Social Groups Stabilize Ecosystems

Not exactly novel news, but an interesting study nonetheless. There is even an application to human evolution and the extinction of the megafauna:

Ecologists have been puzzled for decades over the stability of predator-prey relations, said ecologist Craig Packer of the University of Minnesota, a co-author of the paper. “Traditional ecological models have erroneously predicted that predators would inevitably over-exploit their prey, leading to frequent population crashes. But most highly vulnerable prey species form herds, swarms, schools or flocks, and group living reduces predators’ efficiency to the point where co-existence is likely to be the rule rather than the exception.”
Packer said that while sociality in early humans and in their prey might have allowed long periods of co-existence, we eventually became such extraordinarily efficient hunters that herd formation could no longer protect our prey from mass extinction during the great die-offs in North America and Europe around 12,000 years ago.

If memory serves Meave Leakey has done some work on hominins moving into a carnivore niche…

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26 Responses

  1. is there any actual evidence that humans drove the megafauna to extinction?
    I mean, it’s one of those attractive theories for the Naked Ape types… “Man the Mighty Hunter” and all that, but wasn’t there this thing called climate change going on around about then, too?
    In North America the First Nations were extraordinarily efficient hunters of herds, but they came nowhere near threatening the buffalo with extinction, for instance.

  2. Interesting stuff. It reminds me of a paper in Conservation Biology from earlier this year about prey animals (mostly herding artiodactyls like elk, deer, and bison) quickly learn to pick up on the presence of re-introduced carnivores like wolves, showing that the carnivores would not simply return and wipe out all the herbivores.
    I’m actually writing up something today about scavenging & early hominids, primarily in terms of the relationship of felids to scavenging techniques, but obviously hunting became very important later on. I’ve always had some problems with the “overkill hypothesis,” though, and while I don’t doubt that hunting impacted herding animals to a point I’m not so sure so many animals could have been driven to extinction by hunting alone. Hopefully more work will be done on the Pleistocene impact hypothesis as I would imagine it would have important implications for North American fauna at least.

  3. I’m more familiar with the “prudent predator” hypothesis. The idea that predatores focus on most abundant prey and as abundance declines shift focus to another species. While in graduate school, I heard a presentation on housecats in California. These cats would hunt one species of mouse to extinction before switching to another species. The investigator was puzzled by this. Some years later, I realized that the housecat was an introduced species and that this was not a co-evolved predator-prey system.
    I like Martin’s overkill hypothesis for the decimation of the New World megafauna; however, I suspect the introduced disease via dogs, and the climate change hypotheses also have virtue.
    One night, in North Texas, I came upon a situation where twenty or so coyotes had herded a hundred or so jackrabbits into a ‘baitball’. I don’t know if this is in the literature.

  4. The Overkill arguments generally go something like this: 1) climate change happens all the time, but only when humans arrived did it result in mass extinctions in North America; 2) everywhere else that humans have arrived “recently” – Pacific Islands, Australia – mass extinctions have occurred immediately.
    In addition, our bison bison turns out not to be a direct descendant of ancient North American bison. Instead, it’s descended (with modification) from the Asian bison, which expanded into NA after the other megafauna were gone. This animal,of course, had co-evolved with people for quite some time already.
    Not proof positive, of course, but the circumstantial evidence seems awfully hard to account for other than humans being the major culprit.

  5. I’ve always been somewhat skeptical of Martin’s hypothesis myself. I’m not aware of any human caused extinctions up until after the invention of gunpowder (but I could be wrong). Although as ScottH points out humans and Bison bison did co-evolve to some extent… At any rate, I found this interesting more for what it might say about human evolution as hominins constructed a more overtly carnivorous niche.
    Edit: I forgot to mention that the research is being published in Nature

  6. Take a look at the literature on extensive prehistoric human-caused extinctions on Pacific islands. One of the reasons you don’t hear much about E.O. Wilson’s island biogeography theory these days. It was hailed at the time as the ecological counterpart of the discovery of DNA structure. Wilson was unaware of the prehistoric extinctions, and thus did not include them in his calculations.

  7. I knew there was something out there I was overlooking. I will have to dig into that…

  8. Afarensis wrote, “I’m not aware of any human caused extinctions up until after the invention of gunpowder”. As Jim Thomerson says bird extinctions in the Pacific islands coincide extremely closely with the expansion of humans. They didn’t have gunpowder in those days.
    Graculus wrote, “is there any actual evidence that humans drove the megafauna to extinction? … wasn’t there this thing called climate change going on around about then, too?” If you look at each case individually you can certainly find something other than humans to blame. But extinctions happen at different times in different parts of the world, but always soon after the first arrival of humans. So much so that I believe it’s possible to date human expansion simply by looking at extictions in each region. If the other evidence is inconclusive go with the extinction evidence.
    As well as the examples from the Pacific and America above there is the evidence for megafauna extinction in Australia. By 45,000 years ago. And yes, climate change has been blamed there as well.

  9. I read something once which suggested that early humans in Australia did enough burning that the entire ecosystem changed. In any case, I can’t picture humans accepting the existance of large land crocodiles and 30 ft long monitor lizards. Tastes like chicken, you know.

  10. Yes Jim. Australia’s vegetation has been so profoundly affected by fire it’s often called “an Aboriginal artifact”. I think it was Flinders who wrote that he could tell from a distance that Kangaroo Island was uninhabited when he first saw it. No smoke. On the nearby mainland the sky had been constantly full of smoke for days as he sailed along the south coast. This was long after the land crocodiles and huge lizards had died out.

  11. I think the island extinctions are a different case, in terms of the ecology and opportunities of populations to recover. Nor do I think the limit would be gunpowder. Native Americans drove entire herds over cliffs with nothing more than fire and flags. Extremely efficient, but not technologically advanced.
    Nor do I buy the co-evolved entirely. The horse became extinct in North America, yet thrived in Asia, a far more humanized ecology. Were Eurasians less efficient predators than North Americans?
    The disease hypothesis is interesting, and far more plausible than predation (crunch the numbers sometime to see how many people would have to be doing how much killing, with nothing more than sticks and stones). Climate change is what encouraged human expansion, too. Correlation is not necessarily causation. Human hunting may have driven the final nail into the coffin of species already teetering, but I don’t see how it could, given the technology and population density of the time, have driven healthy populations under.
    One interesting thing I ran into a while ago was a paper that looked into the effects of rising temperatures on the current megafauna. Apparently rising temperatures also effect reproduction rates in modern temperate zone megafauna (cows, for instance).
    The factors involved look far, far more complex than “Thrud the Mighty Hunter did it” (in the library, with an atlatl).

  12. OK Graculus. “Crunch the numbers sometime to see how many people would have to be doing how much killing, with nothing more than sticks and stones”. Doesn’t take many, especially if they’re surrounded by plenty and only eating the choicest parts, as the Moa-hunters were doing in New Zealand.
    Of course we know that environmental change has always been the main cause of extinction but a couple of us have mentioned use of fire. That has certainly changed the environment of Australia over the last 50,000 years. Sure, disease may have come in with humans but when was the last time an infectious disease knocked over a whole species? It may be doing so today for the Tasmanian devil but that species is already under intense pressure from human-caused environmental change. The main reason it’s suffering is because of the consequent inbreeding. I agree the cause is “far, far more complex than Thrud the Mighty Hunter did it”, especially not just “in the library, with an atlatl”. Human expansion brought far more environmental change than just that.

  13. OK Graculus. “Crunch the numbers sometime to see how many people would have to be doing how much killing, with nothing more than sticks and stones”. Doesn’t take many, especially if they’re surrounded by plenty and only eating the choicest parts, as the Moa-hunters were doing in New Zealand.
    Of course we know that environmental change has always been the main cause of extinction but a couple of us have mentioned use of fire. That has certainly changed the environment of Australia over the last 50,000 years. Sure, disease may have come in with humans but when was the last time an infectious disease knocked over a whole species? It may be doing so today for the Tasmanian devil but that species is already under intense pressure from human-caused environmental change. The main reason it’s suffering is because of the consequent inbreeding. I agree the cause is “far, far more complex than Thrud the Mighty Hunter did it”, especially not just “in the library, with an atlatl”. Human expansion brought far more environmental change than just that.

  14. I googled ‘Martin overkill” and got this as the first hit. Looks pretty comprehensive. http://everything2.com/index.pl?node_id=1694655

  15. terryt – that last sentence, I think, is probably more in keeping with the evidence. I have seen a number of studies that indicate the extinctions were not instantaneous (in the geological sense). If memory serves there was a paper on mammoths out last year that indicated that the local extinction of mammoths (I want to say in Alaska, but I am going to have to dig through my PDF’s ) took several thousand years. As Jim’s link points out, there is also some cause for skepticism in that most of the megafauna that went extinct do not show up at Clovis kill sites. So I agree that the situation is a good deal more complicated than “Thrud the Mighty Hunter did it”. As I see it humans are a contributing factor to the megafaunal extinction, not the primary cause. For those who are interested, but don’t know much about the subject follow the links below.
    http://faculty.washington.edu/grayson/jas30req.pdf
    http://www.unr.edu/cla/anthro/download%20Haynes_catastrophicExtinction.pdf
    http://faculty.washington.edu/grayson/jas31contd.pdf
    http://www.unr.edu/cla/anthro/downloadFiedelHaynes_JAS.pdf
    http://quaternary.net/WhitneySmithExtinctionSD2004.pdf
    http://www.unr.edu/cla/anthro/download%20Haynes_MisrepresentedOverkill_QI.pdf
    http://www.amnh.org/science/biodiversity/extinction/Day1/bytes/AlroyPres.html
    http://uwacadweb.uwyo.edu/SUROVELL/pdfs/Extinction%20Proofs.pdf

  16. I think humans were the primary cause, since large animals disappear (and a lot that didn’t lost big chunks of their original ranges) soon after we showed up in any given area…
    And, for that matter, that Africa and southeast Asia (places with millions of years of human presence) lost few species while the Americas and Australia lost virtually all their large animals.

  17. Afarensis, you said, “most of the megafauna that went extinct do not show up at Clovis kill sites.” But the link actually says, “In Blackwater, New Mexico evidence of eight mammoths were found associated with Clovis projectile points carbon dated between 11,630 – 11,040 years ago. In Burning Tree, Ohio the butcher-marked bones of a mastodont were located and dated to 11,660 years ago. In Lehner, Arizona evidence of thirteen mammoths associated with Clovis projectile points and dating to 10,900 years ago were found. The list of mammoth kill sites associated with early Paleoindian culture and dated from 11,700 to as early as 10,670 is long.”
    You may be partly correct when you say, “humans are a contributing factor to the megafaunal extinction, not the primary cause.” BUT, the megafauna would not have died out if not for the presence of humans.

  18. terryt – I should have been more clear when I said that. I was actually thinking of Grayson and Meltzer’s 2002 paper in the Journal of World Prehistory (Vol 16:313-359).
    Also, and this is a comment added later, Jim’s link does say:

    And finally there is the lack of evidence that Clovis hunters were utilizing the entirety of the now extinct animal kingdom. Largely evidence of big game kills has been focused on the mammoth with projectile points found embedded in bone and other definitive evidence. The lack of evidence is what keeps many archaeologists and vertebrate paleontologists from going along with the overkill theory. It’s only recently that data proving that horses were hunted by Clovis has come into light, and other than a few animals aside there are still many creatures un-represented at Clovis kill sites. However, just as the horse data was only recently retrieved, it is possible that in time more sites will be discovered containing the protein residue or bone fragments of various other extinct animals.

  19. This is the paper on mammoths in Alaska that I mentioned in an earlier comment…

  20. Thanks for those links Afarensis. I must admit I loved the way the authors of your first link in your earlier list stuck up for the Irish. They point out that reindeer and Irish elk died out about 10,600 years ago and humans arrived there 10,000 years ago. Leaving out the margin of error in both dates, how are they so certain the dates represent the true last date for the deer and first date for humans? I remember reading something similar regarding mammoth extinction on Catalina. Also humans arrived five minutes after mammoths became extinct there. Same problem. I’ve even seen the same argument used to prove the first Australians weren’t responsible for megauna extinctions. Megafauna died out in one valley in Queensland before evidence in it for human presence. The author ignored the fact humans were well established in the rest of Oz thousands of years before then.
    Years ago I read a book on mammoths. The author dated mammoth extinction on the Russian steppe at 35,000 years ago, North China steppe 20,000 years, Europe 15,000, North America 10,000, Wrangel Island 7000. Fits the human expansion pattern pretty well I thought. You can see from any map why mammoths lasted on Wrangel until relatively recently.
    As for extinctions not happening in Africa. The book “The Age of Mammals” (Kurten 1971) claims by 60,000 years ago three-toed horses, a giant deer, an antlered giraffe, giant baboon, sabre-tooth and scimitar-toothed cats, and many other species had all become extinct there.
    Sounds pretty convincing to me.

  21. Correlation, however, does not equal causation, and as the first article points out the tie between kill sites and megafauna is slim. Only 14 secure cases of mammoth or mastodon associated with Clovis sites (here, obviously, I am referring to North America only). Surely, if the megafauna went extinct because humans hunted them to death there would be kill sites all over the place with every kind of megafauna imaginable?

  22. Correlation, however, does not equal causation, and as the first article points out the tie between kill sites and megafauna is slim. Only 14 secure cases of mammoth or mastodon associated with Clovis sites (here, obviously, I am referring to North America only). Surely, if the megafauna went extinct because humans hunted them to death there would be kill sites all over the place with every kind of megafauna imaginable?

  23. I agree correlation does not equal causation but it’s all quite a startling coincidence don’t you think?

  24. I may have faulty memory on this, but I recall that Martin cited lack of kill sites as positive evidence for his hypothesis. A kill site is the result of processing the kill for furthur use; butchering, drying, cooking, etc. But suppose you kill a mammoth for breakfast, and only eat the tongue. Then you kill a bison for lunch and have some hump meat. Kill a horse for supper and have some liver. There are no kill sites, just largely intact meals for scavengers. I think this is the kind of killing (more or less) Martin has in mind.

  25. Yes, he did make some such claim, but I find that unconvincing. It may explain the lack of processed remains, but doesn’t explain the lack of remains. Whether or not you butcher the mammoth, etc., after you kill it, you still have to kill it. With a spear or some such and that is going to leave traces in the skeleton. As will some of the selective butchery you mention (we can see some evidence of selective butchery along those lines later in cliff jumps). Martin’s suggestion can’t explain that.

  26. I’ve said before that if we focus on just one region we can usually find an explanation for the particular extinction event that doesn’t include humans. After all, as supporters of ID demonstrate all the time, it’s possible to prove anything if we ignore enough evidence. But, if we look at the expansion of humans beyond the ancient Homo erectus range we find the timing of megafauna extinction coincides remarkably with that expansion. How do we explain that?

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