Melanie L. Blanchette

Omnivory and opportunism characterize food webs in a large dry-tropics river system

Abstract: We analyzed basal sources, trophic levels, and connectance in dry-season food webs on 4 rivers in the upper Burdekin catchment in the dry tropics of northeastern Australia. The region is characterized by episodic summer rainfall, and most of the annual river flow occurs in a short period. In the dry season, rivers typically contract into a series of water holes of varying permanence and hydrologic connectivity. We used stable-isotope and stomach-content analyses to identify trophic levels of macroinvertebrates and fish, and we used a mixing model (SIAR) to identify foodweb basal sources at each site. We found substantial variability among sites in basal-source contributions, trophic position of individual taxa, and foodweb structure, and sites from the same river often were as different as sites from different rivers. Important basal sources at different sites included allochthonous tree litter, autochthonous algae and macrophytes, and Fe-fixing bacteria. Many relationships between consumers and basal sources were not resolved in the mixing model, mainly because of extensive omnivory or isotopic overlap among sources. Nevertheless, our results show high variability of dry-tropics river communities that extends beyond previously described macroinvertebrate assemblages to the broader food web. However, the main components of the upper trophic levels were similar across sites, such that different lower trophic levels supported similar assemblages of top consumers. These tropical rivers were defined by omnivory and ecological opportunism, which may be adaptations to seasonal hydrological variability.

Macroinvertebrate assemblages in rivers of the Australian dry tropics are highly variable

Abstract Dryland rivers are globally widespread and regionally important, and understanding their ecology is critical for sustaining ecosystem processes and biodiversity. The dry tropics are characterized by episodic summer rainfall. Most of annual river flow occurs in a short period of time, after which rivers typically contract into a series of waterholes of varying permanence and hydrologic connectivity. We investigated how seasonal environmental factors affected macroinvertebrate assemblage composition in dry-tropics rivers at the river, site, and habitat scales. We assessed biophysical characteristics, including water physicochemistry, riparian-zone condition, and macroinvertebrate assemblage composition at 15 sites on 4 unimpounded rivers in the Burdekin catchment, north Queensland, Australia. We used permutational analysis of variance and multiresponse permutation procedures to identify differences between a priori groups, and illustrated the results with principle components analysis (biophysical data) and nonmetric multidimensional scaling (invertebrate assemblage data). Biophysical variables were spatially and temporally variable, and all rivers and most sites differed significantly. Macroinvertebrate assemblages also differed significantly among rivers, sites within rivers, habitats, and seasons. Assemblages from the same habitat but different sites were more similar than assemblages from different habitats within the same site. Thus, the dynamic environment of dryland river systems drives naturally dynamic and variable macroinvertebrate assemblages across a range of scales. The differences among sites from the same river were of particular importance from a monitoring standpoint because they indicated that extrapolating whole-river condition from a few sampled sites would be difficult.