BUSHFIRES AS DRIVERS OF EXTINCTION: PRESENT & PAST

By Matt McDowell

The recent massive firestorms that incinerated much of Australia’s vegetation has left people reeling, but the impacts on our native flora and fauna may be even more devastating. Several stands of wet rainforests that haven’t been burnt in European memory, and lots of other vegetation communities, appear to have been wiped out, taking with them many of the vertebrate and invertebrate species that depended on them. Other more widely occurring ecosystems have also been impacted and scientists warn that when the smoke clears, the extinction count may be more devastating than people anticipate.

The fires have left several species of mammals, reptiles, amphibians, birds and insects, some of which are considered endangered, potentially homeless and without cover to protect them from predators. In several effected areas the decimation of vegetation has been so severe that there’s nowhere for songbirds to perch. But perhaps worst of all, there is little left for herbivorous survivors to eat, and despite the best efforts of conservationists to supplement food supplies, many animals will probably starve.

The animals most likely to survive the recent firestorms have adaptations that help them cope with fires better than other animals.

For example, they may be amphibious, dig burrow or be able to escape rapidly either by running or flying. Some animals may have sought out shelter in burrows excavated by other species, but despite recent news reports, it is unlikely that burrowing species such as wombats would invite, let alone shepherd, other species into the safety of their homes.

Figure 1. Geological time scale showing time periods mentioned in the text

We may think these fires are unprecedented, and in our experience, they are, but the fossil record tells us they’ve happened time and again, ever since land-based plants evolved and filled earth’s atmosphere with oxygen some 470 million years ago during the Ordovician period (Scott 2000; Figure 1). The first record of fossil charcoal (known a fusain or inertinite) comes from the late Devonian (375-360 million years ago; Figure 2). The fossil record indicates that major fire events occurred throughout the Carboniferous (~360-300 million years ago), but were rare during the Permian (~300-250 million years ago) due to a major reduction in atmospheric oxygen.

Figure 2. Phanerozoic fossil charcoal distribution (= fire) and predictions of past atmospheric oxygen levels (from Hill & Jordan 2016)

But the fossil record shows that the most ferocious fires the earth has ever experienced happened about 66 million years ago when a 10-kilometre-wide asteroid smashed into present-day Mexico leaving a huge crater the size of Tasmania. The resulting fires burnt practically every bit of vegetation on the planet, filling the atmospheres with enough soot, dirt and carbon dioxide to block out the sun for decades. The event, known as the Cretaceous–Paleogene extinction, or fifth great extinction, is marked by huge soot deposits found in rocks all over the globe. The combination of this massive impact, widespread fires and the following period of freezing darkness caused by atmospheric pollution drove 75% of all species on earth to extinction, including the dinosaurs. The only other impact of equal or greater magnitude we know of is thought to have caused the formation of the moon.

Figure 3. An artist’s impression of the asteroid strike that is thought to have killed the dinosaurs

So, what survived?

Most marine animals survived unscathed, but the story was very different for land animals. Like the survivors of Australia’s recent fires, animals that survived of the Cretaceous–Paleogene extinction must have had characteristics that gave them some advantage to deal with the heat: amphibiousness, burrowing or rapid locomotion (running or flight). But even then, survival was not guaranteed.

Crocodiles, aquatic tortoises and burrowing snakes and lizards made it, as did aquatic monotremes and small burrowing placental mammals. Some ground dwelling birds also survived, but tree dwelling, or perching birds, along with their habitat, were decimated. It took millions of years for their equivalent (aka songbirds) to re-evolve.

What perished?

Pretty much every surface-dwelling animal bigger than a Spotted-tailed quoll didn’t make it.

The potential for another extra-terrestrial impact is probably low but completely unpredictable. However, the impact we are having on the earth may be just as damaging (Youngsteadt et al. 2019) We have cleared enormous tracts of vegetation, mined fossil fuels to pump our atmosphere full of carbon dioxide, introduced exotic predators and competitors that hunt and compete with Australia’s endemic flora and fauna, and on occasion, have deliberately persecuted species to extinction. Few people realise that we are in the middle of what many scientists refer to as the sixth mass extinction. Some say it may be as devastating as the Cretaceous–Paleogene asteroid impact and most agree humans are at least partially to blame. The good news is, if we act fast, we may be able to do something about this one. If not, I guess the earth always has pandemics.

All posts are personal reflections of the blog-post author and do not necessarily reflect the views of all other DEEP members

References Hill, R. S. & Jordan, G. J. (2016). Deep history of wildfire in Australia. Australian Journal of Botany 64, 557-563. Scott, A. C. (2000). The Pre-Quaternary history of fire. Palaeogeography, Palaeoclimatology, Palaeoecology, 164, 281–329. Youngsteadt, E., Lopez-Uribe, M. M., & Sorenson, C. E. (2019). Ecology in the sixth mass extinction: Detecting and understanding rare biotic interactions. Annals of the Entomological Society of America 112, 119-121.