A stocktake of the number and size range of flora and fauna species of Magela Creek, Alligator Rivers Region, NT

Department of the Environment and Heritage, 2004

Internal Report 441
Camilleri C
Supervising Scientist Division

About the report

The Magela Creek and associated wetlands is located in the Alligator Rivers Region, which includes World Heritage Area Kakadu, approximately 200 km east of Darwin Northern Territory. Research, monitoring and management strategies in the most part have been directed towards the effects of uranium mining and milling at the Ranger minesite, on animals, plants and people living downstream of the mine. Protection of the rich diversity of flora and fauna of Magela Creek, in World Heritage Area Kakadu is a high priority. The most susceptible area for possible impact from the release of waste water from mining operations, is the wetland ecosystem downstream of the minesite. Magela Creek has been intensively studied for almost 30 years, and represents a unique long-term time-series of freshwater aquatic biological and ecological data.

The purpose of a stocktake of aquatic species and their sizes in the Magela Creek system is to provide life-history data for ecological risk assessment at population and community levels. Risk assessments for ecosystems are generally of two kinds: (1) for structured single populations, and (2) food chains. In both, bioassay results can be linked to population and/or community dynamics using life history knowledge. Structured population models can incorporate: specific dose-response survival and/or fecundity rates for each age/stage class; account for density dependence of selected ages/stages; and, ultimately, summarise population-level responses to toxicants through estimates of population growth rate or extinction risk. Food chain or web models may encompass the effects of toxicant kinetics or bioaccumulation factors incorporating: select dose-response and predator-prey functions; and simulating ecosystem dynamics and estimating the risk of adverse events. Population-level ecotoxicological risk assessments for environmental contaminants use data from standard laboratory bioassays. These data are incorporated into the parameters of a population model, then a risk assessment is performed analysing population-level differences between control and impacted samples. Bioassays on individuals are used to assess the impact of toxins on natural systems, and are usually expressed in terms of individual-level assessment endpoints such as growth, survivorship and fecundity. Stage-structured single-population models and food chain models are therefore critical in predicting population-level effects of toxicants on individuals.

Typically however, there are large gaps in knowledge on the demography of species within ecological communities for any site in the world. Fortunately, life history theory is central to demography and can be used to predict the dynamics of populations or individual-level responses to toxicants. A large number of empirically based relationships describe biological rates as simple functions of body size, such as maximum rate of population increase, animal density, physiology, morphology and sensitivity to toxicants. In the absence of demographic or physiological information, body size relations provide the most extensive and powerful generalisations, and can be used to fill gaps in knowledge. Additionally, this report gathers together for the first time, a comprehensive list identifying to species level all available size class information on the aquatic organisms living in or closely linked and associated with Magela Creek.