When ecologists, policy makers, or the public, think about the visceral impacts of climate change on Australia’s natural systems, World Heritage listed Kakadu National Park (KNP), located in the seasonal tropics of the Northern Territory, is high on the at-risk list. But looking deeper into the human-driven processes now threatening KNP, there is actually a synergy of interrelated problems requiring simultaneous management – a situation common to most biomes threatened with global warming (Brook et al. 2008).
The big issues for KNP are changed fire regimes (impacting savanna and rain forest communities), rising sea levels (affecting the floodplain wetlands), and a suite of invasive weed and feral animal species, operating across all three major ecosystems. All three threats have a climate change component, although for fire and ferals, not wholly.
The savannas, which by area make up the largest part of KNP, are at first glance apparently intact. There has been relatively little clearance of the woody component (dominated by Eucalyptus tetrodonta and E. miniata); indeed analysis of historical aerial photography has documented vegetation thickening linked to elevated atmospheric CO2, which favours the growth of woody C3 species (Banfai & Bowman 2005). However, an emphasis by Park managers on avoiding hot late dry season fires, has meant that a large proportion of KNP is burnt during the dry season, with a return time of 1 to 5 years (Williams et al. 1999).
The impact of regular early season burning on the Park’s biota and on the structuring of understory vegetation, is a topic of ongoing debate and research. Nevertheless, some long-term studies have explicitly linked high fire frequencies to species declines (Pardon et al. 2003; Andersen et al. 2005). Climate change, via increased temperatures or shifts in the timing and intensity of monsoonal rainfall, will likely enhance future fire risk (Parry et al. 2007).
The KNP wetlands support a rich and spectacular biota, including vast flocks of magpie geese (Anseranas semipalmata), which congregate in millions to feed on Eleocharis chestnuts growing on the floodplains of the Alligator Rivers system. These wetlands formed ~6,000 years ago after sea level stabilisation, following a post-glacial rise of 120 metres. Ironically, additional sea level change associated with anthropogenic global warming threatens their future viability (Brook & Whitehead 2006).
About 20cm sea level rise occurred during the 20th century. At least double that amount – and potentially >1m due to accelerated polar ice sheet melt – is predicted by 2100. Rising sea levels, in combination with intense tropical storm surges, increases the regularity and severity with which saline flows penetrate the low-lying freshwater wetlands. At the mouth of the Mary River, to the west of KNP, extensive earthen barrages have already been built in an attempt to alleviate the damage caused by salt water intrusion.
A complex network of low-lying natural drainage channels, enlarged or cross-connected by movement of feral animals such as Asian water buffalo (Bubaus bubalis) and pigs (Sus scrofa), means that even a few tens of centimetres of additional sea level rise may be sufficient to degrade or eliminate a large fraction of the floodplain communities (Traill et al. 2010). What remain will be isolated patches of freshwater wetlands within a mire of brackish swamps and saltwater mangroves.
Beyond their impact in facilitating saline intrusion of the wetlands, feral ungulates help spread weed species such as introduced pasture grasses and Mimosa (Bradshaw et al. 2007), which compete with native plants for space and nutrients. Climate change will also cause shifts in the relative ability of invasives to compete with indigenous species, especially if natives are also under stress from herbivore grazing, changing habitat quality and altered fire regimes (Rossiter et al. 2003; Brook 2008).
KNP inevitably faces a tangled web of mutually amplifying processes associated with global change. Given the global failure to achieve significant carbon emissions mitigation to date, it seems that ‘adaptation’ and ‘building resilience’ will be the buzz words for KNP conservation managers for many years to come.