1. In many intermittent, dryland rivers, fish are confined to isolated waterholes for much of the year. It is only during brief flow events, which typify the hydrology of these systems, that fish are able to move between waterholes and explore surrounding habitat. Because most of the river channel will dry afterwards, there is a strong advantage for selection of persistent waterholes. 2. Two hundred and fifteen individual fish of three common large-bodied species were tagged in two isolated waterholes in the Moonie River (Queensland, Australia) over 3 years. Their movements were monitored to identify the flow events that trigger fish movement between waterholes, differences in response among species and size classes and refuge selection preferences. 3. Some individuals of all species moved during flow events and others remained within the same waterhole. There was no clear upstream or downstream preference, and most individuals used a reach of up to 20 km, although some individuals ranged over more than 70 km in only several days. Above a threshold flow of 2 m above commence-to-flow level, timing of flow was more important than magnitude, with most movement occurring in response to the first post-winter flow event, independent of its magnitude and duration. Many of the fish that moved displayed philopatry and subsequently returned to their starting waterhole either by the end of a flow event or on subsequent events, suggesting ability to navigate and a preference for more permanent refuge pools. Maximising survival in a highly variable environment provides a plausible mechanism for maintaining these behaviours. 4. Modifications to both flow regime and hydrological connectivity may reduce movement opportunities for fish in intermittent rivers. Our findings show that fish in intermittent systems use networks of waterholes and that management and conservation strategies should aim to maintain movement opportunities at large spatial scales to preserve population resilience.
To examine how food resource availability links with natural variation in primary productivity in the Moonie River, south-west Queensland, the diets of two native Australian fish species (Nematalosa erebi and Macquaria ambigua) were examined from fifteen waterholes in February, May and September 2006. N. erebi diets reflected strong ‘boom and bust’ patterns of food consumption, with high concentrations of benthic (non-filamentous) algae during boom (flow) times, moving to higher concentrations of filamentous algae and detritus during bust (no flow) periods. M. ambigua diets were primarily dominated by aquatic insects in all sampling periods. Although there was no clear ‘boom to bust’ pattern in relation to flow, M. ambigua secondary prey consumption revealed a compensatory switch between high energy prey (crustaceans) during more productive periods with terrestrial insects during less productive periods. The ability of both species of fish to switch from high to low concentrations of food quality under a variable environmental background allows them to persist through both high productive and low productive periods. This interaction between native biota and variable ‘boom’ and ‘bust’ conditions, and how changes to the natural hydrology will affect it is an important consideration of any future water resource development plans.
Summary The biogeography of freshwater fish is determined in part by large‐scale filters such as phylogenetic history, the spatial arrangement of catchments and environmental variability. Species are filtered from the regional pool if they possess a combination of functional traits enabling them to persist in the local environment. This article aims to quantify the relative importance of these large‐scale filters in determining spatial variation in freshwater fish life‐history traits and functional trait composition of Australian river basins. We developed a database of 10 life‐history traits for 141 native freshwater fish species and compiled species distribution data for 123 river basins across the Australian continent. In order to partition the variation in the representation of life‐history trait into unique and overlapping components, we also quantified the degree of phylogenetic relatedness among species, the geographical arrangement of river basins throughout the landscape and 12 broad‐scale environmental factors. We then related life‐history trait composition to gradients of environmental variation by constrained multivariate ordination and simple linear regression. Our explanatory matrices accounted for 86.8% of the total variation in life‐history trait composition at the river basin scale, of which 59.4% could be attributed to phylogeny and spatially structured environmental variation. This component represents the overlap among the broad‐scale filtering processes of phylogenetic history, spatial autocorrelation and environmental variability in accounting for the distribution of life‐history traits across Australian river basins. Our analysis showed strong associations between suites of life‐history traits that define generation time and reproductive output and a strong climate–hydrological gradient across the landscape. We also showed significant correlations between specific environmental variables and a number of key life‐history traits that highlight the importance of trait‐mediated environmental filters at broad spatial scales. This study advances our conceptual understanding of broad‐scale community assembly theory and has revealed trait–environment relationships at scales relevant to restoration and conservation of aquatic biodiversity. Our study provides greater insight into the determinants of spatial variation in fish species distributions and potentially addresses key scientific challenges, such as understanding how fish communities are assembled, and identifies the potential threats to, and responses of, these communities caused by environmental change.
Understanding the biogeographic and phylogenetic basis to interspecific differences in species’ functional traits is a central goal of evolutionary biology and community ecology. We quantify the extent of phylogenetic influence on functional traits and life‐history strategies of Australian freshwater fish to highlight intercontinental differences as a result of Australia's unique biogeographic and evolutionary history. We assembled data on life history, morphological and ecological traits from published sources for 194 Australian freshwater species. Interspecific variation among species could be described by a specialist–generalist gradient of variation in life‐history strategies associated with spawning frequency, fecundity and spawning migration. In general, Australian fish showed an affinity for life‐history strategies that maximise fitness in hydrologically unpredictable environments. We also observed differences in trait lability between and within life history, morphological and ecological traits where in general morphological and ecological traits were more labile. Our results showed that life‐history strategies are relatively evolutionarily labile and species have potentially evolved or colonised in freshwaters frequently and independently allowing them to maximise population performance in a range of environments. In addition, reproductive guild membership showed strong phylogenetic constraint indicating that evolutionary history is an important component influencing the range and distribution of reproductive strategies in extant species assemblages. For Australian freshwater fish, biogeographic and phylogenetic history contribute to broad taxonomic differences in species functional traits, while finer scale ecological processes contribute to interspecific differences in smaller taxonomic units. These results suggest that the lability or phylogenetic relatedness of different functional traits affects their suitability for testing hypothesis surrounding community level responses to environmental change.
The ecology of dryland rivers is driven by their highly variable hydrology, particularly flooding regimes, whereby intermittent floods typically generate 'booms' of primary and secondary productivity, including massive fish production. We tested these concepts in the Moonie River, Australia, using the percichthyid, Macquaria ambigua, a dryland river species known to display pronounced 'boom and bust' abundance patterns in response to floodplain inundation followed by extended periods of low to no channel flow. We expected that body condition (as measured by whole body lipid content) and biomass of M. ambigua would be related to prey biomass, and that these factors would all 'spike' following widespread flooding. Instead we found more subtle responses. There were 'booms' in biomass of Macrobrachium and zooplankton, two important food items, whereas M. ambigua maintained relatively low but sustained lipid and biomass levels following flooding. It appears that instead of a 'boom' in fish biomass, abundant invertebrate food resources and sustained lipid levels contributed to high survivorship of this species during the 'bust' period over cool dry months.
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