Australian megafauna extinction

[Isengard]

Hi all,

I've recently been reading up on this and I was interested to hear your views.

I read this article which I felt was very reasonable and balanced giving a range of views an airing:

ngm.nationalgeographic.com/2010/10/dreamtime-animals/achenbach-text

However, I have been reading a book called Sapiens by Noah Harriri in which he gives several pretty convincing reasons why humans must have been the cause. I was fairly convinced but I am open to all opinions!

Fascinated by the megalania article by the way, sounds great.

[Ian Thomas]

The problem with the MF extinction debate is there's no conclusive evidence either way, viz humans dunnit or something else. Which is not new - the debate has been going strong for 200 years. No sign of resolution yet. Which is good news for paleontologists.

Inadequate evidence means the flavour of the month is decided by who is most eloquent in defending one theory or the other. viz. It's not the science decides which side gets media attention and funding - it's who has the loudest megaphone and the slickest media prez.

[molloch]

The analogy I like to use is the "Who Sunk the Boat" one. For those without small children, this is a preschool book about a bunch of animals getting into a boat, from largest to smallest. They all fit happily until the mouse gets on and the boat sinks.

I think we can safely say that without direct hunting by humans, the MF would have survived. Similarly, without climate stress, the MF would have survived, without environmental change through fires, hunting, etc. the MF would have survived. So what killed the MF?

The accumulation of stresses caused by (at least) all of the above factors. They had survived worse climate shifts before, but the climate combined with pressure by humans was insurmountable. The Megalania paper shows there has been a temporal overlap of some time - rather than the Blitzkrieg hypothesis, this is more of a constant pressure. This makes a lot of sense. As the authors say, as dating gets CHEAPER and more refined, I think we are going to see a lot more site dates around that mark. But dating is expensive, it isn't something we do unless we need to and have a few thousand $$ to spend.

I don't think it is necessary to pick any one thing as causal; the humans sank the boat, but if it wasn't for the climate change - it wouldn't have been dangerously low in the first place.

Anyway, that's my (academically unsubstantiated) 2c - it would be almost impossible to prove without a very complex computer model - but we don't know enough about the biology and ecology of the characters in the story to build such a model yet.

[Ian Thomas]

I'd go along with all that except I'd say the stresses applied to MF and to human popuolations were much more episodic and intense than traditional palaeo says. The last interglacial appears to have inflicted much worse climate shocks in the form of mega-aridity spikes than we have seen so far in the Holocene. Will return to that in a minute.

First the idea that MF and humans overlapped is not set in cement. Wroe contests it and is persuasive -->

Quaternary Science Reviews 25 (2006) 2692–2703
A review of the evidence for a human role in the extinction of Australian megafauna and an alternative interpretation
Stephen Wroe & Judith Field. Accepted 3 March 2006

Abstract
Arguments that megafaunal extinctions in Australia were anthropogenically mediated have focused on establishing terminal
appearance ages. This approach has been underpinned by three principle tenets:

(1)
if megafauna disappeared before significant climate change, but after human colonisation, then it can be inferred that extinctions were human mediated;
(2)
climate change within the last glacial cycle was unremarkable relative to previous cycles;
and
(3)
all or most Pleistocene megafauna were present when people arrived on the continent.

We review the evidence for human causation and note mounting evidence suggesting that the last 400–300ka in Australia has been characterised by escalating aridity and climatic variability, culminating in the breach of a hydrological threshold within the last
glacial cycle.

Only 21 species (35%) of megafauna whose disappearance has been attributed to human activity are known to have persisted after the Penultimate Glacial Maximum, a time of undoubtedly severe climate change.

Thus, 39 species of megafauna (65%) cannot be reliably placed within 85,000 years of firm evidence for human arrival, ca 50–43ka. At most eight species (13%) were clearly present at this time. Four or more persisted until the onset of full glacial conditions at ca 30ka.

We argue for a falsifiable model of staggered extinction in which most megafaunal extinctions predated human arrival and with the influence of people as a minor superimposition on broader trends in train since middle Pleistocene times.

There's a robust rebuttal of objections -->

doi:10.1016/j.quascirev.2006.10.009
A reply to comment by Brook et al. “Would the Australian megafauna have become extinct if humans had never colonized the continent?”
Stephen Wroe and Judith Field 11 January 2007.

If Wroe & Field are on the right track most of the MF were long gone by the time modern humans arrived in Sahul. However the oldest rock art does depict MF - mainly diprotodons around the Flinders Ranges and Thylacines + Thylacoleo elsewhere and now we hear Megalania was around at the time. OK, people must have encountered a few tailenders. If during later arid times like the LGM, MF survivors were forced to share dwindling water resources with humans we can guess things would have gone badly.

[Ian Thomas]

Is there any independent evidence that tends to back up Wroe's argument?

There's a paper by Zielinksi & others where they proxied aridity events in Sahul over the last 100kya. Deb put this one up years ago?
Here the key image again, pasted together from separate charts. Note the date of the really big events, how they correspond to estimated MF extinctions. Especially, they correlate with Wroe's suggestion that the main event happened long before humans were around at 60-68kyrs. The big spike at 60kya corresponds to dates I've seen for Lancefield. I've read that the Lancefield setting was a spring surrounded by dune-type desert and the assemblage has no young animals which means they had stopped reproduction.



Note the regularity of the events and their duration - we haven't seen anything like that in the Holocene. To me it's no wonder MF went down and all other human species too. We're the last ones standing.

Is my academically unqualified 2c worth - with a lot of past input from sis. Not proof but circumstantial?

[Ian Thomas]

Stuffed up, sorry. That paper refers to Greenland ice core data -> North. hemisphere. My bad.

It does record global volcanic and atmospheric dust episodes however. Which were of global extent. What's impressive is the likeness of the pre-LGM repeating spikes to the Younger Dryas post LGM - they last about 1000 yr each time.  The YD affected Australia so both NH & SH are involved - there's a paper reports the YD's signature in Buchan Caves here in Victoria. Which means we better expect more of the same in this interglacial.

I'll post the refs if of interest - wrong drive installed on this machine.

[Isengard]

Presumably though humans have prevented this being an interglacial by changing the atmosphere so an ice age cannot happen now?

[Ian Thomas]

Aug 27, 2015 4:54:36 GMT 10 Isengard said:

Presumably though humans have prevented this being an interglacial by changing the atmosphere so an ice age cannot happen now?

I don't think we've changed it much. The carbon cycle is gigantic compared with anything humans do.