Lake mud reveals 7,000 years of Kangaroo Island’s complex fire history
- Written by: Haidee Cadd, Research Fellow, Faculty of Science, Medicine and Health, University of Wollongong
During the summer of 2019–2020, half of Australia’s third largest island was on fire. Kangaroo Island, also known as Karta Pintingga or Karti in local mainland Aboriginal languages, was one of the worst-hit places during the Black Summer fires. Two people lost their lives and almost all the remnant vegetation on the island burned.
In the wake of the fires, fears grew for unique species that live on the island, such as the green carpenter bee, a critically endangered dunnart, and the Kangaroo Island micro-trapdoor spider.
Increasingly unstable climate conditions are exacerbating fire risk across the globe. Since the Black Summer fires six years ago, we’ve seen many more megafires as far north as the Canadian Arctic. Every fire season in Australia brings more devastation as well.
In the months following the 2020 fires, we headed to Lashmars Lagoon on the Dudley Peninsula, eastern Kangaroo Island. Here, trapped within the mud, are thousands upon thousands of charcoal fragments built up over time from ancient fires. By analysing them we could reconstruct valuable long-term context for what’s happening today.
Our findings, now published in Global and Planetary Change, tell a complex 7,000-year-long story of how fire was shaped by the climate, vegetation and people.
A rare case study
To understand environmental change and how ecosystems cope with extreme events, we need perspectives longer than written observational records.
Studies of long-term fire histories from mainland Australia propose that Indigenous management reduced fuel loads, thereby reducing the occurrence and risk of bushfires.
After European colonisation, much of the Indigenous land management stopped. Since then, plant life in many parts of Australia has changed, exacerbating the risk and impact of wildfires.
But these changes also coincided with long-term fire suppression by the colonisers, landscape degradation and anthropogenic climate change. This makes it hard to untangle the exact effect of any one of these changes on fire regimes.
Kangaroo Island provides a rare chance to study the long-term fire history of an Australian environment that wasn’t managed by First Nations people in recent times. Early European colonisers in the 1800s noted Kangaroo Island’s thick scrub and lack of campfire and cultural burning smoke as evidence for lack of human habitation.
Indigenous oral histories also describe the departure of people from the island following isolation from the mainland. Archaeological work further supports the idea that the island was uninhabited for thousands of years.
Kangaroo Island is famed for its high biodiversity and unique ecosystems. There are 45 species of plants not found anywhere else. Have widespread wildfire events in the past contributed to this high biodiversity? Or are increasingly frequent fires threatening these remnant ecosystems?
This is where we come back to a seven-metre-long sediment core (a cylindrical sample) we collected after the fires in 2020.
Painting a detailed picture
We were not the first scientists to examine the mud from this site. Fifty years ago, Australian biologist Robin L. Clark established methods central to research in this field. She used fragments of charcoal and pollen grains found in the sediment of Lashmars Lagoon to paint a picture of past fire and vegetation.
We also used these techniques, combined with scientific advances in sediment dating, analysis and interpretation, to re-evaluate Clark’s hypothesis that fires became bigger after the departure of people from Kangaroo Island.
After a rigorous screening of archaeological data, we found the last reliable evidence for people living on the island was between about five and six thousand years ago. After people left, a more shrubby, denser vegetation established on the island.
Despite this, fire remained relatively rare and subdued in the landscape for a further 3,000 years under relatively wet climates. Then, fires increased over the last 2,000 years, culminating in prominent fire activity between 700 and 900 years ago.
This increase in fire activity coincided with a trend towards the climate becoming dryer, possibly due to changes in the southern westerly winds.
Crucially, this increase in fire activity is at odds with evidence from mainland Australia. Over the same 2,000-year period, fire activity in southeastern Australia was actually lower. This suggests the importance of Indigenous stewardship in suppressing bushfires, even when contending with the impacts of a drying climate overall.
Ultimately, our study has a message of optimism. Biodiversity on Kangaroo Island appears to have weathered major changes in climate and fire regimes in the past. However, questions still remain as to whether this unique environment can continue to withstand decreasing water resources and more frequent intense fires.
One thing is certain. With a rapidly changing climate, there is an urgent need to combine Indigenous wisdom, community engagement and western scientific evidence to conserve these unique ecosystems for future generations.
Authors: Haidee Cadd, Research Fellow, Faculty of Science, Medicine and Health, University of Wollongong
