Weeds present an on-going threat to the values of the Ramsar site, primarily due to their ability to out-compete native plants, leading to displacement of food sources for fauna as well as bush tucker species. Additionally, weeds may change the structure of vegetation communities and fire regimes, thereby altering habitat suitability for functions such as waterbird nesting and roosting.
Walden and Gardener (2008) estimate that Kakadu National Park management’s annual expenditure on weed prevention and control is approximately $1.2 million. Species of concern are listed in Walden and Gardener (2008), and key species are also discussed below and in Section 6.1.2.
Weeds are particularly problematic within the freshwater habitats of the Park (for example, Bayliss et al. 1997, Douglas et al. 2008). In particular, threats posed by exotic pasture grasses continue to increase. The predicted spread of para grass Urochloa mutica is expected to cause a range of negative impacts on freshwater wetlands including almost total displacement of native vegetation (Douglas et al. 2001). New outbreaks of olive hymenachne Hymenachne amplexicaulis are also notable, similarly displacing native floodplain vegetation (Douglas et al. 2008).
From a cultural heritage perspective, weeds including salvinia Salvinia molesta, olive hymenachne, para grass and mission grass Pennisetum polystachion have reportedly led to a decrease in hunting and fishing grounds. Decreased use of traditional hunting and fishing grounds can lead to reduced application and transmission of traditional ecological knowledge.
As described in Section 3.5.2, weeds have also led to changes in fire regimes, making it difficult to maintain Kakadu National Park’s natural ecosystems through fire management based on traditional Bininj burning practices. Also as mentioned in Section 3.5.2, exotic grasses increase fuel loads and the intensity of fires, which may threaten native fauna such as turtles (Douglas et al. 2008) and fire sensitive flora species and/or propagules (Petty et al. 2008).
Principal future threats to ecological character from weed invasion relate to introduction of propagules (of weed species that are currently problematic or new species that may become problematic) from adjoining pastoral land on the Park’s west, from Arnhem Land and from visitors.
Exotic fauna will continue to be a threat in the absence of management intervention. Control options for large herbivores are generally restricted to broad-scale helicopter shooting campaigns. As part of the Kakadu National Park Feral Animal Management Program 2008-2009, approximately 468 helicopter hours were deployed and almost 11 000 large non-indigenous herbivores culled during that program (S. Atkins pers. comm. 2009; see Table 5 -25). From year to year, extent and location of aerial culls can vary considerably (for example, 2008-2009 program was the largest in recent years and provided particular attention to the southern and western parts of the site; B. Salau pers. comm. 2009).
Despite feral animal management programs, efforts to control damage within the site are likely to be compromised by entry of animals from neighbouring regions where land managers either lack interest or funds to implement broad-scale control, have reservations given the lack of evidence for general density-damage relationships, or have fundamentally different management goals (Bradshaw 2008). Furthermore, for some species (for example, horses), there are competing cultural, ethical and political interests that render the decision to reduce non-indigenous animal densities controversial (Gardner et al. 2002; Director of National Parks 2007; Bradshaw 2008).
Cane toads Rhinella marina are a relatively recent arrival in Kakadu National Park, first recorded in 2001 but have since become well-established, and are likely to colonise every habitat type present (van Dam et al. 2002b). Cane toads are regarded as a potentially significant threat to many native fauna species within the site (Gardner et al. 2002; Director of National Parks 2007). Key concerns are linked to negative impacts arising from direct consumption, competition for resources, and toxic effects on toad predators (van Dam et al. 2002b; Bradshaw et al. 2007, also refer Section 6.1.2). There is currently no known method of cane toad control over large spatial areas (Director of National Parks 2007).
Impacts of exotic fauna are further discussed in Section 6.1.2.
Table 5 25 Summary of Aerial Feral Animal Management Program 2008-2009