Habitat loss = extinctions??

[SkepticLance]

In Bjorn Lomborg's book : "The Skeptical Environmentalist", he makes the statement that loss of natural habitat is a minor cause of extinctions. Two examples are given of places where massive natural habitat loss occurred with very little in the ways of extinction of species.

1. Puerto Rico.

2. Atlantic coast of Brazil.

 

Yet in environmentalist literature, habitat loss is almost invariably described as a major cause of extinctions.

 

Does anyone have any unambiguous examples of cases where habitat loss has caused substantial extinctions? Please try to use examples where other causes are unlikely.

[theCPE]

Not entirely on topic (although pretty close) rent the Penn and Teller episode of B.S. on the ESA. It is quite interesting:)

[Sayonara]

A recent high profile extinction was that of the Baiji, or Yangtze dolphin. It was in serious decline since the 1950s due to entanglement in nets etc, but the destruction of its habitat through pollution accelerated matters.

 

The final nail in the coffin was the completion of the Three Gorges Dam in Sandouping, and the species is now considered wiped out.

 

http://en.wikipedia.org/wiki/Baiji

 

One of the problems the dolphin had was a lack of refuge spaces. This is not always a problem for terrestrial or oceanic animals, which can sometimes migrate and - depending on their ability to adapt - make do with slightly different habitats.

[herpguy]

Well, this isn't an extinction, but it is a serious decline in numbers.

 

The eastern massasauga rattlesnake has been in decline since the 1970s when much of its wetland habitats were starting to be destroyed. It's pretty resistant to pollution, so that can't be the cause of the decline. People do kill a few, but not enough to cause this big of a decline.

[herpguy]

Oh, a thought just occured to me.

 

Do you consider loss of habitat by pollution loss of habitat?

 

Example: There's a pond that isn't destroyed, but fertilizer from a neighborhood close to it pollute it. Is that loss of habitat?

[Glider]

I would say so, because pollution makes that habitat no longer habitable.

 

You don't have to destroy a house to make someone homeless, you just have to make it impossible for them to live in it.

[Paralith]

A pretty good case for habitat loss is Amur leopards - again, not extinct, but almost close enough. They used to range all throughout Russia and nothern China and India etc when it used to be all forest. Now they're restricted to remote Siberia and they number less than 50. Poaching has exacerbated their situation, but I think we can agree that the loss of a huge amount of habitat, for a predator that requires a large range, was the real killing blow.

 

It's really kind of stupid to say habitat loss is a minor cause of extinction. Animals adapt to their environment, and many specialize to their specific habitat. And if that habitat is pulled out from under them in an extremely short amount of time? There's no way they can evolve quickly enough, unless we're talking about mice or something, but they're generalists anyway. Of course they won't survive, not well at least. And if they're competing with people? Psht.

[Sayonara]

It's not stupid, it's just so highly conditional as to be a poor generalisation.

[SkepticLance]

It's pretty tough. I can name a heap of extinctions caused by the introduction of an alien predator. And there are a whole lot more caused by over-hunting or over-fishing by people. There are, without doubt, lots of cases where loss of habitat was a contributing factor. But I don't think Lomborg was denying that.

 

The question is whether there are any cases which are undeniably caused by loss of habitat, with no significant other contributing factors.

[Sayonara]

I doubt that there are many extinctions caused solely by a single factor, and in this respect I don't see why habitat destruction as a cause should stand alone.

 

There are so many species which are completely adapted and specialised to survive in a single limited habitat that it is inevitable that many will be destroyed when that habitat is altered.

 

We should remember that many species will be destroyed along with their habitats without us even noticing. It could be as lowly as a moss that only grows around one particular type of tree, but it still "counts".

 

We also need to be careful about what we consider to be "habitat destruction". We might say "habitat destruction didn't kill the spotted trout, pollution did", but if the pollution did not affect the spotted trout directly, yet altered its habitat to such an extent that the species could not survive there, then that could be interpreted as habitat destruction. Perhaps "habitat denial" would be a less ambiguous term?

[Dak]

loss of habitat = extinction is sort of a tautology, if you take habitat as 'place where you can live'.

 

introduction of predetors (including humans), competing species, pollution, phisical distruction of local geography, etc, all count as loss of habitat if the effect is that your habitat is no longer inhabitable* by you (habitat denial, as sayo said).

 

from that, i'd assume 'making it so that all the places they're trying to live are uninhabitable** to them' would be the number 1 cause of extinction

 

 

* hmm... does that mean 'can inhabit it', or 'unhabitable'? I meant 'can inhabit it'.

**this one means 'unhabitable'. sorry

[SkepticLance]

Sayonara said :

 

I doubt that there are many extinctions caused solely by a single factor, and in this respect I don't see why habitat destruction as a cause should stand alone.

 

I have a nice story to illustrate my point. The Stephens Island Rock Wren. This was a small, largely flightless bird living only on Stephens Island (north end of New Zealand's South Island). It was discovered by the lighthouse keeper on Stephens Island. Every morning his cat would deposit a small and somewhat undistinguished looking bird at his feet. After a few months of this, his curiosity was aroused, and he sent one of the sorry corpses to the museum in Wellington. They identified it as a new, previously unknown species, and immediately sent a scientific team to Stephens Island to investigate. When they got there, the species was gone. Since that day, not a single Rock Wren has ever been seen. This extinction was caused by a single introduced predator. More than that. By a single individual predator - the lighthouse keepers cat.

 

So the truth is that extinctions are, in fact, from single causes on occasion. Predation by an introduced species is a common one. But direct human hunting/fishing is another.

 

So is Lomborg right in saying that habitat loss is a rare cause?

 

Incidentally, I think we should agree that pollution can be classified as habitat loss. If the habitat is changed to the point where survival is impossible, then that is the same thing.

[Sayonara]

So the truth is that extinctions are, in fact, from single causes on occasion. Predation by an introduced species is a common one.

Yes, but there is also the fact that other bird species could/would have avoided this extinction by migrating and expanding their range. If the prey species is too specialised to occupy refuges where the predator cannot reach them, then the simple fact that the predator eats them is not the sole reason for their demise: it's also because they are not adaptive enough to cope. This is what I meant about "habitat denial" - it's the aspect of the niche becoming untenable rather than being destroyed.

 

But direct human hunting/fishing is another.

Yes it can happen, but this doesn't mean it has to. The dodo is a good high profile example of a species whose extinction was (near enough) due to hunting, whereas the woolly mammoth is a similarly high profile example of a species which went extinct for a blend of reasons, one of which may have been over-hunting.

 

It's helpful to remember that hunting by humans usually applies moderate selective pressure to species. Not normally enough to force large adaptations, but sometimes enough to force range expansion.

 

So is Lomborg right in saying that habitat loss is a rare cause?

Dunno. What's his evidence? Does he give any basis from ecological evolution, or is it mainly by example?

 

Incidentally, I think we should agree that pollution can be classified as habitat loss. If the habitat is changed to the point where survival is impossible, then that is the same thing.

Agreed. Although this means we have to then differentiate between "pollutants", which change the habitat but do not directly affect the species in question, and "toxins", which enter the habitat and directly kill individuals.

[SkepticLance]

Sayonara's response to Lomborg.

 

Dunno. What's his evidence? Does he give any basis from ecological evolution, or is it mainly by example?

 

In science it is well known that it is virtually impossible to prove anything. Thus we have the Popper Falsification Principle. That is : any hypothesis is put up to others to attempt to falsify through argument (based on empirical data), experiment or observation. When something survives many attempts to falsify it, it is considered to be a good scientific model.

 

So let's consider Lomborg's idea as a candidate for falsification. Can anyone demonstrate it to be false? If not, we have to consder it a good scientific model.

 

So far, several examples have been raised which suggest that habitat loss may contribute to extinction of species. However, nothing yet to show that it can, as a single cause, drive a species to extinction.

[Sayonara]

As is indicated by our destruction/denial discussion, the magnitude of our examples' significance in such a falsification will be directly related to what he considers "habitat loss" to be.

[SkepticLance]

To Sayonara.

Re magnitude of habitat loss.

 

Lomborg talks of two examples : Puerto Rico and the Atlantic coastal forest of Brazil.

 

In both cases, massive destruction of forest took place. In neither case was the forest totally destroyed, although in Puerto Rico, more than 100% was felled. It was just that it recovered sufficiently to ensure that 5 to 15% of the original forest area remained, albeit as new regrowth. About 12% of the Atlantic Brazilian coastal forest remains, though in very fragmentary state.

 

I know of a number of cases of extinctions that can be put down pretty much to single causes - mainly alien predators and to direct human hunting - but I do not know of a single extinction event that can clearly be laid at the door of Habitat loss alone. The Yangtse River dolphin comes close. The only thing about that is the propensity of the Chinese people to kill and eat anything that moves. This leaves a large space for doubt when pondering the assertion that this extinction was totally caused by pollution of the river - hence habitat loss.

 

In New Zealand, 1000 years ago, there were 11 species of large flightless birds - the moas. They ranged from turkey size to 3 metres tall. Within 150 years of the first human contact, all 11 species were gone. A clear cut example of extinctions by human hunting.

 

Do we have any such clear cut examples of extinctions by loss of habitat?

[Sayonara]

I know of a number of cases of extinctions that can be put down pretty much to single causes - mainly alien predators and to direct human hunting - but I do not know of a single extinction event that can clearly be laid at the door of Habitat loss alone. The Yangtse River dolphin comes close. The only thing about that is the propensity of the Chinese people to kill and eat anything that moves. This leaves a large space for doubt when pondering the assertion that this extinction was totally caused by pollution of the river - hence habitat loss.

No assertion was made that the dolphin was made extinct by the pollution, although it was affected by this to some degree. It's possible that it might have eventually adapted. What finally killed the species was the completion of the Three Gorges Dam, as I mentioned before, which changed the habitat permanently.

 

(Interestingly enough there were 1 million people living in what was to become the reservoir: their habitat was also destroyed, but they were able to migrate and adapt).

 

In New Zealand, 1000 years ago, there were 11 species of large flightless birds - the moas. They ranged from turkey size to 3 metres tall. Within 150 years of the first human contact, all 11 species were gone. A clear cut example of extinctions by human hunting.

Not necessarily; at the time we had a habit of taking disease-ridden rats with us everywhere we went, which had a hand in many extinctions. But for all intents and purposes, yes, this example is "human interference".

 

Although you could easily go the "it's habitat denial" route, which is why...

 

Do we have any such clear cut examples of extinctions by loss of habitat?

 

...is still a useless question until we decide what range of reasons qualify as "loss of habitat".

[SkepticLance]

Sayonara said :

 

at the time we had a habit of taking disease-ridden rats with us everywhere we went, which had a hand in many extinctions. But for all intents and purposes, yes, this example is "human interference".

 

Kind of stretches credulity to suggest that a bird that reached 3 metres tall could be driven to extinction by small rats. We have not encountered diseases carried by these polynesian (hence small) rats that might affect birds. We already know the early humans hunted this bird. We need not look for alternative explanations.

 

is still a useless question until we decide what range of reasons qualify as "loss of habitat".

 

I suggested we accept any change to the habitat that changed it sufficiently to make it non survivable. Is this idea not sufficient for purposes of discussion?

[Sayonara]

Kind of stretches credulity to suggest that a bird that reached 3 metres tall could be driven to extinction by small rats.

Consider the Asian black rat. It has notoriety in the archives of history for sparking off some of the most horrific plagues mankind has ever witnessed.

 

Perhaps you think it stretches credulity to think that "small rats" could be responsible for the death of a third of the population of Athens in the 5th century, a quarter of the population of the east Mediterranean in the 6th century followed by another wave that left 25 million dead, a third of the population of all of Eurasia during the 14th century, and 12 million people in China and India in the 19th century.

 

No? Then why not for the Polynesian rat?

 

Do not play the fool by ignoring the potential effects of such a potent and well-known vector organism. The Polynesian rat was introduced to New Zealand by humans, and is widely acknowledged as being a major participant in several South-Pacific extinction events, including many bird species.

 

We have not encountered diseases carried by these polynesian (hence small) rats that might affect birds.

I can't honestly imagine that you have checked that for factual accuracy, however there is the issue that we would not expect to find diseases vectored by the rat to be present in the population almost a millennium after the host species has been wiped out.

 

Also, consider that they do not need to affect the birds directly. They need only compete for, infect, or destroy their food supply. Were you aware that the Polynesian rat was a major factor in the deforestation of Easter Island, for example? Look how that turned out.

 

We already know the early humans hunted this bird. We need not look for alternative explanations.

I am not looking for "alternative explanations". I am pointing out that what you attribute in a blinkered fashion to simple human hunting was actually almost certainly brought about by several contributing factors, as these things so often are. Ecology is rarely cut-and-dried.

 

Regardless, I already stated that the rats could be written off as human interference, so I don't see any need for an objection. Or indeed, how any can be seriously attempted without ecological evidence.

 

I suggested we accept any change to the habitat that changed it sufficiently to make it non survivable. Is this idea not sufficient for purposes of discussion?

We seem to be going in circles now. If we consider habitat loss to be "any change to the habitat that changed it sufficiently to make it non survivable", then Lomborg is clearly wrong and there is nothing more to discuss.

 

This is why I enquired about his specific criteria.

[SkepticLance]

To Sayonara.

 

The reason I called it a small rat is because it is a small rat. The Polynesian rat is a different species to the ship rat or the Norwegian rat, which are both much bigger and both more destructive. We know from current day ecological studies that the Polynesian rat can harm small species of bird, but the larger ones (Kiwi, Takahe, Kakapo) are not affected. I have a friend who is a zoology technician who works with researchers, including some who have been studying the ecology of the Polynesian rat, and I have, whether I wanted to or not, listened to a lot of the findings.

 

Easter Island is an example of an ecological disaster of substantial proportion about which an enormous amount of uninformed garbage has been written. The truth is that there are many conflicting theories unresolved. There is little doubt that the native polynesians presided over a big deforestation event. Their own loss of population afterwards is a bit controversial. Some researchers believe it was due to the terrible practise of 'blackbirding' (a euphemism for taking slaves) combined with the introduction of European diseases. Others claim it was due to their own destruction of the ecology of the island.

 

The polynesian rat was carried right across the Pacific and doubtless caused the extinction of 1000 or more bird species. But only small birds.

 

Sayonara said :

 

If we consider habitat loss to be "any change to the habitat that changed it sufficiently to make it non survivable", then Lomborg is clearly wrong and there is nothing more to discuss.

 

Yet we still have not received any clear cut examples. Unless we can demonstrate that it happens, and frequently, then Lomborg is right. There are about 20 known extinctions per year. That is : species known to have existed a few years ago which are now known to be gone. If habitat loss is a major cause of extinction, we should be able to find a reasonable number of these that are almost totally caused by habitat loss.

[Paralith]

Sayonara said :

 

If we consider habitat loss to be "any change to the habitat that changed it sufficiently to make it non survivable", then Lomborg is clearly wrong and there is nothing more to discuss.

 

Yet we still have not received any clear cut examples. Unless we can demonstrate that it happens, and frequently, then Lomborg is right.

 

Well, now we're getting into exactly what we mean by habitat. Correct me if I'm wrong, but I've always understood an animal's habitat to be the physical environment which it inhabits, including plant life. In this case, habitat loss can be actual removal of all the plant life and/or a change to the physical aspects of the area, such as pollution, aka the introduction of chemicals harmful to the animal or the habitat itself.

 

But does habitat also include the other animals that live in that habitat, the whole community, in other words? Because if it does, then you're covering a lot more ground. Then habitats can become non-survivable if a new and effective predator (including humans) or competitor is introduced, or a viral or bacterial disease is introduced.

 

At this point you can probably take almost any of the multiple causes of extinction and say that it somehow counts as habitat loss, the habitat having become non-survivable for a given species. If this is how we define habitat, I would have to agree with Sayonara - clearly Lomborg is wrong if nearly every cause of extinction can be attributed to this definition of habitat loss.

 

However, if habitat is more limited to the physical aspects of a species' environment, then I would agree more with SkepticLance. While it seems obvious to me on an intellectual level that sudden removal of an animal's physical habitat would be more than enough on its own to drive a species to extinction (the one in which it was designed to survive, particularly if the animal is a specialist), I have to admit that I don't know of any clear cut obvious examples where habitat loss is the only (or almost only) contributing factor.

[SkepticLance]

Paralith.

Thanks for the reply.

I think you are correct. I think we must exclude such factors as new predators (including humans) and new diseases. However, removal of plant cover, or a change to the physical parameters, including pollution, can be included.

 

The other change that is often touted is global warming. Again, I am unaware of any extinction that can, as yet, be laid solely or mainly at the door of global warming. However, I think, for this question, we need to exclude global warming, which is a separate and controversial subject.

 

I still have seen nothing to lead to the conclusion that Lomborg is wrong.

[Sayonara]

Paralith.

Thanks for the reply.

I think you are correct. I think we must exclude such factors as new predators (including humans) and new diseases. However, removal of plant cover, or a change to the physical parameters, including pollution, can be included.

If we are including the latter and disregarding the former, I would err on the side of Lomborg being wrong, but see below:

 

Yet we still have not received any clear cut examples. Unless we can demonstrate that it happens, and frequently, then Lomborg is right. There are about 20 known extinctions per year. That is : species known to have existed a few years ago which are now known to be gone. If habitat loss is a major cause of extinction, we should be able to find a reasonable number of these that are almost totally caused by habitat loss.

Paralith has made a very concise argument, and it echoes my sentiments exactly.

 

To give clear cut examples, we need to know what we are supposed to give clear cut examples of. If we are to determine whether Lomborg is right or wrong, then we must evaluate his arguments, so we need know what his criteria are for habitat loss.

 

Personally, I do not own his book, which is why I asked you (twice).

 

One example I can think of which would probably satisfy all definitions of habitat loss is that of volcanic eruptions within isolated island ecosystems - these happen all the time and have been going on since before life even arose. It is a safe bet that tens of thousands of species of plant, animal, and perhaps even micro-organisms have met their ultimate fate this way.

 

 

The other change that is often touted is global warming. Again, I am unaware of any extinction that can, as yet, be laid solely or mainly at the door of global warming. However, I think, for this question, we need to exclude global warming, which is a separate and controversial subject.

Do we consider that warming at the end of ice ages is effectively the same as global warming? If so, we may be able to find candidates relatively easily.

 

 

I have replied to your later post first because my reply to your earlier post is going to be a bit off-topic

 

The reason I called it a small rat is because it is a small rat. The Polynesian rat is a different species to the ship rat or the Norwegian rat, which are both much bigger and both more destructive.

True, but the size of a rat is not any indicator of the risk posed to ancient species by any pathogens it might have been carrying during a historical encounter.

 

We know from current day ecological studies that the Polynesian rat can harm small species of bird, but the larger ones (Kiwi, Takahe, Kakapo) are not affected. I have a friend who is a zoology technician who works with researchers, including some who have been studying the ecology of the Polynesian rat, and I have, whether I wanted to or not, listened to a lot of the findings.

What you need to bear in mind is that, in ecological terms, present-day studies can give a very precise idea about how the modern form of a species interacts with the current ecosystem. Unfortunately models that we derive from those studies do not extrapolate backwards very well at all, particularly where there are extinct animals that formed part of the historical ecosystem that the rat was involved in, and especially where the ecosystem was so isolated (these tend to have strange rules and a higher demonstration of interspecial vulnerabilities).

 

The Polynesian rat arrived in New Zealand with the humans, about 1,000 years ago. Yes, humans hunted the moas and doubtless did massive damage to the population, but this alone does not mean we can rule out that the rat attacked the staple food source of the moas, whether by competition or by vectoring disease, nor can we rule out that the rats did not attack the moas themselves (again, by vectoring or by predation). A full-grown moa would likely make mincemeat of a rat, but what of an egg, or a chick?

 

I would not waste too much time with this personally - I have already said twice it can be written off as human interference (mainly because we took the rat to New Zealand). I am simply trying to get you to think about these scenarios in a more ecologically eclectic way, so that you are less likely to make the mistake of assuming the most easily identified cause for population decline is the only one being enacted.

 

Simply put, commentary on the current ecology of a species is not commentary on the historical ecology of a species. Consider that the reason that large birds today are unaffected by the Polynesian rat (assuming this is true, which I am going to) is because these are the species that were able to adapt to its presence. One thousand years is more than enough time for selection to push ecological interactions that way.

 

Easter Island is an example of an ecological disaster of substantial proportion about which an enormous amount of uninformed garbage has been written.

This is true, there certainly is some rubbish. But there is some good evidence as I recall that the rats had a grave impact on the ability of the palm trees to maintain a workable population, by eating the nuts (i.e. seeds) that they dropped. The reason for mentioning this was to illustrate that even a small rat can have a devastating effect, and that we should not be so quick to dismiss them from our consideration of the ecological battleground.

 

The polynesian rat was carried right across the Pacific and doubtless caused the extinction of 1000 or more bird species. But only small birds.

The moas, also hunted by humans, would have been placed under a different set of selective pressures which could easily have caused adaptive conflict. In these situations, adaptation through selection can be seriously impaired, if not downright counter-productive.

 

But, like I said, the only reason for discussing all of this is to show you that we need to think about this problem in a more ecologically-oriented fashion (it is, after all, a question of ecology), and to demonstrate that we should keep an open mind about historical extinctions that pre-date accurate naturalistic record keeping (what you said about Easter Island applies to New Zealand too, don't forget).

[Paralith]

I think Sayonara is making an important point about the ecological complexity of extinction. We may never find an example of habitat loss being the one and only reason for an extinction, except maybe in cases like Sayonara's example of a sudden volcanic eruption simply wiping out all life on an island.

 

Let's talk about a hypothetical example. The snuffles are a big cat that live in forests. It starts with the forest area being severely reduced, which in turn reduces the number of snuffles that can be supported at any given time. After sighting snuffles for the first time while cutting down the forest, people start poaching them for their beautiful coats. Then as a result of their reduced and continually shrinking population size, they suffer more from effects like demographic stochasticity (too many males and not enough females are born) and environmental stochasticity (a drought reduced the prey density), which even further reduces their population size, sending them into an extinction vortex.

 

So what is to blame? Had the forest not been cut, would the snuffles have been able to survive occassional culling by poachers? Had there been no poachers, would the higher ratio of males to females have been such a big deal? Would they have been able to squeak past even given the lower prey density? Is it the final blow or the first blow that counts? What if the poaching came first, then the drought, and then the habitat loss? Few species live in such a bubble that they will not suffer mutliple "attacks," as it were, as their population size shrinks for one reason or another.

[Sayonara]

It's a shame you had to kill the poor snuffles to illustrate that point, but yes - that's exactly the problem. Extinction pressures are more often than not due to a complex and specific set of circumstances.

 

We need to know more about Lomborg's thinking.