Sahul - Part II: Climate Change

Climate Change

Australia is dominated by a vast desert, where only the most resilient creatures can cling on to survive. Yet such was not always the lay of the Sahulian land: Where there are now only flats of salt lay rich lakes; dry riverbeds ran wide with water, and lush rainforests spanned huge distances. Over time the green plains of yesterday have given way to the red dunes of today, fertility clinging desperately on at the edges of the continent. Considering these transformations, it would not be surprising, perhaps even expected, that the herds of giant creatures vanished with them. But such simple narratives are often more complex under scrutiny, and the question of climate change and its effect on the megafauna is no exception. To shed light on this fact we examine the changes occurring in Sahul not just around the time of the extinction some 40-50 thousand years ago, but also in the greater context of the Pleistocene (26.mya – 11.7kya) and figure out how devastating the impact of climate change was on the diverse assemblage of beasts inhabiting the land down under.

The extinctions probably place around 40-50kya in the middle of a warm period known as Marine Isotopic stage (MIS) 3 (57-29kya) a period of global warming which saw an increase of about a degree celsius since the preceeding cool period MIS4 (See table 1) (23), for comparison this is somewhat less than the 1.5 degree increase observed in Australia since 1850 (4). This comes as a successor to a series of gradually hotter warm periods since MIS 11 (34), even so it can’t be said that the extinction window was uniquely hot, MIS 5 attained temperatures two degrees warmer (23, 34), therefore temperature in and of itself doesn’t correlate with the extinction (23). instead we must look to another climatic factor, aridity. Quantifying the dryness of the continent is a difficult business, but we will examine three prominent areas of evidence: The variation in the El Niño Southern Oscillation, examination of the major Australian bodies of water and lastly the changes in vegetation composition.

Marine Isotopic Stage	Chronology (kya)
1	14-Present
2	29-14
3	57-29
4	71-57
5	130-71
6	191-130
7	243-191
8	300-243
9	337-300
10	374-334
11	424-374

Table 1. Marine isotopic stages (MIS) are alternating warm (odd numbers) and cool (even numbers) periods in Earths history inferred from oxygen isotopes obtained from ice cores. The estimated chronology is shown for each period within the last 400,000 years.

The El Niño Southern Oscillation (ENSO) is a weather patten observed in the Pacific Ocean south of the equator, and it consists of three phases: The neutral, El Niño and La Niña. During the neutral stage the trade winds in the tropical South Pacific run from East (South America) to West (Australasia) bringing with them warm air and precipitation, this makes the South American west coast dry and the Australian North-East wet. When an El Niño occurs, there is a weakening of these trade winds so that instead warm air and moisture accumulate in the South Eastern Pacific. After a while this will be interrupted by a La Niña, which reverts the trade winds back to the default state. This pattern oscillates, which is known as an El Niño Southern Oscillation, usually an annual or multi-year phenomenon. We can make inferences about how much the ENSO varied during the Pleistocene by comparing fossil corals to modern day corals. Corals are longliving and the annual growth bands are affected heavily by salinity and temperature which are both altered drastically during El Niños (26). The frequency and/or duration of each phase of an ENSO seems to have fluctuated over time with Sahul experiencing much lower rainfall in some periods (28). This variability was comparatively high around the time of the extinction window, but significantly lower than during during MIS5, especially around 130kya (26). Additionally, the extinction window does not correlate with any period of abrupt change in this cycle (23). This calls the claim that lower rainfall caused the extinction into question.

Another keyway of measuring the aridification of Sahul is through the history of its freshwater bodies. Sahul was once home to vast, lush lakes and riverine systems. Largest of all was the Eyre-Frome super lake, which during late MIS5 covered an area of over 25,000 Km2, making it comparable to Lake Erie in North America. Eyre-Frome appears to start drying up 47kya (3) and vanishes altogether by 35kya (7), before partially refilling in the following millenia (3). Today it exists only as a series of seasonal salt lakes reaching no more than half the area, though some of this deterioration occurred during the MIS1 (3). Similar patterns are observed in other large ancient lakes (7) across the continent such as Lake Gregory 2), these seem to corroborate a hydraulic collapse around the time of the extinctions (2, 7). The lakes were full since at least late MIS5, records dating further back are lacking somewhat. The records from Lake gregory extends to about 300kya and show a reduction in area taking place sometime during either MIS 6 or early MIS 5 (2), but the jury is still out on whether this was a local event. So, the evidence at present suggests that the extent of the waterloss is unique to MIS 3, though lake drying may also have occurred at MIS5. The timeline is not inscrutable though, hydraulic collapse in some locations such as Wolf Creek Crater don’t appear to have occurred until 35kya, which is after the estimated extinction age (12). The disappearance of lakes and rivers could explain local extinctions in much of the interior from habitat loss as nearby fossil sites (e.g. South Water Downs & Cudde Springs) are rich with megafauna including potentially amphibious species. (7). Unfortunately, this does not constitute a continent-wide explanation, much of the Australian periphery contains its own water catchments and similar lake diminishments have not been demonstrated to have occurred in New Guinea or Tasmania.

Fig 1. Map showing the borders of the Pleistocene continent of Sahul (Dark grey) and modern-day landmasses (Light grey). The location of each site mentioned is indicated (Red dots).

Finally, the aridity of the continent has been inferred from a profound shift in the vegetation composition around the time of the extinction window, especially to fire and drought tolerant species. Vegetation structure is measured using pollen records preserved in layered sediments, either marine or terrestrial. Based on the representation of various plant groups in each layer we can construct a timeline of habitat change. Perhaps one of the most apparent examples is the collapse of rainforest habitats in the North-Eastern Australia, these are observed both at Lynch’s Crater (21) and South Walker Creek (7). At Lynch’s crater high resolution pollen data has enabled us to determine that the vegetation shift occurred in a matter of centuries, indicating a rapid and irreversible change in conditions (21). A similarly rapid change at South Walter Creek has been linked to the hydraulic collapse (7), however Lynch crater belongs to a separate catchment which lacks evidence of drying. The retraction of the Australian rainforest during dry periods is nothing new, at Lynch’s Crater an even more dramatic transition from rainforest to dry tolerant forest ocurring at at the end of MIS5 (21). Similarly, a rainforest ecosystem at Mt Etna collapsed during MIS7 (6). Strangely, pollen studies from the Baliem River in New Guinea show the opposite trend with a transformation from an open landscape to woodland (8)

Elsewhere the picture becomes even more puzzling. A study of the diet of the emu (Dromaius novahollandiae) and common wombat (Vombatus ursinus) analysing carbon-14 concentrations in eggshells and teeth respectively, revealed a rapid shift from C4 plants which largely grow in lush habitats to C3 plants which thrive in arid terrain, the shift occurred in both species suddenly during MIS3 (1). The sudden transition is unique to MIS3 and is not in MIS5 despite being the warmest period (23) with the most variable ENSO (26)

Fig 2 The Common wombat (Vombatus ursinus) is a strict grazer found in South-eastern Australia and Tasmania. Prior to the Late Pleistocene extinction its diet comprised primarily of rich C4 grasses, but around 45,000 years ago it incorporated a lot of C3 plants into its diet.

Terms of use: This image is licensed under a Creative Commons Attribution-Share Alike 4.0 International. It is attributed to Dmitry Brant. The image is unedited.

Perhaps even weirder is the fact that at other sites this rapid turnover in vegetation structure doesn’t occur at all. For instance, at Tight Entrance Cave there is a continuous transition between mid MIS5 and late MIS3 from C4 to C3 plants (18). Off the coast of Devil’s lair, the change was at the beginning of MIS4 from woodland to dry shrubland with no noticeable change during the extinction window (29). Likewise records at Lake Selina in Tasmania show an abrupt transition from rainforest and woodland to grass and heathland around the start of MIS4 and then no major change until the start of MIS1. (28).

Can we even accept a transition towards a dry-tolerant shrubland as a suitable proxy for a drier climate? The link appears quite compelling, it’s logical that such a floral ansemble would thrive in a dry environment. As outlined, some sites show drastic vegetation changes during several past climate upheaval on the continent. Studies have also tracked the link directly, for instance the amount of grass pollen correlates very well with ENSO variability patterns (27), so dryness does impact the vegetation composition. However, climate alone doesn’t always correlate as has been found at Lynch’s crater (21). Alternative factors could affect the vegetation composition such as a change in fire regimes or the megafaunal loss itself. (We will explore these hypotheses in part 3).

Faunal Response

With the notable exception of the Australian interior, the evidence suggests a climatic shift in line with past periods of warming and drying in Sahul. Even so, we can consider whether the Australian megafauna would be sensitive to such waves of climate change. Firstly, lets examine how communities have responded to past periods of climatic change, two sites are well studied for this: Tight Entrance Cave (18) and Naracoorte Caves (17). At the former the records stretch from Late MIS6 until Late MIS3. The overall species richness of megafauna remained constant, even during early MIS5 the warmest period of the late Pleistocene. There appears to be a faunal change around 70kya where the relative abundances of each species changed, but species richness remained constant. Interestingly, the shortfaced-kangaroo Simosthenurus became more common during this time, indicating it may have been adaptable to the climate change. Naracoorte Caves by comparison spans as far back as 530kya and until 200kya and underwent a different rapid change in climate during MIS 8 from a wet to a dry climate, this transition was associated with a slight decline in megafauna, both in density and species richness. It is worth noting however that much like at Tight Entrance Cave there was an increase in some species of megafauna, in this case the giant kangaroo Procoptodon goliah. The species lost at the site are known from later deposits elsewhere indicating the extinctions were only local.

This is interesting as it raises the suggestion that some Sahul megafauna was well adapted to aridity. One would expect such species to thrive with the aridification of the continent. Aside from the aforementioned sites there is ample evidence of such species existing at the Nullarboor plains (16). This site was arid between 400kya and 200kya with annual estimated rainfall during dry periods ranging between 100 and 260mm per year. Even so, it boasts 21 extinct species between 10kg and 200kg, including species of Procoptodon, Protemnodon, Phascolonus and Thylacoleo (16). Evidently at least a proportion of the Sahul fauna could adapt to arid environments comparable to much of the interior. Perhaps though – as suggested by Wroe and Field 2006 – it was the complete disappearance of freshwater sources that explained the targeted disappearance of megafauna, as smaller organisms can obtain water through their diet whereas megafauna traverse large distances (33). The problem with this idea is that current arid megafauna such as the Red Kangaroo (Macropus rufus) is also constrained to ranges within about 10-15km of water sources (9) yet persisted into the modern day. Indeed, the first human colonisers of the interior cluster around water sources that exist to this day (1), indicating they also yielded water at least 30-40kya. If this is true, then in principle arid adapted taxa should be capable of acquiring water.

Even if we conceed that the Australian megafauna were unable to adapt to the arid interior, a climate hypothesis still does not account for the variety of refugias that existed in Sahul. Notably, Tasmania does not appear to have been affected by climate substantially during the extinction window (28). Likewise, though New Guinea does show a transition to forest, (8) but there is no reason to expect this would not keep supporting the diverse assemblage of megafauna present in the area (after all rainforest taxa are known from other parts of Sahul (6, 7). On the Australian mainland we can demonstrate the presence of refugias by examining the genetic history of various species of the Cypress-Pine Callitris. As it turns out even in the Australian interior forests of Callitris have been able to survive the waves of climatic change in the past. Bottlenecks have occurred but these pre-date the megafaunal extinctions, indicating that the integrity of the forests was maintained in numerous locations (22). Other species of Callitris, which span the major Australian climatic zones appear to have been largely unaffected in the past periods of climate change, including in the East and South-West. The tropical North also showed bottlenecks which predated the extinction window (22). So at least some habitats in even the arid interior appear to have survived and there’s no reason to expect megafauna would be incapable of seeking these out, in fact the graduality of the climatic change between MIS 4 and 3 would only cause species to have to migrate an estimated 10-150m each year (23).

Fig 3. The Cypress-Pine Callitris columellaris, part of a species complex which spans Australias major climatic zones, ranging from the arid interior to the tropical North.

Terms of use: This image is licensed under a creative commons attribution 2.0 Generic. It is attributed to djpmapleferryman. The image is unedited, and the original can be found here.

Conclusion

So where does this leave climate change as the driver of extinction in Sahul? As demonstrated the extinction window around 40-50kya was at a time of warming and drying but was unremarkable in the context of the Late Pleistocene with greater changes occurring around 70kya and 130kya. This is corroborated both in temperature and El Niño Southern Oscillations. Arguments about climate change based on vegetation change should be questioned as alternative hypotheses may explain these, and even if we accept it as a climate proxy significant vegetation change is absent from several key areas. Climate change appears to have had a somewhat limited impact on megafauna prior to the extinction, with the only noticable impacts being local extinctions and alterations in species composition at various sites. Furthermore, the evidence of megafauna adapted to arid conditions undermines any argument that the fauna was inherently vulnerable to extinction by climate change. It seems quite apparent that climate change provides an insufficient explanation for the demise of the Sahul megafauna.

What remains a more open question is whether it was a significant contributor to the extinction. I would offer a hesitant yes…and no. On the one hand, the disappearance of major water bodies in the interior is unique to the extinction window and it is difficult to imagine that this didn’t affect local fauna significantly. We may examine the modern-day Saltwater Crocodile (Crocodylus porosus), this species was once found as far south Cuddie springs yet has disappeared altogether from the Australian interior and is now limited to the North of the continent. The loss of habitat probably drove it to extinction in the area as it is a water sensitive species. The range of other water-dependent megafauna such as Paludirex and possibly Quinkana and Meiolania are unknown, but the hydraulic collapse may have wiped some of them out completely. Even species which were not directly extirpated from these climatic changes may have been more vulnerable to extinction by having their ranges reduced to refugia. On the other hand, megafaunal extinctions still occurred even in areas such as South-West Australia and Tasmania where climate change has been shown to have had little impact on the local communities. Furthermore, as demonstrated the resilience to aridity of some faunal assemblages, such species ought to thrive in much of the deteriorating regions. As such it appears as if climate change contributed to extinctions in some cases, perhaps even driving a few select species extinct, but that it was not a necessary ingredient in the extinction of the Sahul megafauna. So then what could explain the demise of Sahul giants? The main alternative theory is a human cause either from overhunting or landscape burning and we shall examine both these hypotheses in part 3.

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Taxa List

Table 2. animal species and genera mentioned in text. ‘†’ Denotes a globally extinct taxon. It should be noted most species are known only from limited fossil material, so classifications, sizes and ecology might be unclear or wrong. Do note there are some additional genera and species not included in this essay, particularly some dubious genera of Kangaroos and Diprotodonts.

Mammalia
Thylacoleonidae†
Thylacoleo carnifex †	Thylacoleo was a unique Marsupial with no comparable animals known. It weighted in at about 100-150kg (32) and probably constituted a hyper carnivore using powerful forearms and massive incisors to hunt prey (30).
Vombatidae (Wombats)
Phascolonus gigas †	A large species of wombat potentially reaching 460-730kg in weight (19), though other estimates places it as significantly smaller (31). Unlike modern wombats Phascolonus probably did not burrow, but probably it also had a grazing diet like modern wombats (31).
Vombatus ursinus	Common Wombat.
Macropodidae (Kangaroos and relatives)
Macropus rufus	Red Kangaroo.
Procoptodon	A genus of shortfaced kangaroo, which is very variable in size from about 50kg in P. gilli to 230kg in P. goliath which is the largest known kangaroo (5). Much like other shortfaced kangaroos it has a short and robush face, well adapted for powerful cheek muscles used in browsing (13).
Protemnodon	A genus large wallaby-like kangaroos with about a dozen species (5) ranging from 45kg to about 165kg (5). It was a versatile browser-grazer (15).
Simosthenurus	A species rich genus of shortfaced kangaroo, reaching about 120kg but with smaller species as well (5). It was probably a browser like other shortfaced Kangaroos (13)

Aves
Casuariiformes
Dromaius Novahollandiae	Emu.

Reptilia
Crocodylia (Crocodiles)
Crocodylus porosus	Saltwater crocodile.
Paludirex vincenti †	A genus of mekusuchine crocodile, formerly known as Pallimnarchus (20). It was a broad snouted crocodile larger than C. porosus (20) associated with freshwater fossil locations (7).
Quinkana fortirostrum †	A genus of mekusuchine crocodile. The size of quinkana is unclear, but has been proposed to be about 3m, with weight estimates as high 200kg It bears similarities to other suggested terrestrial crocodiles and may have been land adapted (14), though it has mostly been found in sites associated with water and the morphological evidence has been challenged (32).
Testudina (Turtles)
Meiolania sp. †	An unnamed species of the horned turtle Meiolania, belonging to the ancient gondwana clade of Meiolanidae (24). It was probably herbivorous (25). Meiolania had a bony club, with unknown function, but could be for defence or have a social function. It was probably terrestrial (25), but another species found on Lord Howe Island has been found to be semi-aquatic (10).

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