Do food chain lengths in tropical floodplain rivers of northern Australia fit with expectations?

The lengths of food chains observable in nature are believed to be driven by the amount of energy available at the base of the food web (productivity hypothesis), the areal extent of the food web (ecosystem size hypothesis), or some combination thereof (productive-space hypothesis). While larger, more productive ecosystems are believed to maintain longer food chains due to more available energy to be transferred to higher trophic levels, a counter hypothesis invokes widespread omnivory to be linked to high productivity (due to greater feeding opportunities at low trophic levels), thus diminishing the length of food chains in productive systems. Tropical floodplains of northern Australia are inherently assumed to be highly productive compared with temperate systems due to differences in temperature and light regimes. To compare food chain lengths with published estimates from temperate regions, we sampled producers and consumers from a total of 60 sites in three large river systems of Australia's wet-dry tropics. Samples were analysed for stable nitrogen isotopes and the trophic position of the top predator in each system was calculated. Across sites, food chain lengths showed a similar distribution (range = 2.8 to 5.0, primary consumers = trophic level 2) and mean (3.8 ± 0.6 SD) when compared with estimates from elsewhere, with piscivorous fish barramundi (Lates calcarifer) and longtom (Strongylura krefftii) occupying top predator positions when present. These findings suggest that omnivory and seasonal limits to ecosystem size (late dry season contraction of available habitat) balance any positive effects of higher productivity on the ability of these systems to support long food chains. Further analyses will examine factors (size, ecosystem type, etc.) that may be responsible for the observed variation among sites.