Did Indigenous fire change Australia's environment?

Discussion paper, May 2014; written for a subject in BSc course.

Many Australian plant species and ecological communities have evolved to cope with, benefit from or even rely on fire (Bowman, 2012). There is little debate that the Indigenous peoples of Australia prior to European colonisation also used fire widely including for clearing vegetation and hunting, and that they effected some changes on the ecology of the continent.

The exact nature of those changes, and the role of fire specifically, however, is a more contentious topic, and hard to settle conclusively with the evidence currently available (Kirkpatrick, 1999; Kohen, 1993).

This paper will explore several of the key lines of argument and dispute, examine some of the evidence they are based on, and suggest how well current research may be able to answer the question (or not).

1. The initial impact of Indigenous fire

While many reports by early European explorers note the frequency with which Indigenous people set fire to country, little scientific recognition of Indigenous fire management was extended by mainstream Australia until the 1960s (Flannery, 1994; Kohen, 1993).

Current recognition of the importance of Indigenous fire management, as in Gammage (2011), has not been matched by a growing clarity as to the specifics of Indigenous fire regimes in times past, nor their exact impact on the environment.

It has been postulated that the arrival of the First Peoples, currently understood to have been at least 40ka [40,000 years ago] (Gott, 2005), was followed by a more or less rapid conversion of ecosystems by the use of fire (Flannery, 1994).

According to various versions of this argument, fire regimes after the arrival of the first people stimulated the rapid spread and dominance of the fire-tolerant genera of plants (Flannery, 1994, pp. 230-1); and even their speciation and radiation (cited in Bowman, 2012).

Recent work on molecular phylogenies suggests that the evolution of Australia's fire-adapted species and ecosystems happened in the order of 55-60Ma [million years ago], and that they became more dominant after 15 Ma. Features that make species fire-tolerant or dependent, such as oil glands in the leaves, may have evolved for reasons not related to fire, such as to deter browsing animals (Bowman, 2012).

This evidence does not rule out a significant impact by Indigenous fires on the distribution of species and ecosystems (Bowman, 2012; Kirkpatrick, 1999).

Evidence of the long-term fire and vegetation record is largely drawn from analysis of charcoal residue and pollen in core samples from sedimentary deposits. Not all sites show unequivocal evidence for a significant increase in burning with the presumed arrival dates of the first people, and fire has a long and definite history on the continent prior to human occupation (Bowman, 2012).

Complicating factors influencing fire include drying out in glacial periods, and faster vegetation growth in warmer, wetter periods. These complex factors of interaction make it hard to distinguish Indigenous fire's effects (Bowman, 2012; Kohen, 1993).

Some sedimentary analyses have found (for example) charcoal increases at the boundary between earlier layers dominated by rainforest, and later layers dominated by sclerophyll vegetation. It has been difficult to conclusively date these layers, however (Bowman, 2012), and given the continuing uncertainty about the exact time of arrival of the first people in Australia, and other factors mentioned above hard to know the cause of such fire events.

Understanding the background rate of natural fires (from lighting strikes) and their impact on vegetation is key to understanding how Indigenous fire may have moderated, prevented, or increased the background fire rate in any given ecosystem, and how much of a change the Indigenous arrival presented, or how much change was caused by the end of Indigenous fire in recent times (see for example Fensham, 2012). Lighting remains the major cause of fire in some parts of tropical Australia (Kohen, 1993).

One soils study found strong evidence that erosion increased significantly around 35-30ka with evidence of fire at the same time, in Tasmania where lighting fires are relatively rare. This is around the time Indigenous people are most commonly thought to have arrived there. This lends some weight to the idea that significant impact on the landscape occurred, but it is not conclusive. (McIntosh et al, 2008).

On the other hand, genetic analysis has been used to show that fire-sensitive Callitris cypress-pine populations have not experienced significant population crashes as would have been caused by widespread hot fire on the arrival of Indigenous people (Sakaguchi et al, 2013).

2. Measuring Indigenous impact by its absence

While the scale of the initial impact of the first peoples' fire is hard to judge from existing records, its ongoing influence on the ecology of the continent is easier to find evidence for.

Along the east coast and northern Australia, rainforest ecosystems have spread to areas which were open woodland or sclerophyll forest at the time of European arrival (Flannery, 1994; Kirkpatrick, 1999; Fensham, 2012; Bowman, 2012). Mechanisms for the return of rainforest appear to include protection from fire, including lighting-lit fires (Fensham, 2012), but possibly also changed rainfall and atmospheric CO₂ for some locations (Bowman, 2012).

The spread of rainforest can be verified by comparing current vegetation to historical records (Flannery, 1994; Gammage, 2011). It is known that some areas now rainforested were, at the time of European arrival in Australia, kept as open woodland by Indigenous burning (McIntosh et al, 2009).

The knowledge of Indigenous hunting includes widespread reference to the use of fire (Flannery, 1994; Gammage, 2011). Indigenous use of food plants in many areas also depended on fire. For example, without frequent fire (every 3-5 years in many locations), grassland and grassy woodland regenerates a thick sward that smothers small tuberous food plants that provided a large proportion of the diet for people living in those areas (Gott, 2005).

It is clear that in many areas, including areas still under Indigenous fire management in the arid centre and northern Australia, Indigenous fire management enhanced and enhances local biodiversity by preventing hot wildfire (fuel reduction), and by maintaining a fine patchwork or mosaic where each patch has been burned at different times, with some patches burned infrequently or not at all (Kohen, 1993; Bowman, 2012; Altman and Kerins, 2012; Fensham, 2012).

It has been argued that the cessation of Indigenous burning (and the introduction of a new, different fire regime) has caused ecosystem changes that have led to some mammal species becoming endangered or extinct (Flannery, 1994). While this hypothesis is very plausible, it requires further testing for specific locations and species instances (Fensham, 2012).

Whether Indigenous fire resulted in widespread extinction of mammals or other biota following the First Peoples' arrival is also uncertain, as the exact date of the megafauna extinctions, and the cause, is still hotly debated; some scientists have even argued that evidence supports long co-existence of megafauna alongside people for at least 15000 years (Trueman et al, 2005).

3. Conclusion

There is some evidence to suggest that the arrival of the First People to the Australian continent resulted in fairly rapid changes to ecosystems as a result of fires. The exact degree of the changes is difficult to quantify. At the least, it can be reliably assumed that change to the distribution, boundaries, and diversity of many ecosystems was caused by Indigenous fire, possibly including protecting species that would otherwise have become extinct (or which may now become extinct, having adapted to the Indigenous fire regime – Fensham, 2012). There is some evidence that the impact was much greater, initially, including widespread erosion, and maybe even regional climatic change.

Difficulties in assessing the ongoing impact of Indigenous fire arise from the paucity of reliable fossil or archaeological record, uncertainties in dating samples, and lack of understanding or record of the diverse practices of Indigenous people prior the British conquest. Nevertheless, the historical record (including Indigenous knowledge) has provided enough information for ongoing debate.

At one extreme, it has been argued that almost all of Australia was burned regularly under Indigenous management, perhaps as often as every 2-5 years (Gammage, 2011). On the other hand, evidence from current Indigenous practice and knowledge directly contradicts this uniformitarian generalisation (Prober et al, 2013; Morgan, 2013). It seems most likely that Indigenous fire management varied across the different climatic and vegetation regions of the continent, but even then the degree of variation is not certain. Current research available cannot tell us whether Gammage's unfortunate generalisation is nevertheless widely applicable, or in fact wildly inaccurate.

Further evidence from investigation in social and physical sciences is needed to make significant progress in answering this question. Doing so will not only shed light on the events and practices of the past, but provide a scientific basis for ecological fire management for conservation into the future.

References

Altman, J. and Kerins, S. (eds) (2012). People on Country: Vital Landscapes Indigenous Futures, Sydney: The Federation Press.

Bowman, D.M.J.S, Murphy, B.P., Burrows, G.E., & Crisp, M.J. (2012). Fire regimes and the evolution of the Australian biota, in Bradstock, R.A., Gill, A.M. & Williams, R.J. (eds) (2012). Flammable Australia, Melbourne, Australia: CSIRO Publishing.

Fensham, R. (2012). Fire regimes in Australian tropical savanna: perspectives, paradigms and paradoxes, in Bradstock, R.A., Gill, A.M. & Williams, R.J. (eds) (2012). Flammable Australia, Melbourne, Australia: CSIRO Publishing.

Flannery, T.F. (1994). The Future Eaters, Sydney, Australia: Reed New Holland.

Gammage, B. (2011). The Biggest Estate on Earth, Sydney, Australia: Allen & Unwin.

Gott, B. (2005). Aboriginal fire management in southeastern Australia: aims and frequency, Journal of Biogeography 32, 1203–1208. doi:10.1111/j.1365-2699.2004.01233.x

Kirkpatrick, J. (1999). A Continent Transformed, Melbourne, Australia: Oxford University Press.

Kohen, J. (1993). The Impact of Fire: An Historical Perspective. Retrieved May 21, 2014, from http://anpsa.org.au/APOL3/sep96-1.html

McIntosh, P.D., Price, D.M., Eberhard, R., & Slee, A.J. (2009). Late Quaternary erosion events in lowland and mid-altitude Tasmania in relation to climate change and first human arrival, Quaternary Science Reviews 28/9-10, 850-872. doi:10.1016/j.quascirev.2008.12.003

Morgan, J. (2013). Ngadju kala: fire management in woodlands. Retrieved May 21, 2014 from http://morganvegdynamics.blogspot.com.au/2013/12/ngadju-kala-fire-management-in-woodlands.html.

Prober, S.M., Yuen, E., O'Connor, M.H., & Schultz, L. (2013). Ngadju kala: Ngadju fire knowledge and contemporary fire management in the Great Western Woodlands, Floreat, WA: CSIRO Ecosystem Sciences.

Sakaguchi, S., Bowman, D.J.M.S., Prior, L.D., Crisp, M.D., Linde, C.C., Tsumura, Y., & Isagi, Y. (2013). Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia, Proc R Soc B 280(1773) doi:10.1098/rspb.2013.2182

Trueman, C.M.G., Field, J.H., Dortch, J., & Wroe, S. (2005). Prolonged coexistence of humans and megafauna in Pleistocene Australia, PNAS 102(23), 8381-8385. doi:10.1073/pnas.0408975102