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That seems to be the conclusion of a mostly European research:
Mathias Stiller et al. Withering Away—25,000 Years of Genetic Decline Preceded Cave Bear Extinction. Molecular Biology and Evolution 2010. Pay per view.
Abstract
The causes of the late Pleistocene megafaunal extinctions are still enigmatic. Although the fossil record can provide approximations for when a species went extinct, the timing of its disappearance alone cannot resolve the causes and mode of the decline preceding its extinction. However, ancient DNA analyses can reveal population size changes over time and narrow down potential causes of extinction. Here, we present an ancient DNA study comparing late Pleistocene population dynamics of two closely related species, cave and brown bears. We found that the decline of cave bears started approximately 25,000 years before their extinction, whereas brown bear population size remained constant. We conclude that neither the effects of climate change nor human hunting alone can be responsible for the decline of the cave bear and suggest that a complex of factors including human competition for cave sites lead to the cave bear's extinction.
Cave bears went extinct roughly at the time of the Last Glacial Maximum, 25-18,000 years ago.
See also:
· Article at Science Daily
· Leherensuge: Mammoths died because of forest expansion.
23 comments:
"a complex of factors including human competition for cave sites lead to the cave bear's extinction".
But they suggest the final blame does rest with humans, because even 'the decline of cave bears started approximately 25,000 years before their extinction' could have been a product of competition with Neanderthals. I don't think anyone would blame the megafauna extinctions at the Last Glacial Maximum on 'human hunting alone'. Habitat destruction is always the main cause of extinction. It's unlikely that hunting alone led to the near extinction of the North American bison, and certainly not the passenger pigeon and the carolina parakeet. And the moa was presumably not hunted to extinction in New Zealand. Fire, and introduced predators were primarily to blame. But humans were responsible for those vectors.
"It's unlikely that hunting alone led to the near extinction of the North American bison"...
Well, in this case, destructive disrespectful hunting was surely the cause. But no hunter-gatherer people would or even could do what "the white man" with automatic rifles, railroads and large numbers could do.
The comparison is most unfortunate.
"... introduced predators were primarily to blame".
I watched a documentary on cats in Australia several times these last months and the conclusion seems to be that rather than cats it is the extinction of local competitor predators (Tasmania's devil) and specially the alteration of the landscape by sheep herding what may be the main factor to blame. Cats can hit the final blow but they are not responsible for cornering species and depriving them from their habitat. This happens because modern "hyper-intensive Neolithic" human practices.
"Cats can hit the final blow but they are not responsible for cornering species and depriving them from their habitat".
I agree that alteration of the landscape is the most important contributor, and was probably so in the Pleistocene/Holocene extinctions. But given the habitat alteration, in New Zealand the survival and increase in native bird species is dependent solely on the control of introduced predators such as cats, rats, mustelids and dogs. In areas where they are controlled or exterminated endangered birds proliferate. So it seems that the makers of the documentary on cats in Australia you have been watching have gone out of their way to avoid blaming cats. Probably so they can still sell their program to cat lovers.
I believe the documentary is good and looks at all sides of the problem and it's not the film makers but the interviewed, all experts in ecology and fauna, some Aboriginals, who produce the analysis. It's not about cat lovers when you eat cat at a Pintubi hearth, is it?
The problem is loss of habitat and numbers. Predators alone (which are also food, btw) can hardly get a species extinct. Predators essentially keep their prey dynamic, evolutionarily dynamic, by killing the less perfect individuals.
Cats in the desert, where no farming happens, have not driven a single species extinct, regardless that they are excellent at living without water. They only cause such extinctions in refuges where a few survivors from human colonization are probably doomed anyhow.
They are essentially a scapegoat to hide the guilt of sheep and shepherds.
"it's not the film makers but the interviewed, all experts in ecology and fauna"
Do they interview Tim Flannery? I'd take seriously anything he says on the subject.
"Predators alone (which are also food, btw) can hardly get a species extinct".
Which is why I couldn't make sense of the comment:
"the extinction of local competitor predators (Tasmania's devil)"
How would the extinction of predators lead to the extinction of the prey?
"Cats in the desert, where no farming happens, have not driven a single species extinct"
Not actually true. Many desert-dwelling species have become extinct. I agree cats are not entirely to blame. Foxes are widespread.
A short article on cats in Australia. Seems that Tim Flannery actually claims the influence of cats has been exagerated, but this article disagrees:
http://feral.typepad.com/feral_thoughts/2008/12/the-feral-cat-versus-the-environment-denying-past-impacts-will-lead-to-future-problems.html
"Do they interview Tim Flannery?"
Can't remember. I always forget names unless written and related to something I'm working in. Try with a photo? I used to be good at remembering faces (devoid of name).
"How would the extinction of predators lead to the extinction of the prey?"
Breaks the balance, and removes a competitor for cats too. No cat-related extinction has been reported in Tasmania.
"Not actually true. Many desert-dwelling species have become extinct".
I remember they said: cats eat almost only lizards in the dry season (only cats and lizards seem able to survive there) and they have not driven any single species of lizard extinct nor anywhere near risk.
"I agree cats are not entirely to blame. Foxes are widespread".
Foxes can't live in the desert like cats do.
Anyhow your attitude is simplistic: predators kill prey, hence prey is driven to extinction by predators (when it's in fact kept healthy and in the appropriate numbers for ecological balance). This is simply false: damage to the environment such as the destruction of forest (or other natural environments) for farming, shepherding or mere human construction is the key factor, along the introduction of many alien wild animals, like rats, rabbits and goats, which compete with native fauna for the same resources.
The situation that may happen in a small island is not the same as what may happen in a large island or in a whole continent, even in one as fragile as Australia.
Btw, I will try to remember tomorrow and post something on these but, fyi, these two news items are very much related to the main theme and the discussion:
- carnivores shrank in global warming following herbivores
- human predation may have boosted numbers of a sea mollusk
We are arguing over whether cats or sheep have been the cause of extinctions. This is the wrong end of the argument. Both were introduced by humans anyway. So, like the cave bears and the mammoths, humans are responsible for the extinctions.
"Breaks the balance, and removes a competitor for cats too".
But that balance is simply a result of time. Given time cats in New Zealand and Australia will fall into balance with their prey too, but many extinctions will have occurred by then.
"No cat-related extinction has been reported in Tasmania".
I'm fairly sure that is incorrect. Anyway, what was the cause of the extinction of the native predators? I'm fairly sure cats and dogs have been blamed for tasmanian devil huge decrease in numbers.
"cats eat almost only lizards in the dry season"
OK. And what do the native mammals in the region eat?
"Anyhow your attitude is simplistic: predators kill prey, hence prey is driven to extinction by predators (when it's in fact kept healthy and in the appropriate numbers for ecological balance)".
Introduced predators change that ecological balance. The fact is well recognised by every ecologist I know. Cats were eliminated on Little Barrier Island in New Zealand and since that time bird life has increase astronomically. Draw your own conclusions.
"damage to the environment such as the destruction of forest (or other natural environments) for farming, shepherding or mere human construction is the key factor, along the introduction of many alien wild animals, like rats, rabbits and goats, which compete with native fauna for the same resources".
I agree. But introduced predators are part of the same suite of factors. Habitat destruction is always the main cause of extinctions, including that of the megafauna.
"The situation that may happen in a small island is not the same as what may happen in a large island or in a whole continent, even in one as fragile as Australia".
The situation in Australia is exactly the same as that in New Zealand, which is exactly the same as that in Hawaii, and so on.
Regarding size reduction in warmer periods. The authors answer their own riddle:
"Mammals in warmer climates today tend to be smaller than mammals in colder climates"
And the conch story is not what it appears to be either:
"Fitzpatrick believes the size increase is likely related to an increase in nutrients in the conch's waters, stemming from increased agriculture and other human activities".
Shells in early middens in New Zealand are generally much larger than are those of the same species today.
"But that balance is simply a result of time".
Not really: it is the result of the overall health and balance of the ecosystem, including each of its pieces in any given time. Obviously, provided that there are no further disturbances, the ecosystem tends to re-stabilize itself but that is not the result of time as such but of atemporal internal balance at every instant.
I think that more than waiting for time to do its job, we should think on what really happens through that time.
"We are arguing over whether cats or sheep have been the cause of extinctions. This is the wrong end of the argument".
Not really. Herbivores and such are the center of the universe, carnivores are just their regulators.
"Both were introduced by humans anyway".
But sheep would not survive feral, nor in most of the environment naturally.
Dingos were also introduced and now they are considered a central piece of the ecosystem and a non-destructive species. For the Pintubi both animals are "native" (have been there since they can remember).
"The situation in Australia is exactly the same as that in New Zealand, which is exactly the same as that in Hawaii, and so on".
Gross simplification. No two situations are "exactly the same". Plus the size and ecosystem typology of those "islands" is totally not comparable between each other.
You are using "magical thought" where things are not for what they objectively are but by means of simplistic (and non-realistic) comparison.
"The authors answer their own riddle:
"Mammals in warmer climates today tend to be smaller than mammals in colder climates"".
Fair enough. So it makes sense that when there's global warming, largest animals have problems, right?
"And the conch story is not what it appears to be either:
"Fitzpatrick believes the size increase is likely related to an increase in nutrients in the conch's waters, stemming from increased agriculture and other human activities"".
Maybe, though I can't imagine what kind of nutrients might human Neolithic activity produce other than a little pile of trash. Could it be because of the spread of pigs, who work the soils a lot (and are in fact used as "living ploughs" in permaculture)?
"No two situations are 'exactly the same'".
No. But they're close enough to make very valid comparisons.
"Maybe, though I can't imagine what kind of nutrients might human Neolithic activity produce other than a little pile of trash".
So where do you imagine that island people usually have their latrines?
"So it makes sense that when there's global warming, largest animals have problems, right?"
No. They get smaller.
"Herbivores and such are the center of the universe, carnivores are just their regulators".
And when the herbivores dissappear what do the carnivores eat?
"Dingos were also introduced and now they are considered a central piece of the ecosystem and a non-destructive species".
Yes. After the'd caused the extinction of the Tasmanian devil on the mainland, not to mention the marsupial wolf, and presumably various herbivores.
"For the Pintubi both animals are 'native' (have been there since they can remember)".
So now you're taking myths seriously and ignoring scientific evidence? It is you who is 'using "magical thought" where things are not for what they objectively are'. I should have seen it all along.
"But they're close enough to make very valid comparisons".
Doubt it.
"So where do you imagine that island people usually have their latrines?"
You can't be serious?! Other animals also have to, you know. There's no particular reason for human manure to be more productive than any other.
"No. They get smaller".
They get smaller because larger animals are selected against, what is the same.
"And when the herbivores dissappear what do the carnivores eat?"
What are you talking about? Each carnivore unit (individual or pack) usually manages a territory and keeps other carnivores at bay. Normally there are enough prey. If not, they die and the herbivores can recover. A balance is spontaneously kept and restored.
If all rabbits die, the Iberian lynx goes extinct of course. However the real reason is human encroaching of the territory (at a scale that is many orders of magnitude greater than not just Neolithic... but anything before the late 19th century). No hunter-gatherer group could do anything of the like.
"You can't be serious?!"
Sorry to disillusion you, but I am. Most island comunities place their latrines over the open sea. I guess it avoids having to step on it. So the manure is not particularly productive, it's where its placed that has the noted effect. Of course they don't eat the shellfish collected near their latrines but the nutrients would spread more widely.
"They get smaller because larger animals are selected against, what is the same".
No it's not. The species survives quite readily.
"Normally there are enough prey".
But werten't we talking about occassions when the herbivores become extinct?
"No hunter-gatherer group could do anything of the like".
I'm afraid the evidence is pretty overwhelming that they did. So take your blinkers off.
But you mean that crabs, octopuses, fish, etc. don't crap? I don't make any sense of your "latrine theory".
"The species survives quite readily".
Not necessarily if it's a large-sized species. Whatever the case, it seems that warming acts against oversized animals.
"But werten't we talking about occassions when the herbivores become extinct?"
I don't really think herbivores can become extinct in normal circumstances (i.e. excepting maybe mass extinctions, which is not the case). Some hervibores may die off or become too rare but others will thrive instead. If wolves can't hunt bison, they will hunt horse, and if they can't hunt horses either, they will hunt deer, boar or whatever is at hand.
"I don't make any sense of your 'latrine theory'".
No wonder you have trouble understanding so many other things. It's basic ecology. Why not enrole in a course somewhere?
"Whatever the case, it seems that warming acts against oversized animals".
And cooling acts against undersized animals, both processes are widely accepted biological facts.
"I don't really think herbivores can become extinct in normal circumstances"
Mostly when hunted by newly arrived predators or when habitat was severely altered. Humans have long been agents of both phenomena.
"No wonder you have trouble understanding so many other things. It's basic ecology. Why not enrole in a course somewhere?"
If you are going to launch insults instead of providing meaningful discussion/information, then please do it somewhere else.
I would have thought it was obvious what the cause was. Shitting onto the water at the same place all the time would surely raise the local nutrient level, allowing the shellfish to grow larger. Shit from crabs, octopuses, fish, etc. tends to be distributed more widely.
Well, for me it's quite obvious that there are many many other animals feeding the life cycle, so to say. Humans, hunter-gatherer humans specifically, could only have a very limited impact that way.
The reason must be some other.
"The reason must be some other".
Certainly. We can't possibly blame humans. The implications would be far too great. Just as many people today deny humans can possibly have any role in global climate change.
It's not that we cannot blame humans, it is that your logic makes no sense. A handful of humans cannot produce even a tiny fraction of the manure of any natural ecosystem.
You compare too much across ages, when these are each very different. The economy, ecology and densities of the Paleolithic certainly cannot be compared with the Neolithic, the Metal Ages nor much less the Industrial Era. It's a radical change of conditions, economy, densities... everything!
Humans now are surely causing global warming but some humans are causing most of it while the vast majority only has a miscellaneous impact. It is industrialization what causes that and not even the mere sheer numbers of people but the unsustainable consume of a fraction of them alone.
This is called ecological footprint and hunter-gatherers' ecological footprint is almost indistinct from zero.
In your latest two comments you adopt contradictory perspectives.
"A handful of humans cannot produce even a tiny fraction of the manure of any natural ecosystem".
In the discussion on genetic flow across the strait of Gibraltar you're arguing in favour of relatively dense ancient human population. So much so that you claim that incoming Neolithic populations could not have had any marked effect on the (presumably large) Paleolithic population.
"A handful of humans cannot produce even a tiny fraction of the manure of any natural ecosystem".
I'd remind you that in this case we are not talking 'a handful of humans'. We're talking specifically about the influence on sea mollusks of a Neolithic population on a Pacific island.
"This is called ecological footprint and hunter-gatherers' ecological footprint is almost indistinct from zero".
OK. Is that so? Let's look at the extinction evidence:
New Zealand 800 years ago
Madagascar 2000 years ago
Cyprus 10,000 years ago
America 12,000 years ago
Northern Eurasia 12,000 years ago
Central Eurasia 35,000 years ago
Australia 46,000 years ago
Spot anything significant? Humans not responsible? Then why do extinctions throughout the world coincide remarkably well with human arrival in each particular region?
No. I do not think that it was the density of European foragers what weakened the immigration of similarly European farmers some 7,000 years ago. What I think is that the archeological record shows that there were not many immigrants and instead many many foragers-turning-farmers. This is particularly true in Mediterranean Europe.
But, anyhow, even early farmers densities would have more ecological impact by means of tree cut and the sheep and cattle manure than because of human faeces. And these were not just denser than foragers but also more intensive in their usage of the land. Whatever the case, we have no clear evidence that farmers affected the ecology at large scale but they were certainly more potentially harmful than foragers, because they were sedentary, cleared fields for crops and got their sheep, pigs, cows, goats... grazing around.
And also lived in quite higher densities.
"... why do extinctions throughout the world coincide remarkably well with human arrival in each particular region?"
I suspect those dates are conveniently selected to support your thesis: among many possible extinctions you have picked some, and not others. These few may indeed have been damaged by overhunting. But still human hunters alone had not the option of genociding a whole species, they were just an additional element in the doom of some species, which surely had other problems of its own.
You know: animals used to go extinct before humans arrived too.
"I suspect those dates are conveniently selected to support your thesis"
Complete rubbish Maju. Have a search for the information yourself, instead of just hanging onto your few, strongly held beliefs.
"You know: animals used to go extinct before humans arrived too".
Correct, but not all at once, unless an asteroid hit.
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