Consumer–resource coupling in wet–dry tropical rivers

Abstract

Summary1. Despite implications for top-down and bottom-up control and the stability of food webs, understanding the links between consumers and their diets remains difficult, particularly in remote tropical locations where food resources are usually abundant and variable and seasonal hydrology produces alternating patterns of connectivity and isolation. 2. We used a large scale survey of freshwater biota from 67 sites in three catchments (Daly River, Northern Territory; Fitzroy River, Western Australia; and the Mitchell River, Queensland) in Australia's wet-dry tropics and analysed stable isotopes of carbon (d 13 C) to search for broad patterns in resource use by consumers in conjunction with known and measured indices of connectivity, the duration of floodplain inundation, and dietary choices (i.e. stomach contents of fish). 3. Regression analysis of biofilm d 13 C against consumer d 13 C, as an indicator of reliance on local food sources (periphyton and detritus), varied depending on taxa and catchment. 4. The carbon isotope ratios of benthic invertebrates were tightly coupled to those of biofilm in all three catchments, suggesting assimilation of local resources by these largely nonmobile taxa. 5. Stable C isotope ratios of fish, however, were less well-linked to those of biofilm and varied by catchment according to hydrological connectivity; the perennially flowing Daly River with a long duration of floodplain inundation showed the least degree of coupling, the seasonally flowing Fitzroy River with an extremely short flood period showed the strongest coupling, and the Mitchell River was intermediate in connectivity, flood duration and consumer-resource coupling. 6. These findings highlight the high mobility of the fish community in these rivers, and how hydrological connectivity between habitats drives patterns of consumer-resource coupling.

Temporal variation in benthic primary production in streams of the Australian wet–dry tropics

“…This lag in the peak of macroinvertebrate biomass supports the idea that benthic algae are an important food resource during the dry season (Jardine et al, 2013). In addition, tracking of benthic chl a biomass by macroinvertebrate biomass suggests strong consumer-resource coupling, which is supported by an extensive d 13 C and d 15 N stable isotope study across a broad range of sites (including intermittent and perennial) in northern Australia (Jardine et al, 2012b). Following the peak in the early-dry season, macroinvertebrate biomass steadily declines as the dry season progresses, likely due to habitat contraction and disconnection from potential upstream colonisers, increased predation pressure, and emergence (Garcia et al, 2011).…”

“…Seasonal changes in flow and hydrological connectivity are key factors regulating ecological structure and processes in large floodplain tropical rivers (Douglas et al, 2005;Jardine et al, 2012b). However, little is known about how this distinctive seasonal hydrology, and therefore connectivity, is reflected in benthic productivity in low-order streams.…”

“…Seasonal patterns in low-order streams (tropical and temperate) are largely unquantified, generally anecdotal and/or extrapolated from landscape-scale patterns . Understanding the changes in benthic productivity is vital for identifying potential production 'bottlenecks' when parts of the landscape are reliant on local energy sources (Jardine et al, 2012b) and hence more susceptible to local anthropogenic impacts.…”

The persistence of subsistence: qualitative social-ecological modeling of indigenous aquatic hunting and gathering in tropical Australia

“…Firstly, at a general level, modeling is an increasingly prevalent tool in bureaucratic decision-making processes, and so understanding how models may be generated and applied with respect to indigenous peoples (who are often the subject of such decisions) is an important issue (Barber and Jackson 2015). Secondly, there is a tendency in the conceptual modeling of aquatic environments to limit the representation of socioeconomic systems, especially human decision-making (Douglas et al 2005, Jardine et al 2012, and instead treat humans as a type of boundary condition without reciprocal links to floodplain dynamics, food webs, flow ecology, etc. (Di Baldassarre et al 2013).…”

Maintaining customary harvesting of freshwater resources: sustainable Indigenous livelihoods in the floodplains of northern Australia

“…This energy transfer is not just for the wet season but persists into the dry season (Jardine et al 2012b). The East Alligator has a relatively long period of inundation (Ward et al 2014) and recent work from across northern Australia has found fish from systems with longer inundation periods are less dependent on local carbon sources, rather than those from the floodplain (or estuary) (Jardine et al 2012a). Thus maintenance of floodplain condition and connectivity is crucial to support these harvested species.…”