Body size drives allochthony in food webs of tropical rivers
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Food web subsidies from external sources (“allochthony”) can support rich biological diversity and high secondary and tertiary production in aquatic systems, even those with low rates of primary production. However, animals vary in their degree of dependence on these subsidies. We examined dietary sources for aquatic animals restricted to refugial habitats (waterholes) during the dry season in Australia’s wet–dry tropics, and show that allochthony is strongly size dependent. While small-bodied fishes and invertebrates derived a large proportion of their diet from autochthonous sources within the waterhole (phytoplankton, periphyton, or macrophytes), larger animals, including predatory fishes and crocodiles, demonstrated allochthony from seasonally inundated floodplains, coastal zones or the surrounding savanna. Autochthony declined roughly 10% for each order of magnitude increase in body size. The largest animals in the food web, estuarine crocodiles (Crocodylus porosus), derived ~80% of their diet from allochthonous sources. Allochthony enables crocodiles and large predatory fish to achieve high biomass, countering empirically derived expectations for negative density vs. body size relationships. These results highlight the strong degree of connectivity that exists between rivers and their floodplains in systems largely unaffected by river regulation or dams and levees, and how large iconic predators could be disproportionately affected by these human activities.
KeywordsConnectivity Estuary Floodplains Waterholes Savanna autochthony
The research team would like to thank Traditional Owners and other Bininj/Mungguy (local Indigenous people) for their support and assistance in undertaking this research. In particular, we would like to thank Steve Winderlich, Anne O’Dea, Jonathon Nadji, Sean Nadji, Fred Hunter, Calvin Murakami, Rob Nobobbob, William Alderson, Samantha Deegan, Kakadu National Park staff, and the Gundjeihmi Aboriginal Corporation as well as Quentin Allsop, Michael Hammer, Peter Kyne, Brad Pusey, Mark Kennard and staff of the RV Solander in the Alligator Rivers Region, and Richard Hunt, Alisha Steward, Stephen Hamilton, Viv Sinnamon and the Kowanyama Aboriginal Land and Natural Resource Management Organization in the Mitchell River. Rene Diocares and Vanessa Fry assisted with sample processing and analysis, and Prabin Rokaya assisted in creating figures. All handling and humane treatment of crocodiles was conducted under Australian government and Kakadu National Park codes. This research was funded by the Australian Government through the National Environmental Research Program’s Northern Australia Hub.
Author contribution statement
TJ, DW, MD, and SB conceived the idea. TJ, TR, NP, and SB refined the field study design. TJ, TR, NP, DV, GL, and SB conducted field work, with DV and GL leading the crocodile component. DV conducted laboratory analyses. TJ wrote the manuscript with editorial input from all authors.
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