The mystery of our missing megafauna

By Rachel Chan

Giant ground sloths, super-sized beavers, three metre tall kangaroos. Like something from a Lewis Carroll novel, these are a few animals of the extensive megafauna that walked (and flew and swam) the Earth tens of thousands of years ago. Generally, any animal weighing above 40kg constitutes as megafauna (Roberts, 2001). As Alfred Wallace put it, “we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared”. Megafauna extinction events have taken place over the last 40,000 years, occurring in pulses, each in different regions (Kolbert, 2014). The current debate centres around the two main theories for this: human arrival (and consequently hunting and/or habitat alteration) and natural changes in climate (Stuart, 2014). 

The first pulse of extinction occurred around 40,000 years ago in Australia, where 23 of 24 genera of land megafauna went extinct. (Miller, 2005). This was followed by North and South America 15,000 years ago (Alroy, 2001). Up until the Middle Ages, Madagascar’s giant lemurs and pygmy hippos were still thriving. This may not support the climate change claim, as it is difficult to perceive how a sequence like this can match a single climate change event. On the other hand, this sequence seems to almost match patterns of human settlement. 

The megafauna extinction event of Australia has been widely debated, not just because of its dramatic megafaunal losses but because of its lack of reliable evidence. In favour of the climate change argument is the Last Glacial Maximum (LGM), which introduced a period of increased aridity (Roberts, 2001). While humans are also a culprit, a human-climate synergy has also been suggested and that human pressures were the nail in the coffin for populations already compromised by climate change (Saltré et al., 2016). Elusive evidence has prevented clear conclusions from being drawn, as the initial arrival of humans and megafaunal extinction took place close to the limit of radiocarbon dating (Miller, 2005). Data describing the timing of megafauna extinctions comes from the estimated ages of fossilised remains, but bias is introduced by incomplete sampling – the Signor-Lipps effect (Prideaux et al., 2010). 

Recently, a newly revised model has been produced to estimate the duration of the human-megafauna coexistence in Australia, using only reliably dated archaeological records and correcting for the Signor-Lipps effect (Saltré et al., 2016). The refined model concluded that megafauna went extinct within about 13,500 years of human arrival (Saltré et al., 2016), implying that human pressure may be responsible for these extinctions. This is consistent with previous estimates claiming that megafauna coexisted with humans for about 10,000 years (Roberts, 2001). Furthermore, physical evidence suggests that the climate stayed relatively stable during this period (Brook et al., 2007). Burial ages also suggest that the extinctions occurred around 20,000 years before the height of the Last Glacial Maximum, strongly (but indirectly) suggesting that human arrival was the demise of megafauna in Australia (Miller, 2005). 

A main pillar of the human activity argument is the overkill hypothesis; that megafaunal extinctions were caused by overhunting. At first glance, it is hard to imagine how small numbers of technologically primitive ancient humans could be responsible for this. Could realistic human population growth result in a realistic number of extinctions? Models of human activity in North America affirm this. A computer simulation of the North American end-Pleistocene population dynamics accurately predicts the fate of 32 out of 41 megafauna species (Alroy, 2001). Assuming an initial human population of 100 and modest rates of hunting, hunting alone is a plausible explanation for the North American extinctions (Alroy, 2001). Of course, climate alteration could have been in the mix, but this model shows that overhunting alone was enough to cause an extinction of that magnitude.

Similarly, models like this have also shown that the overkill hypothesis is plausible in Australia (Brook and Johnson, 2006). Ultimately, the overkill hypothesis is made possible because of the slow reproductive rates of megafauna. Superior size and a lack of predators were the formula to megafauna’s survival until human predation was concerned (Brook and Johnson, 2006). Ironically, ancient humans would not have had a clue about the disappearing megafauna. The extinctions happened in a geologic instant but were undetectable in relation to a human lifespan (Kolbert, 2014). 

Ultimately, our progress is hindered by imperfect evidence. This is true for Africa and Southern Asia, where we have the least available data, and Australia, where radiocarbon dating still limits conclusions to some extent. To fully draw conclusions, we would need to establish an extensive database of reliable radiocarbon dates for each region, based on megafaunal remains (Stuart, 2014). Until we improve the quality of our data, debate over imperfect evidence will continue. 

Beyond being intriguing mysteries, these extinctions offer us excellent insight into the alarmingly rapid biodiversity loss at present. Megafaunal extinctions left us more than a mark in the fossil record because they possibly changed entire landscapes. In North America, vegetation changed as megafauna went extinct. Sporormiella spores offer evidence to suggest that changes in vegetation followed megafauna extinctions. These spores, found in herbivore dung, declined as the megafauna did (Gill et al., 2009). 

Vegetation changes were dramatic; Eurasia and North America saw increased tree cover, contributing to important positive feedback impacts (Stuart, 2014). The replacement of boreal grasses by forests decreased surface albedo, causing local warming of up to 6°C (Bala et al., 2007). Megafaunal disappearance seems to play a part, as reduced mammoth populations would allow the expansion of dwarf deciduous trees, decreasing surface albedo and accelerating climate change (Stuart, 2014).

Equally significant ecological changes were seen in Australia, where a shift from rainforest to sclerophyll vegetation began around 45,000 years ago (Turney et al., 2001). Increased charcoal deposits indicate fire, either anthropogenic or caused by megafauna loss. Without large herbivores to eat forest vegetation, fine fuel would have built up (Flannery, 1990). Consequently, there were more frequent and intense forest fires, resulting in vegetation shifting towards more fire tolerant species (Rule et al., 2012). 

Megafaunal extinctions of the past offer a grave warning to us. Of the 362 surviving megafauna species, 70% are in decline and 59% are threatened with extinction (Ripple et al., 2019). If small bands of ancient humans had the capacity to wipe out much megafauna, what damage can we do? The ecological consequences following megafauna loss draws terrifying parallels with ours. From the change in Australian vegetation 40,000 years ago to the Amazon forest currently at the tipping point of becoming a savannah (Staal et al., 2020), megafaunal extinctions show how considerably landscapes can change. While megafauna extinctions were imperceptible to ancient humans, we know now more than ever the ecological havoc our species can cause. 

References:

Bala, G., Caldeira, K., Wickett, M., Phillips, T., Lobell, D., Delire, C. and Mirin, A. (2007) Combined climate and carbon-cycle effects of large-scale deforestation. Proceedings of the National Academy of Sciences, 104(16), 6550-6555. Available from: doi:10.1073/pnas.0608998104

Brook, B. and Johnson, C. (2006) Selective hunting of juveniles as a cause of the imperceptible overkill of the Australian Pleistocene megafauna. Alcheringa: An Australasian Journal of Palaeontology. 31, 39-48. Available from: doi:10.1080/03115510608619573

Brook, B., Bowman, D., Burney, D., Flannery, T., Gagan, M., Gillespie, R., Johnson, C., Kershaw, P., Magee, J., Martin, P., Miller, G., Peiser, B. and Roberts, R. (2007) Would the Australian megafauna have become extinct if humans had never colonised the continent? Comments on “A review of the evidence for a human role in the extinction of Australian megafauna and an alternative explanation” by S. Wroe and J. Field. Quaternary Science Reviews. 26(3-4). 560-564. Available from: doi:10.1016/j.quascirev.2006.10.008

Flannery, T. (1990) Pleistocene faunal loss: implications of the aftershock for Australia’s past and future. Archaeology in Oceania. 25(2), 45-55. Available from: doi:10.1002/j.1834-4453.1990.tb00232.x

Gill, J., Williams, J., Jackson, S., Lininger, K. and Robinson, G. (2009) Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America. Science. 326(5956), 1100-1103. Available from: doi:10.1126/science.1179504

Kolbert, E. (2014) The Sixth Extinction: An Unnatural History. New York, Henry Holt and Company.

Miller, G. (2005) Ecosystem Collapse in Pleistocene Australia and a Human Role in Megafaunal Extinction. Science. 309(5732), 287-290. Available from: doi:10.1126/science.1111288

Prideaux, G., Gully, G., Couzens, A., Ayliffe, L., Jankowski, N., Jacobs, Z., Roberts, R., Hellstrom, J., Gagan, M. and Hatcher, L. (2010) Timing and dynamics of Late Pleistocene mammal extinctions in southwestern Australia. Proceedings of the National Academy of Sciences. 107(51), 22157-22162. Available from: doi:10.1073/pnas.1011073107

Ripple, W., Wolf, C., Newsome, T., Betts, M., Ceballos, G., Courchamp, F., Hayward, M., Valkenburgh, B., Wallach, A. and Worm, B. (2019) Are we eating the world’s megafauna to extinction? Conservation Letters. 12(3). Available from: doi:10.1111/conl.12627

Roberts, R. (2001) New Ages for the Last Australian Megafauna: Continent-Wide Extinction About 46,000 Years Ago. Science. 292(5523), 1888-1892. Available from: doi:10.1126/science.1060264

Rule, S., Brook, B., Haberle, S., Turney, C., Kershaw, A. and Johnson, C. (2012) The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia. Science. 335(6075), 1483-1486. Available from: doi:10.1126/science.1214261

Saltré, F., Rodríguez-Rey, M., Brook, B., Johnson, C., Turney, C., Alroy, J., Cooper, A., Beeton, N., Bird, M., Fordham, D., Gillespie, R., Herrando-Pérez, S., Jacobs, Z., Miller, G., Nogués-Bravo, D., Prideaux, G., Roberts, R. and Bradshaw, C. (2016) Climate change not to blame for late Quaternary megafauna extinctions in Australia. Nature Communications. 7(1). Available from: doi:10.1038/ncomms10511

Staal, A., Fetzer, I., Wang-Erlandsson, L., Bosmans, J., Dekker, S., van Nes, E., Rockström, J. and Tuinenburg, O. (2020) Hysteresis of tropical forests in the 21st century. Nature Communications. 11(1). Available from: doi:10.1038/s41467-020-18728-7

Stuart, A. (2014) Late Quaternary megafaunal extinctions on the continents: a short review. Geological Journal. 50(3), 338-363. Available from: doi:10.1002/gj.2633

Turney, C., Kershaw, A., Moss, P., Bird, M., Fifield, L., Cresswell, R., Santos, G., Di Tada, M., Hausladen, P. and Zhou, Y. (2001) Redating the onset of burning at Lynch’s Crater (North Queensland): implications for human settlement in Australia. Journal of Quaternary Science. 16(8), 767-771. Available from: doi:10.1002/jqs.643

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s