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

The trophic ecology of a desert river fish assemblage: influence of season and hydrologic variability

“…High hydrologic variability has been shown to correspond to a generalist pattern of feeding ecology among fishes (Poff and Allan , Blanchette et al. ) and may be a key factor mediating the trophic ecology and resultant foodweb structure associated with fish assemblages (Winemiller and Rose , Winemiller , Mims and Olden ), particularly those of the desert southwest of the United States (Olden et al. ).…”

“…Recent work utilizing stable isotope techniques has indicated terrestrial resources can be important contributions to food webs in some arid streams and rivers and that this integration may be mediated by seasonal and hydrologic variability (Zeug and Winemiller , Blanchette et al. , Sabo et al. ).…”

Spatial and seasonal variability in the trophic role of aquatic insects: An assessment of functional feeding group applicability

“…Studies that combined different approaches such as stable isotope and gut content analyses also show considerable dietary variation within taxa among sites, and concluded that omnivory and opportunistic feeding were widespread (e.g. Blanchette et al., ; Paice et al., ). Furthermore, our study showed that dissimilarities among the diet of each taxon at each site were not explained by the availability of trophic resources.…”

Condition factor variations over time and trophic position among four species of Characidae from Amazonian floodplain lakes: effects of an anomalous drought

“…Some authors have proposed that the present dominance of omnivorous species within the Amazon Basin may be an evolutionary response to the large diversity of food items and to the seasonality of their abundance and availability (Lowe-McConnell, 1999;Mérona and Rankin-de-Mérona, 2004;Blanchette et al, 2014). An extreme drought could result in a severe deterioration of the water quality, with high temperature and low dissolved oxygen, which would in turn have negative consequences for the quality of their food sources.…”