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.
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Imperial Bioscience Review 