environmental characteristics, such as substrate composition and water depth, vary within riverine landscapes to create a complex array of habitats that support diverse fish assemblages (Wellborn et al., 1996; Tokeshi & Arakaki, 2012; Booker & Graynoth, 2013).”

“Diverse habitats across catchments support diverse fish assemblages, and this diversity is important for community resilience which supports the provision of ecosystem services (Hooper et al., 2005; Oliver et al., 2015).”

“Habitat specialisation, dispersal limitations, and biological interactions are also likely to influence where species are found (Jackson et al., 2001).”

“Freshwater ecosystems are already experiencing greater rates of biodiversity loss than terrestrial systems (Tickner et al., 2020) and face many current and future anthropogenic pressures globally, including water resource development and climate change (Reid et al., 2019).”

“To examine the individual and shared roles of hydrological connectivity and environmental characteristics in predicting fish species composition (abundance CPUE), we used Generalized Dissimilarity Modelling (GDM; Ferrier et al., 2007).”

“…from impacts of anthropogenic activities, such as farming and mining (which are occurring or have occurred in the past in the upland reaches of the Mitchell River and its tributaries), is therefore important for conserving fish diversity in the catchment (Meyer et al., 2007; Wantzen & Mol, 2013).”

“The habitat requirements of fish are influenced by physiological and functional traits (Tilman, 2001).”

“…a wide range of water quality and habitat conditions and possess a reproductive strategy that allows them to rapidly establish in a new environment (Canonico et al., 2005; Russell et al., 2012), with a substantial amount of potential habitat identified in the Mitchell River catchment (O’Mara et…”

“…in this study, but it is important to consider that taxonomic composition may also be influenced by life-history niche overlap, predator–prey dynamics, and competition for food between organisms occupying similar trophic niches (Jackson et al., 2001; Olden et al., 2006; Mac-Dougall et al., 2018).”

“…and position along the river network, changes to flow may alter the environment through sediment erosion and deposition, changes to depth, bank shape, and vegetation via scouring, and changes to aquatic vegetation cover (Bunn & Arthington, 2002; Schleiss et al., 2016; Spurgeon et al., 2019).”

“Reductions to flow from climate change and water resource developments are likely to alter hydrological connectivity in rivers throughout the world, including the Mitchell catchment (Döll & Zhang, 2010; Jaeger et al., 2014; O’Mara et al., 2021).”

“Reductions to flow from climate change and water resource developments are likely to alter hydrological connectivity in rivers throughout the world, including the Mitchell catchment (Döll & Zhang, 2010; Jaeger et al., 2014; O’Mara et al., 2021).”

“Spatial and temporal variation in hydrological connectivity that is driven by the natu-ral flow regime provides a diversity of instream habitat conditions (Opperman et al., 2010; Carrara et al., 2014).”

“This is important for the conservation of habitat specialist species and unique headwater assemblages, which may be most at risk from habitat change (Buisson et al., 2008; Nyboer et al., 2019).”

“Equally, finer-scale environmental characteristics, such as substrate composition and water depth, vary within riverine landscapes to create a complex array of habitats that support diverse fish assemblages (Wellborn et al., 1996; Tokeshi & Arakaki, 2012; Booker & Graynoth, 2013).”

“The proportion of habitat generalist and habitat specialist species in the community is likely to influence the strength of the biological–environmental relationship (Büchi & Vuilleumier, 2014), which is likely to be spatially variable across the catchment according to species turnover.”

“…from impacts of anthropogenic activities, such as farming and mining (which are occurring or have occurred in the past in the upland reaches of the Mitchell River and its tributaries), is therefore important for conserving fish diversity in the catchment (Meyer et al., 2007; Wantzen & Mol, 2013).”

“Water resource developments that alter the timing or magnitude of the ‘flood pulse’ may therefore impact the lateral movements of these migratory species (Hurd et al., 2016).”

“Rapid declines in elevation downstream of head-water sites can result in low connectivity between reaches close in river distance, limiting dispersal and contributing to turnover (Catalano et al., 2007; Hitt & Angermeier, 2008; White et al., 2012; Allan et al., 2020).”

“Functional traits, including trophic guild and spawning substrate preference, determine the habitat conditions required for processes such as feeding and reproduction (Blanck et al., 2007; Zeni & Casatti, 2014).”

“Spatial and temporal variation in hydrological connectivity that is driven by the natu-ral flow regime provides a diversity of instream habitat conditions (Opperman et al., 2010; Carrara et al., 2014).”

“Spatial replacement of fish species occurs in response to conditions along these environmental gradients and can be measured as turnover (Soininen, 2010; Zbinden & Matthews, 2017).”

“Depth, substrate type, and presence of woody debris were amongst the strongest predictor variables, which is consistent with observations for these species from other tropical Australian river systems (Kennard et al., 2007; Keller et al., 2019).”

“These relationships could be used by managers to predict assemblage responses to habitat changes caused by altered flow, such as predictive studies on individual species by Stewart-Koster et al., (2011), Chan et al., (2012), and Turschwell et al., (2019).”

“…traits, along with biological interactions and biogeographic features (e.g. natural rock barriers) in the environment, lead to spatial variation in species composition filling the variety of ecological niches that occur across environmental gradients (Chase & Myers, 2011; Zeni & Casatti, 2014).”

“Consistent with previous research, turnover in headwaters was found to be high (Carvalho & Tejerina ‐ Garro, 2015; Richardson, 2019), suggesting that upland sites select for unique biological assemblages compared to downstream reaches.”

“…the influence of hydrological connectivity on habitat characteristics and fish community composition is important for habitat conservation to maintain fish diversity in the face of climate change and other anthropogenic pressures that will alter hydrological connectivity (Petchey et al., 2009).”

“Spatial replacement of fish species occurs in response to conditions along these environmental gradients and can be measured as turnover (Soininen, 2010; Zbinden & Matthews, 2017).”

“Habitat change will likely cause changes in the fish community of the Mitchell River due to altered habitats selecting for different physiological and functional traits via environmental filtering (Erős et al., 2009).”

“…via variations in river flow, elevation, and landscape topology which may further mediate fish assemblage composition (Arthington et al., 2014; Liu & Wang, 2018; Rogosch & Olden, 2019; Wang et al., 2020), particularly in systems with extreme hydrological variation such as the wet–dry tropics…”

“Indigenous Australians from several communities make up 23% of the catchment population and harvest fish, turtles and other resources from wetlands, creeks and river channels as an important part of their diet (Jackson et al., 2014).”

“The trait classifications that were used included those thought to be most likely to influence where a species is found (Hitt & Angermeier, 2008; Sternberg & Kennard, 2013), including classifications of trophic guild, reproductive movement requirements, and spawning substrate and frequency.”

“…patterns that produce high wet season flows and low (or no) dry season flows, where wet season flows facilitate longitudinal and lateral connectivity and low dry season flows create a river network that is highly spatially variable in connectivity (McJannet et al., 2014; Boulton et al., 2017).”

“Environmental gradients occur along river networks from headwaters to the river mouth and are evident in correlations between elevation or distance to river mouth and changes in environmental characteristics (Stewart-Koster et al., 2007; Esselman & Allan, 2010; da Silva Almeida et al., 2022).”

“The trait classifications that were used included those thought to be most likely to influence where a species is found (Hitt & Angermeier, 2008; Sternberg & Kennard, 2013), including classifications of trophic guild, reproductive movement requirements, and spawning substrate and frequency.”

“Environmental gradients occur along river networks from headwaters to the river mouth and are evident in correlations between elevation or distance to river mouth and changes in environmental characteristics (Stewart-Koster et al., 2007; Esselman & Allan, 2010; da Silva Almeida et al., 2022).”

“…dispersal is still likely to occur for some common species and species-specific responses to altered connectivity downstream of headwater sites can therefore result in composition changes to both headwater and broader catchment fish communities (Slawski et al., 2008; Kashiwagi & Miranda, 2009).”

“Species responses to changes in environmental and hydrological conditions may be impacted by the presence of invasive species (Dominguez Almela et al., 2021).”

“Migratory species play a key role in this energy transfer by using the ‘flood pulse’ to access wetlands, but are likely to experience a trade-off between accessing wetlands to utilize abundant food sources and contributing to the reproducing population (Roberts et al., 2019).”

“Biodiversity plays a critical role in the long-term resilience of ecosystems, allowing them to maintain the enhanced state through which the production of ecosystem services occurs, including social and economic functions (Elmqvist et al., 2003; Douglas et al., 2019).”

“…of water quality and habitat conditions and possess a reproductive strategy that allows them to rapidly establish in a new environment (Canonico et al., 2005; Russell et al., 2012), with a substantial amount of potential habitat identified in the Mitchell River catchment (O’Mara et al., 2023).”

“…in river flow, elevation, and landscape topology which may further mediate fish assemblage composition (Arthington et al., 2014; Liu & Wang, 2018; Rogosch & Olden, 2019; Wang et al., 2020), particularly in systems with extreme hydrological variation such as the wet–dry tropics of northern…”

“This is important for the conservation of habitat specialist species and unique headwater assemblages, which may be most at risk from habitat change (Buisson et al., 2008; Nyboer et al., 2019).”

“Hydrological connectivity affects fish movement opportunity, and highly connected sites are often used as movement corridors (Hitt & Angermeier, 2008; O’Mara et al., 2021).”

“Turnover of fish species is often higher in headwaters, suggesting that upland reaches contain unique fish assemblages in response to the different environmental conditions at higher elevation (Tondato & Súarez, 2010; Herrera-Pérez et al., 2019).”

“Climate Climate change is also expected to modify hydrological connectivity through changes to the flow regime in regions with seasonally variable flow, including an increase in low or no flow spells in regions where rainfall is predicted to decrease (Jenkins et al., 2011; Jaeger et al., 2014).”

“Depth, substrate type, and presence of woody debris were amongst the strongest predictor variables, which is consistent with observations for these species from other tropical Australian river systems (Kennard et al., 2007; Keller et al., 2019).”

“…and position along the river network, changes to flow may alter the environment through sediment erosion and deposition, changes to depth, bank shape, and vegetation via scouring, and changes to aquatic vegetation cover (Bunn & Arthington, 2002; Schleiss et al., 2016; Spurgeon et al., 2019).”

“Physiological traits determine the kind of physical habitat a given species requires, such as fish body shape and preferred flow velocity, with fish in high flow velocity streams having smaller, more stream-lined bodies than those in low flow velocity streams (de Barros et al., 2019).”

“…patterns that produce high wet season flows and low (or no) dry season flows, where wet season flows facilitate longitudinal and lateral connectivity and low dry season flows create a river network that is highly spatially variable in connectivity (McJannet et al., 2014; Boulton et al., 2017).”

“…by hydrological connectivity via variations in river flow, elevation, and landscape topology which may further mediate fish assemblage composition (Arthington et al., 2014; Liu & Wang, 2018; Rogosch & Olden, 2019; Wang et al., 2020), particularly in systems with extreme hydrological variation…”

“…dispersal is still likely to occur for some common species and species-specific responses to altered connectivity downstream of headwater sites can therefore result in composition changes to both headwater and broader catchment fish communities (Slawski et al., 2008; Kashiwagi & Miranda, 2009).”

“Rapid declines in elevation downstream of head-water sites can result in low connectivity between reaches close in river distance, limiting dispersal and contributing to turnover (Catalano et al., 2007; Hitt & Angermeier, 2008; White et al., 2012; Allan et al., 2020).”

“…and landscape topology which may further mediate fish assemblage composition (Arthington et al., 2014; Liu & Wang, 2018; Rogosch & Olden, 2019; Wang et al., 2020), particularly in systems with extreme hydrological variation such as the wet–dry tropics of northern Australia (Pusey et al.,…”

“These relationships could be used by managers to predict assemblage responses to habitat changes caused by altered flow, such as predictive studies on individual species by Stewart-Koster et al., (2011), Chan et al., (2012), and Turschwell et al., (2019).”

“Depth, substrate type, and presence of woody debris were amongst the strongest predictor variables, which is consistent with observations for these species from other tropical Australian river systems (Kennard et al., 2007; Keller et al., 2019).”

“Turnover of fish species is often higher in headwaters, suggesting that upland reaches contain unique fish assemblages in response to the different environmental conditions at higher elevation (Tondato & Súarez, 2010; Herrera-Pérez et al., 2019).”

“…were no invasive species caught at the sites sampled in this study, the river is currently threatened with the spread of newly established populations of invasive cichlids Oreochromis mossambicus and Pelmatolapia mariae , which have inhabited the upper Walsh River since 2017 (O’Mara et al., 2023).”

“Environmental gradients occur along river networks from headwaters to the river mouth and are evident in correlations between elevation or distance to river mouth and changes in environmental characteristics (Stewart-Koster et al., 2007; Esselman & Allan, 2010; da Silva Almeida et al., 2022).”

“Equally, finer-scale environmental characteristics, such as substrate composition and water depth, vary within riverine landscapes to create a complex array of habitats that support diverse fish assemblages (Wellborn et al., 1996; Tokeshi & Arakaki, 2012; Booker & Graynoth, 2013).”

“These mac-rophyte-dense areas offered a complex habitat structure and may have restricted phytoplankton development due to reduced light penetration and nutrient competition, resulting in lower phytoplankton densities (O’Mara et al., 2024).”

“Such multi-scale responses can weaken simple relationships between fish community composition and individual longitudinal or environmental gradients, particularly because the relative influence of local environmental and spatial processes can change with hydrological conditions 9,17 .”

“Other endemic Malagarasi fishes include cichlids from the genera Oreochromis and Serranochromis , together with endemic representatives of other fish groups, including catfishes, cyprinids, killifishes, and spiny eels (Bigirimana et al. 2024).”

“…while ponds and impoundments embedded within drainage networks can retain portions of the stream fauna, especially when exchange occurs through flooding, outlet connections, or recent formation from formerly flowing reaches (Schlosser and Kallemeyn 2000; Bower et al. 2019; O’Mara et al. 2024).”

“Whether regulated tropical rivers retain sufficient hydrological connec-tivity—through spill events, seasonal drawdown, and fish movement—to buffer these effects remains incompletely resolved (Walsh et al., 2022; O’Mara et al., 2024).”

“However, spatial predictors in fluvio-marine landscapes may simultaneously reflect hydrological connectivity, habitat configuration, dispersal pathways, and unmeasured environmental gradients (Carrara et al. 2012; O’Mara et al. 2024).”