Hypothesised? No, hypothesised about it, M. We know of many megafauna hunters, sabre-tooth tigers being the most famous example.
I don't really see why kill-sites (meaning a site showing the remains of vast numbers of prey species, yes?) are expected. A Mammoth carries a lot of food, so I would expect killing individuals and moving to the corpse to be a sane strategy for nomadic tribes peoples.
I got this hyopthesis from an excellent web-site, unfortunately I can't find the thing now. I'll probably have another search later.
However, it is doubtful that sabre toothed cats would have had much of an impact on mammoth populations. They might get lucky and pick of a few juveniles separated from the herd or isolated males in a weakend state but much like modern elephants, it is doubtful mammoths had serious natural predators other than humans.
quote:I don't really see why kill-sites (meaning a site showing the remains of vast numbers of prey species, yes?) are expected. A Mammoth carries a lot of food, so I would expect killing individuals and moving to the corpse to be a sane strategy for nomadic tribes peoples.
This would be a sane strategy but it is not what is proposed by overkill. If human hunters killed a couple of mammoths each season you would not expect kill sites..on the other hand you would not expect mammoths to be extinct either. If humans slaughtered all the mammoths across all of Asia and America you would expect to find kill sites because it would have required massive slaughter of a massive number of animals over a short period of time.
[This message has been edited by Mammuthus, 10-15-2003]
They might get lucky and pick of a few juveniles separated from the herd or isolated males in a weakend state but much like modern elephants, it is doubtful mammoths had serious natural predators other than humans.
Mammoths are by no means the only mega fauna of the age. I think it's highly likely that the very large carnivores that lived in the same period as the very large herbivores ate said herbivores. Otherwise I can see no explanation for their also increased size. I was also under the impression that adult mammoth remains had been found with teeth marks consistent with those of sabre-tooth tiger, although this might have been after-death scavenging.
This would be a sane strategy but it is not what is proposed by overkill. If human hunters killed a couple of mammoths each season you would not expect kill sites..on the other hand you would not expect mammoths to be extinct either.
quote:Mammoths are by no means the only mega fauna of the age. I think it's highly likely that the very large carnivores that lived in the same period as the very large herbivores ate said herbivores. Otherwise I can see no explanation for their also increased size. I was also under the impression that adult mammoth remains had been found with teeth marks consistent with those of sabre-tooth tiger, although this might have been after-death scavenging.
No, they were not the only megafauna. But they were a keystone species and there is precious little evidence that they had natural predators. The large size of elephantids, and tusk size in mammoths in particular, is better explained by sexual selection as opposed to predator avoidance. Elephantid size variation is rather pronounced and does not seem to correlate with presence or absence of predators. There is evidence of sabre-tooth killing of mammoth babies. Like with both living genera of elephants males once they reach puberty get booted from the group (which consists of adult females and juveniles). They then go off and either form bachelor male groups or wander by themselves. (This was also likely the case for mammoths). Individual males in a weakened state might be susceptible to predation but elephants in groups would not. Taking an occassional juvenile or weak and injured male mammoth would not consitute full scale predation and there is no evidence for any predator that would specialize on mammoths. Hard to imagine predators of a highly intelligent, massive, group living herbivore other than humans.
Nope. I prefer a combination of introduction of novel pathogens by the newly invading species (not just humans) affecting keystone species such as mammoths along with climate change allowing for faster breeding herbivores to outcompete keysoone species into extinction. Absence of the keystone species would have drastically changed the ecology in their former habitat which may have resulted in even more exinctions including sabre-tooth etc which also had competition from other carnivores who may have been better at switching prey.
The appeal to me of pathogens is not necessarily that it has to be correct but at least one can potentially collect evidence for introduction of novel pathogens that correlates with the arrival of humans. Positive supporting evidence does not seem to be available for overkill. Climate change as a singular cause of end Pleistocene mass extinction is also full of holes.
I think it's likely that it was a combination of factors. And yeah, I know there's no way to test the idea because there's no way to account for the interaction of all the variables. However, combination looks like the only possible solution. All three of the hypotheses have problems when taken in isolation. Neither overkill or overill have smoking-gun evidence, and in neither case does the pattern look promising. Overchill doesn't work because it simply doesn't make sense - there have been a number of extinction pulses over the last few million years, none of which match up with documented large-scale climate change (i.e. again the pattern is wrong). What does make sense is a combination.
One possible scenario would look something like this:
Climate change becomes a framework. Based on what we've seen happen in modern cases on a smaller scale, populations of organisms facing environmental disruption have the choice of either exploiting the newly available resources, moving (habitat tracking), or dying out. In the Late Pleistocene, we see evidence that the entire continental biome was shifting northward (grasslands expanding, conifer forest moving north and upslope, etc). Since this was a (relatively) gradual change - we're not talking an Alvarez event here - it would be relatively easy for at least a fair selection of each of the now-extinct species to track with it. Local populations might disappear, but the fairly widely distributed species we're talking about wouldn't suffer much more than a population decline in overall numbers - and probably not enough to cause a crash. Eventually, all other things being equal, a new equilibrium would be established further north.
However, during a transition period like this, metapopulation dynamics are in flux. The old source-sink equilibrium is disrupted. Source population distributions are shifting, and normal dispersal patterns no longer hold. Dependent sink populations - rather than being replenished over time - simply disappear. Again, all other things being equal, the dynamics would ultimately be re-established elsewhere.
What happens when (an) additional, large-scale disruptive factor(s) is/are introduced into a dynamic system which is already precariously balanced? This could cause the entire edifice to come crashing down. Overkill may not have been necessary. Simply the introduction of a new major selection pressure into this system capable of culling a sampling - whether through disease, hunting or both - of those northward-tracking, already-reduced populations could cause a ripple effect that escalates over a fairly brief period into a large-scale metapopulation extinction vortex. And just like any other extinction pulse, the secondary and tertiary effects ripple up, down and horizontally through the food web. In an already stressed ecosystem, the ripple effects could be highly magnified. I think this does account for the extinction pattern in North America, anyway.
Humans may have been the proximate cause - but not in the way the overkill or even overill hypotheses propose. They were simply an additional strain on a disequilibrium system that sent it over the edge. A form of natural disaster like a flood or volcanic eruption.
I'm wondering about why you are looking viruses or whether you have counted out bacteria for some reason I'm not picking up on. What about Anthrax? Has it been ruled out for any reason? Then what about diseases such as brucellosis, it may not kill the host, but may effectively render them sterile.
I also have a few more questions about primers, but will serve them up later.
Here is a link for anyone to consider the overkill hypothesis. It's only been around since the 1960s and the major proponent of it is Paul Martin at the U of Arizona. (Grayson and Martin are good friends incidently.)
quote:Also because of the thickness of the skin and the high muscle to fat ratio the meat of an elephant is not as good as other large herbavores
But what can't be eaten can be used for other quite useful purposes, from shelter to clothing.
quote:This still does not address the extreme lack of kill sites
I find it surprising that you would expect to find even a significant percentage of total kill sites from a species hunted in such a vast range for such a (geologically) insignificant time. We don't hold anywhere close to such a standard for other fossils - why would we expect it for mammoths? A much more reasonable stance would be to compare the number of mamoth fossils found at kill sites during this time period to those not found at kill sites.
quote:Aside from the fungi that live on Choloepus
And insects. Which live in the hair, not the skin (or more importantly, the muscle).
quote:I can hardly imagine anyone subsisting on sloth meat
Why? Because of the size? If so, then Mylodon isn't addressed.
quote:In any case, Mylodon, Northrotheriops to a lesser extent, was full of ossicles throughout the skin and probably not so easy to kill.
Humans are inventive. Depending on species, you may find mammoths that were upwards of 10 tons. Mylodon was only the size of an ox. Yes, it's better armored - but so? Humans have fought off far more fearsome armored creatures than that - for example, Megalania prisca. Throughout history, humans have killed crocodiles and alligators; small families of islanders alone have fought off komodo dragons, which have some pretty impressive armor; etc. I have little doubt that humans could have killed mylodon. Our chiefest hunting ability has been to learn the weak points of our prey.
quote:This assumes that the immigrants practiced a more non-economical form of subsistance i.e. killing more than you need in the lands they came from. I don't know that there is any evidence for this.
There's tons of evidence that early natives to different areas employed incredibly ecologically destructive practices. When westerners first found Easter Island, there wasn't a tree on the island more than 10 feet tall, and a devastated people in constant warfare; this is why the presence of giant statues (which would have needed scaffoldings to make) was so surprising. Digs on the island have revealed that it used to be almost completely forested.
Likewise with the Anasazi. For a while, it was a puzzle how the Anasazi developed such a large civilization in the middle of a desert. However, archaeology has revealed that initially, Chaco Canyon wasn't a barren desert - arid, yes, but it was forested with pine and fir trees. As logging increased, they responded not by reducing consumption, but increasing the range that they brought in resources from. They built elaborate log-roads to get their wood in - from as far as 80 kilometers. Being a fairly delicate area, the region was inexorably altered.
I find it amazing that many people have trouble accepting that, given the track record of humans, that this would happen to the megafauna. What we're dealing with is a "land of plenty" situation. Humans move into an area where animals are not adapted to survive with humans hunting them or competing for their food supply, and where the flora is in rough equilibrium with its environment. Humans encounter what seems like a limitless hunting and harvesting paradise. A human population explosion occurs, and a native species population bust occurs, with some species going extinct. It has happened across the entire planet. If too many species go extinct in a region, the human populations there will decline or die out as well. As a consequence, over time the more balanced resource-utilizing societies are selected for.
And I agree with Speel-yi - there is far too wide of a range of species that go extinct to be explained by disease alone. I don't buy into the concept of a lethal disease that happens to jump species, but never made it over before we did, despite all of the migrations by other species of animals. Disease may be part, but wasn't the cause itself.
I think you have to back up and consider that the examples that you are using are from agricultural settings. The use of optimal foraging technique would demand that a hunter be as opportunistic about getting kills, so if you have a species that is overhunted, the hunter switches to a more easily found prey simply because they go after things that are more easily found. In this way, you can see that a new equilibrium will be reached.
There is no doubt that humans modified the environment, but whether and how this modification led to the extinction of species is something we should figure out soon because it seems to be a continuing problem.
------------------ Bringer of fire, trickster, teacher.