An ecological risk assessment of the major weeds on the Magela Creek floodplain, Kakadu National Park


Internal Report 439
Bayliss P & Walden D
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts

About the report

The negative impact of invasive weeds on ecosystem services and biodiversity is probably the most significant threat facing tropical wetlands today. Across the tropics there are many wetland weed species, some of them, including Mimosa pigra, Salvinia molesta and Urochloamutica being widely distributed, if not pan-tropical. Such species have attracted a great deal of attention with the expenditure of large sums of money and effort on control techniques (Finlayson & Mitchell 1981, Storrs & Finlayson 1997, Douglas et al 1998). Fourteen of the top 18 environmental weeds in Australia invade wetlands (Humphries et al 1991). Twelve of these species are currently found in the Northern Territory. For Kakadu National Park, Storrs (1996) lists 15 species of high priority weeds that exist in small to large infestations and which are capable of significant impacts. KNP is thought to have up to 98 naturalised alien plant species (Brennan 1996); species which have become accepted and which have reproduced for several generations. This represents about 5.4% of the total flora but is relatively low when compared with an average of 21% in other Australian conservation areas (Lonsdale 1992a). The number of alien plant species in Kakadu has increased at the rate of 1.6 species per year since 1948, and is expected to continue as a result of increased tourism and development (Cowie & Werner 1993). For an area like Kakadu, apart from the problems of controlling existing weeds, there is the seemingly inexorable advance of major potential invaders (Storrs 1996).

The extent of invasion of wetlands by weeds has been described for some species although often incompletely. In many instances, vital information on the ecological changes wrought by these species is often confined to a few isolated studies or to anecdotal evidence. Economic analyses of the losses caused by pest species are not common. Additionally, studies on the social and cultural impacts of weeds have not been done (Finlayson & Spiers 1999). A Global Biodiversity Forum held prior to the 1999 Ramsar Conference addressed invasive species and agreed upon the following definition: ‘An invasive species is a species, often alien, which colonises natural or semi-natural ecosystems, is an agent of change, and threatens native biological diversity’ (Pittock et al 1999). We accept this concept in our initial risk assessment with the additional recognition that invasive species also impact upon socio-economic values.

Given that weeds are an increasingly serious problem in tropical wetlands, there is a need for management prescriptions to be developed at several levels. Critically, for managers and users of wetlands, practical techniques and options are required that take into account local differences, priorities and resource levels for control. However, for localised effort to be effective a strategic framework is required that provides the necessary options and places particular weed infestations and their control into a regional perspective. A means of ensuring that the above aspects are not forgotten is through the adoption of ecological or wetland risk assessment procedures as the basis for effective and strategic weed management.

Within this context, information on the biology, ecology and management of Mimosa pigra (mimosa), Salvinia molesta (salvinia) and Urochloa mutica (para grass) has been collated and analysed in a risk assessment of the weeds in the regional context of Kakadu National Park, with particular emphasis on the Magela Creek catchment.