European impacts on ‘pristine’ Kimberley vegetation

Often considered pristine, in reality the sparsely populated and isolated Kimberly ‘wilderness’ is already highly altered. While those working in the region know this to be true, quantifying changes to vegetation structure and composition has been difficult due to an absence of critical baseline data.

New collaborative research just published in the Journal of Vegetation Science has utilised soil cores from a lagoon in the remote north-west Kimberley to assess direct landscape changes associated with the arrival of Europeans and particularly the introduction of cattle. For the first time, clear landscape-level vegetation changes have been identified, demonstrating a significant impact on pre-European vegetation. The research was lead by Simon Connor at the University of Melbourne, in collaboration with Larissa Schneider, Jessica Trezise, Susan Rule and Simon Haberle at the Australian National University, Russell Barrett at the Royal Botanic Gardens, Sydney, and Atun Zawadzki at the Australian Nuclear Science and Technology Organisation.

Finding continuous sediment cores in the high rainfall zone of the Kimberly is challenging, but a lagoon near the Mitchell River proved to be ideal. A range of geochemical properties and the pollen record were analysed and dated using Lead-210 and Carbon-14 techniques.

Significant changes were identified across the sediment core, which spanned the last 200 years. Significant declines in the trees Antidesma ghaesembilla and Banksia dentata following the introduction of cattle were identified. This was soon followed by an increase in grass density, combined with geochemical and biological evidence that reflect increased grazing, local fires, erosion and eutrophication.

The combination of cattle, donkeys, horses and pigs, introduced by Europeans, and altered fire regimes, is a crucial mix to understand. Realistic benchmarks for ecosystem conservation and restoration are required to ensure that landscapes are managed effectively.

Cattle were first bought to the Kimberley in 1880’s, rapidly increasing to over 600,000 by 1914 (Bolton 1953; McGonigal 1990). Donkeys were initially bought in as pack animals, but were often released once they had served their purpose, and proved well-adapted to the Kimberley rangelands. Trial piggeries likewise were short-lived, and again some animals were released, breeding quickly and spreading widely across the Kimberley, with significant impacts on riparian and wetland communities (below right).

Fire has long been a controversial topic in landscape ecology, particularly in savannah ecosystems of northern Australia. In an ancient landscape where natural fires are common, determining the role of human-induced fire, and particularly changes due to post-European settlement, are difficult to determine.

According to the Bureau of Meteorology, the Kimberley has one of the highest densities of lighting strikes in the world. Lightning strikes create many natural fires at the onset of the wet season when there is a naturally high fuel load from dead annual grasses. While some lightning fires can spread widely in the landscape, many of these fires are short-lived, being put out by rain from the same storm, so may not have the same impact as fires lit earlier in the year.

In a landscape with a high number of natural fires, determining the additional impact of European-induced fires requires a careful and critical approach. Savannah ecosystems undergo massive seasonal fluctuations in biomass due to the brevity of wet season monsoonal rainfall. Vegetation grows rapidly over the wet season, especially annual grasses, creating the appearance of rapid post-fire recovery at a landscape level.

Look closer at key markers in the landscape, and all is not what it first seems. A useful indicator of savannah health is the mixed age of trees and large shrubs in the landscape. Across much of the northern Australian savannah, the over-story remains intact (for now), but the level of recruitment is dangerously low, and poorly represents what is required for long-term sustainability of natural canopy densities. Recruitment decline has been well documented in the tropical cypress pine Callitris intratropica.

Another key indicator is the presence or absence of ‘seeder’ shrubs and trees, those that are killed by fire, but usually require >5 years post fire to attain reproductive maturity. Some of these species probably require 20–30 year fire intervals to attain their maximum reproductive potential.

This is easier to see in the unique Pindan ecosystems spanning the Dampier Peninsula and east to Fitzroy crossing, which are dominated by ‘seeder’ tree species, mostly in the genus Acacia. Frequent fires are leading to rapid and major shifts in vegetation structure and composition leaving landscapes that appear as if mechanically cleared when frequency is too high. A review paper on this fragile ecosystem is being prepared by the author.

A third marker is the iconic mistletoes (Loranthaceae), with 26 species found in the Kimberley. As soft-stemmed parasites, mistletoes cannot survive being scorched by fire, and are thus one of the most fire-sensitive plant groups in the landscape. Decaisnina signata subsp. cardiophylla is particularly rare in the Kimberley, where it grows only on Banksia dentata which is also fire sensitive.

With this in mind, it is sobering to realise that the remote Kimberley landscape is burnt in its entirety on average every three years, with some regions experiencing 10 or more fires in the last 15 years (http://www.firenorth.org.au/nafi3/). In the southern Kimberley, towards the Great Sandy Desert, landscapes denuded by annual fires are regularly experiencing large dust storms, a recent phenomenon in the region and a warning sign for a potential future.

The absence of people and the remote nature of the Kimberley ‘wilderness’ has much in its favour for conservation, but it also presents very unique challenges for landscape management. We can no longer assume that the landscape will look after itself. European impacts have had a significant detrimental impact on vegetation structure, impacts that will lead to a further decline in vegetation quality without intervention. The EcoFire project is one such attempt that is producing promising early results, but even this requires long-term commitment of very significant amounts of funding, and results will need to be assessed against a baseline that is yet to be determined.

References

Bolton, G.C. 1953. A survey of the Kimberley pastoral industry from 1885 to the present. University of Western Australia, Perth.

Connor, S.E., Schneider, L., Trezise, J., Rule, S., Barrett, R.L., Zawadzki, A. & Haberle, S.G. 2018. Forgotten impacts of European land-use on riparian and savanna vegetation in North-Western Australia. Journal of Vegetation Science 29(3): 427-437. https://doi.org/10.1111/jvs.12591

McGonigal. D. 1990. The Australian Geographic Book of the Kimberley. Australian Geographic, Terrey Hills NSW.