title: Body size drives allochthony in food webs of tropical rivers.
abstract: 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.
authors.affiliation: School of Environment and Sustainability, University of Saskatchewan, Saskatoon, SK, Canada. tim.jardine@usask.ca.|Australian Rivers Institute, Griffith University, Nathan, QLD, Australia. tim.jardine@usask.ca.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Jardine
authors.affiliation: Charles Darwin University, Darwin, NT, Australia.
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Rayner
authors.affiliation: Edith Cowan University, Perth, WA, Australia.
authors.forename: Neil E
authors.initials: NE
authors.lastname: Pettit
authors.affiliation: Australian Rivers Institute, Griffith University, Nathan, QLD, Australia.
authors.forename: Dominic
authors.initials: D
authors.lastname: Valdez
authors.affiliation: Australian Rivers Institute, Griffith University, Nathan, QLD, Australia.
authors.forename: Douglas P
authors.initials: DP
authors.lastname: Ward
authors.affiliation: Department of Sustainability, Environment, Water, Population and Communities, Parks Australia, Jabiru, NT, Australia.
authors.forename: Garry
authors.initials: G
authors.lastname: Lindner
authors.affiliation: School of Earth and Environment, University of Western Australia, Perth, WA, Australia.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Douglas
authors.affiliation: Australian Rivers Institute, Griffith University, Nathan, QLD, Australia.
authors.forename: Stuart E
authors.initials: SE
authors.lastname: Bunn
mesh_terms: D000818:Animals; D049628:Body Size; D017753:Ecosystem; D005399:Fishes; D020387:Food Chain; D045483:Rivers
keywords: Connectivity; Estuary; Floodplains; Savanna; Waterholes; autochthony