Scientists: Are we in 6th Great Extinction?

Great Debates
21

Colibri answered this well. As long as a breeding pair/ fertilised female exists, the species isn’t extinct. Long-term survival is another kettle of fish entirely.

There are various theories regarding the number of individuals required for a species to have a viable population, ranging from “one breeding pair”, to “'50”, “500”, through to “unknown”.
The “50” figure is derived from observations that imply that a loss of genetic variance of 1% per generation causes no genetic problems. The “500” figure is (from memory) based on the lowest population level at which heritability will likely remain at 0.5 (which is in turn assumed to reflect a genetically ‘healthy’ population). While attempts have been made to quantify a minimum population size, these are arbitrary and purely speculative. We do not know.

Even if a minimum population size for viability was determined for a species, it is still a probability only, not a guarantee, as the environment and populations are not static. Many of the pressures that are affecting long-term viability of many species will remain unchanged. Even if you gathered the required number of individuals, embarked on an expensive breeding program and successfully ‘saved’ a species, what then? Do you release the population back into the same environment that drove the population into an endangered state? Do you concentrate on saving a species, or saving the habitat of many species? How do you choose which species to save - there isn’t enough cash to save all threatened species. How much is a given species worth?

22

How many of those species were:

  1. Significantly different from a similar subspecies so that the fossil record could tell them apart? The Iberian Ibex is an example- it isn’t all that different from the African Ibex- some call them subspecies. The Aurochs of course is still around, we have millions of them- we call them “cows”. True, our modern “cattlebeast” looks a lot different, but they were able to breed back to get an Aurochs.

  2. Occupied a Large ecologically significant role? A species where there was never very many, and all of them on one island, isn’t something we’re going to notice in the fossil record.
    The point being- when they counted 'species" that became extinct in the last big die-off, they were able only to could significantly different, and large populous, ecologically significant species. Ones that left a fossil record.

So far- we haven’t put much of a dent in those species. The fossil record wouldn’t show very many extinctions in the last 4000 years or so. Many of those were marginal to start with, or a race/subspecies that still exists.

Thus, even though we know that “so many” species have been killed off- we can’t compare that to a true “Great Extinction”.

We have certainly killed off quite a few marginal species, and put great big dents in some very common and important species, yes.

23

That’s not a particularly good list. Many many clearly distinct extinct species do not appear on it. At the same time it includes quite a few marginally distinct subspecies, such as various subspecies of tigers. However, just to answer the question, of the 57 forms listed on the page, from a quick perusal I would estimate that at least 42 could be clearly recognized as separate species if they were known from fossils alone.

No they weren’t. The “Heck Cattle,” while resembling the Aurochs externally, are much smaller, and are certainly distinct genetically. But this is a trivial issue with regard to the larger picture.

You are completely ignoring the megafaunal extinctions that took place when modern humans spread throughout the world. While it is controversial how much much of a role climate changed in these extinctions, certainly a significant number of them were due at least in part to overhunting. More than 200 genera (that is, very distinctive groups of animals, not just species or subspecies) became extinct, 33 of them in North America alone. The vast majority of these were very large and ecologically important animals, including elephants, rhinos, large cats, bears, giant ground sloths, horses, other large ungulates, etc.

As was pointed out above, we don’t have good data for most small animals in previous mass extinctions, so we can’t compare directly. However, it seems that as much as 20% of all bird species that were extant 4000 years ago are now extinct due to human activity (admittedly, most of them on oceanic islands).

The really serious issue, I think, is that from 12-25% of terrestrial vertebrates (depending on group) are presently considered to be threatened with extinction, and are likely to disappear within the next few centuries if present trends continue. This ignores many other species that are currently considered to be near-threatened. These figures include not just island species, but also many in mainland areas.

As I said in my first post in this thread, we don’t know exactly where the current extinction event ranks (or will rank by the time its over) relative to the Big Five, partly because we lack detailed data on some of the earlier ones (and lack information on groups such as insects and plants in the present one). However, it is (as I said) quite possible that this is the biggest such event since the end of the Cretaceous (which was the most recent of the Big Five).

And that, in my view, is a very serious concern, and not one to be dismissed lightly.

24

Very interesting and well-thought-out responses (though I’m not exactly heartened by them), Colibri. Thank you.

25

I am ignoring them- because we have really no idea how much of a role Humans played in them. The evidence is mainly “well humans showed up, and animals went bye-bye, thus humans killed all the animals”. But- the humans showed up because of climatological changes. If you look at North America- the claim is that human humters- with flint-tipped spears- eradicated dozens of large species. However, there are several large arguments against that:

  1. Simple Hunting has never wiped out a sucessful species

  2. The Bison. Why so many millions upon millions of them left, when they are much easier to hunt and kill than- say- the cave bear, or Mastodons, or Mammoths.

  3. Africa, where humans had the longest impact- there were few extinctions, and the local Elephant was not a common prey animal at all. Why were fewer humans able to wipe out many mnay more species in such a short period?

  4. And, of course- the Pleistocene mega-fauna disapearred around the world all about the same time- such as in Europe (where man had been for hundreds of thousands of years) and in America- where man had just arrived. Odd that Man took hundreds of thousands of years to wipe out the Mammoth in Eurasia, but only a thousand years or less in America.
    :dubious:

The extinction of the Pleistocene megafauna being primarily caused by Man is something hotly debated. Besides, although that wasn’t so very long ago, Geologically speaking, it still was another period.

26

I really don’t want to get into a detailed debate over the whole issue of the Pleistocene and post-Pleistocene megafaunal extinctions. It is, as you say, a hotly debated topic, and we could argue for pages about the details of the whole thing, and which specific species might have been affected by climate, and which by overhunting. However, in my opinion, it beggars belief that humans did not play a significant role, since most of the megafauna had survived a series of major climatic changes during previous glacial-interglacial cycles without going extinct. Humans at least seem to have tipped the balance.

However, I will comment on a few specific points:

This is just plain wrong. The Passenger Pigeon, Great Auk, and Carolina Parakeet were all widespread and highly successful species for which the overwhelmingly most important cause of extinction was overhunting, overexploitation, or direct persecution. For the Passenger Pigeon and Carolina Parakeet habitat alteration may have played some role as well, but of course in the case of the megafauna habitat alteration by humans by burning may very well contributed. The Auk and Parakeet in particular almost certainly wouldn’t have become extinct without direct and severe persecution by humans.

Steller’s Sea Cow is another example. It was once widespread and successful all around the North Pacific Rim. Climatic change does not seem to have played a role in its extinction, because it survived in the remote and unihabited Commander Islands while disappearing on all mainland and island shores accessible to aboriginal hunters. It received the coup-de-grace from overhunting by Europeans after the discovery of the Commanders by Vitus Bering in the late 1700s.

This is completely irrelevant to the question. Its recognition as a separate period is largely arbitrary, since we are actually probably in an interglacial period now. In any case, it is partly defined as a separate period because of the large number of faunal extinctions that took place at its end.

27

Well, the point of the OP was not to ask whether humans are causing a mass extinction, but simply if we are in another Great Extinction comparable to the previous 5.

Btw, what makes a species “marginal” for our purposes? Isolation? I can see if a species is, for example, stuck on an island, its loss would have little or no impact on the grand scheme of things. But it seems to me entirely possible that the loss of a small (in size and population) species might have a domino effect if it were symbiotic in important ways with other species.

28

Frankly, I think that loaded terms such as “marginal” or “successful” are thrown into the debate just to make the problem seem less important than it actually is. We have no good way to evaluate which species are going to be “marginal” or “successful” in geological or evolutionary time. Many widespread species today were “marginal” not so long ago, restricted to disturbed habitats along riverbanks for example. By disturbing natural environments, humans have made much more habitat available to these species, and made them outstandingly “successful.”

And while many ecologically important species are not (yet) extinct, they have had their ranges or numbers knocked back by 90-99% or more, and hence can be considered ecologically extinct in terms of their impact on the environment. Human activity has converted these once important and “successful” species into ones that are now “'marginal” and unimportant.

29

Most experts say that the Passenger Pigeon extinction was primarily caused by man, true- but by the cutting down of the great hardwood forests that made up it’s home. In other words, we destroyed it’s habitat. It was a bird that needed vast forests to exist, and also seemingly needed to live in vast flocks. Thus, cutting down the trees doomed it. The hunting made no significant difference.

The great Auk was finished off by man also, no doubt. However, the main cause for it’s extinction was the fact that in 1830 the island of Gerifuglasker- the great Auk’s egg laying home near Iceland- disappared entirely - likely because of some seismic disturbance.

Stellers Sea cow was already rare when discovered by Bering in the 1700’s. True, it was finished off by Man. Perhaps early man had a hand in making it rare- but we don’t know.

Yes, a "marginal species’ might still be locally important- but that’s not the point. The point is- if we are going to try and decide whether or not we are in another “Great extinction” we have to have something to compare. What sort of species did they count to determine the other “Great extintions”? Those that were successful enough to leave a significant fossil record, and those that were different enough from other close sub-species so that their extinction could be determined by fossils. So they *didn’t *count “marginal species” for the other “5”- because they couldn’t. Nor could they see that the "Iberian Ibex’ was made extinct if the bones of that sub-species weren’t significantly differnt from it’s close cousin the African Ibex.

Thus, if we are to determine if we are in the middle of another “Great extinction” we need to see how many “large, significant & distinct” species have disappeared- the* type *of species that have been counted in the other 5 “Great Extinctions”.

In Historic times- there have been very very few of such species eliminated. There have been many more brought to the edge, yes- so it is possible that we may be at the brink of the “6th great extinction”. But so far- not enough “large, significant & distinct” species have become extinct to say we are in “the middle of”. We may very well be at the brink, or even at the start, yes.

30

This is simply wrong. Most experts say that hunting, especially market hunting, had a huge impact on the species. One of the major factors was its need to live, and to breed, in enormous flocks. Once these flocks were reduced by hunting, and its colonies constantly disrupted by massive harvest, it probably would have been doomed even if its habitat had remained intact.

This is also flat-out wrong. The Great Auk had a very widespread distibution all across the North Atlantic. Its existence was certainly not dependant on a single colony. Most of the accessible colonies were gradually destroyed by exploitation, until the species only survived in remote areas. One of the last big colonies, estimated at more than 100,000 individuals was on Funk Island, off Newfoundland. The birds there were exterminated there about 1800. The species would surely have survived the destruction of Gerifuglasker if all the rest of its major colonies hadn’t already been destroyed by overhunting. And it did in fact survive that event - after it had already become rare, the last few individuals were hunted for scientific collections, the last one being killed in 1844. If that hadn’t happened, the species might conceivably had recovered. So to blame its demise on the loss of a single colony is absurd.

Since it was wiped out everywhere accessible to aboriginal hunters, and survived abundantly in the one place within its large range they didn’t reach, it’s a pretty reasonable supposition that hunting was responsible.

Look, I said in my first post that we didn’t really know where the current extinction lies in relation to the Big Five. Whether we are in the middle vs the beginning of a a large scale extinction event depends in part as to whether you accept the megafaunal extinctions as human-caused or not. I believe they largely were, and so we’re well into one.

What is not in question is that we have lost a large number of species over the past few thousand years, and these have become extinct at a rate far exceeding the average over most of the Tertiary. Whether you consider them marginal or not, these represent a substantial percentage of all vertebrate species that existed in the recent past (perhaps 20% of all birds, for example). And a large percentage of all vertebrates are likely to become extinct within the next few centuries, including many that are “important” and “distinctive” even by your criteria. This has to be considered a significant extinction event, whether or not it reaches the level of the Big Five, or whether we are in the middle of it or just at the beginning.

Man, its getting involved in interminable nit-picking arguments like this that makes me usually stay out of GD.

31

Ok, fine- let us compare then the total number of species alive now, and how many have known to be wiped out in the last 4000 years. There are currently 10- 100 MILLION species now alive on this earth. We have named less than 2 Million. That list shows a paltry 150 or so eliminated species- that’s 1-100th of 1% of the Named species. A “mass extinction” kills off something like 50% or so.

But- let’s just look at birds- no doubt, they have been getting it the worst. There are a little over 10,000 species of birds- and there have been - by that count- well under 100 species eliminated in historical times. (Many of which were minor niche species in Oceania). That’s - after a lot of rounding off UP- almost 1%. Not 20%, or even close. In other words- even amoung the worst hit of all Classes- the birds- so far we have made extinct maybe 1%. A “great extinction” is a kill off of 50 times that or so. In other words- just with the worst case scenario of all- birds- we’re 1/50th of the way along to a “great extinction”. :rolleyes:

We are thus certainly NOT “in the middle of” “a great extinction”.

32

FWIW this review from Oct 1 2004 Science on the megafaunal issue (about 65% gone by 10,000 years ago)

33

And July 27 2001 same mag on the effects of hunting:

Get your hands on the whole actual article for how severe the effects have been. A few outright extinctions. Many decreased many many fold.

Bottom line: many species are, like the man in Monty Python and the Holy Grail “Not dead yet.” but pretty damn close and rates are likely to increase rather than decrease. (Would Terry Schaivo be a better analogy? She’s still 100% alive but i think I’d call her more than half way to death; so it is with many extant species.) The megafauna example illustrated the effects of a convergence of direct human effects and climate change on species loss. In current times we have both direct human effects and many very significant indirect effects on climate and habitat.

It’s kinda ironic. A period of rapid climate change is when you’d most like to have a wide range of genetic diversity “in the bank”, so to speak, and yet our actions have resulted in the exact opposite even among extant species populations.

34

  1. That list is very incomplete and inaccurate

  2. We have very little idea of how many named, and especially unnamed, species of insects, invertebrates, and plants have gone extinct

  3. Once again, I must repeat what I said in my first post: We don’t know how the current extinction event compares (or will compare by the time its over) with the five Great Extinctions. This is because we lack comparable data, something you yourself have also pointed out. For both ancient and current events, we only have data on (mostly) the larger taxa, so to base a comparison on the total number of species that are currently described is bound to be misleading.

My figure, as I said, is based on archeological data from Oceania. The key article is here (warning: pdf)

That’s where my figure of 2,000 extinctions comes from. If you want to quibble, that’s 17% of the total of about 12,000 bird species that there would have been previous to the extinction. In my book, this must be regarded as a pretty significant extinction event, even if it doesn’t rise to the level of the “Big Five.”

And we don’t know if birds “have been getting it the worst.” We merely have the best data on them. Other groups could also be being hit very badly, but we lack good data on them.

I’ve never claimed we were, and neither has anyone else in this thread, so I don’t know why you keep going on about it.

If you look at my first post, what I said specifically is that “we are in the midst of a major extinction event.” In my book, anything that has already knocked off 17% of one of the best known groups, and and is likely to knock off a lot more, has to be considered major event, even if it’s not on the scale of one of the Big Five. And since this extinction event started as much as 50,000* years ago (and Sample indicates the megafaunal extinctions are valid for inclusion under his OP even if they were not caused by humans), it’s fair to say that we are well into the current extinction event, even if we may not be in the “middle” of it.

*Yes, I remember I said 11,000 in my first post. I should have said 50,000.
And kindly stop using the rolleyes smiley at me, or I’ll start using it at you.

35

The key quote from the article:

I agree entirely with this, and the rest of the article. I haven’t been claiming that no other factors were involved in the megafaunal extinctions, but that humans were important in many of them. Climatic changes also played a role.

And at one point you seemed to agree with me:

But maybe not:

Frankly, at this point I’m not sure what side of the megafaunal question you are arguing on.

36

[QUOTE=DSeid]
And July 27 2001 same mag on the effects of hunting:

Get your hands on the whole actual article for how severe the effects have been. A few outright extinctions. Many decreased many many fold.

[quote]

If that’s the Jeremy Jackson et al. article on the collapse of coastal ecosystems, I know it well. Several of the authors are friends or colleagues of mine.

I agree entirely.

But are you sure two different people aren’t posting from your computer?

37

Damn. Coding fixed to avoid confusion

If that’s the Jeremy Jackson et al. article on the collapse of coastal ecosystems, I know it well. Several of the authors are friends or colleagues of mine.

I agree entirely.

But are you sure two different people aren’t posting from your computer?

38

But of course- we have no idea how many Bird species have been on this earth since 50,000 years ago, and the line “People have lived on tropical Pacific islands over the past 30,000 years or 3000 to 1000 years. Their activities have led to the loss of many thousands of populations and as many as 2000 species of birds that probably otherwise would exist today.” is pretty well 100% educated guesswork.

We do know with some certainty that in the last couple/few thousand years, there has been a decline of less than 1% in Avian species. It certainly is possible that some 30000 years ago, there was a huge dip in the number of small isolated Oceania species- and perhaps caused by or triggered by man. But if so, that one time dip hasn’t been repeated. Most of the Great Extinctions seem to have been rather short- not a burst 30-50,000 years ago, then a trickle off. The whole thing seems mostly to have been over & done with in a few thousand years- or even a year or two in the case of the Dinosaurs.

I don’t doubt there was a significant dip in the number of species 30-50000 years ago- the megafauna alone would show that. So, perhaps that was a “mini-not-so-great extinction”. :smiley: But that was certainly brought on to a large extent by the late Pleistocene climatic change, although I certainly agree us Humans did our part. Or, perhaps since the Humans are thought to have spread because of the late Pleistocene climatic change, perhaps it can all be laid on that climatological doorstep. But that (the “late Pleistocene climatic change”)was a distinct event, and we are no longer in it. Thus, those extintions- even those caused by man- can’t be properly counted as part of any recent “Great Extinction”. “That was then, this is now”. Yes, 50000 years isn’t a lot Geologically speaking, but it’s not insignificant, either.

Like I said- in recent years there has not been a significant decrease in the number of species in any easily countable Classes. Yes, many species are “on the brink”, and thus it is not impossible at all that we are thus *on the brink *of another “Mass extintion”. But we aren’t in the middle of one.

We aren’t that far apart. I do not disagree that Humans played a part in the Megafauna- even a significant part. But- not by simple hunting. Predation does not kill off a sucessful species. I agree that the late Pleistocene climatic change & megafaubal kills signalled a significant dip, and could be considered a “mini-not-so-great extinction”. I also agree that the current situation is precarious- not only are many species on the brink, but we may be starting a climatological change.

I just don’t agree that we can lump the late Pleistocene and the last 1000 years together and call it all one great big “great extintion”.

39

Nonsense. It’s a synthesis of a very large and comprehensive body of data, including >26,000 fossils from >60 islands. Pretty much all of our scientific knowledge about the past, including the Great Extinctions, is “educated guesswork.”

This is just utter nonsense. It’s obvious that you haven’t even read the article. This was a extended process that took place over tens of thousands of years. Many of the extinctions occurred within the last few thousand years (and of course they have continued on through European contact up until the present - there was never a extended period in which they stopped.) It is not something that happened in a single burst 30,000 years ago.

And we know almost nothing about the detailed time course of the Great Extinctions, certainly not over the scale of a few tens of thousands of years. Even in the case of the K-T event, while many extinctions may have taken place during the first few years after the impact, many others probably took place over a period of several thousand or tens of thousands of years afterward.

We are well into one, since there is no reason to exclude the megafaunal extinctions.

I just gave several examples of cases in which hunting by humans was the decisive factor in killing off successful and widespread species, which you have not refuted.

You haven’t given anything resembling a logical or conceptual reason to exclude them. They and the current extinctions fall well within the kind of time frame that a typical “great extinction” comprises. And as Sample indicated, they fall within the scope of what he was asking about in the OP.

In any case, if you are going to continue to ignore cites, repeat statements that have already been refuted without providing additional evidence to support them, and apparently randomly change your position on issues without explanation or clarification, there’s not much point in debating you.

40

Colibri, yes that is the Jackson article.

And some of your confusion arises from mixing up my and DrDeth’s posts. I never said that I am ignoring the megafaunal extinctions, that quote was from one of DrDeth’s posts.

I’ve been quite consistent and we are pretty much in agreement. Reread my first post.