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roundtable:a modern mass extinction Watch Show 3:
"Extinction!" on PBS
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First, a general question. Are we now in a mass extinction, and if so, why does it matter?
Some Web visitors wrote to say they have heard it said, especially of tropical and marine organisms, that we are losing species before we even know of them. And the question is, if we haven't discovered a species yet, how do we know that we've lost it?
Would Darwin support the Federal Endangered Species Act? Millions of dollars are spent by corporations and government for habitat conservation plans, recovery plans, etc., for species that seem doomed for extinction. Is this a wise use of money? Why or why not?
If extinction and mass extinctions are a natural part of Earth's history, should we really be concerned about our effects on endangered species? Are we trying to fight something that's inevitable anyway and that is much larger than us, as a single species ourselves?
I'm concerned with the loss of genetic diversity because it affects food supply, drug discoveries, and the stability of ecosystems on which we depend. And yet, I'm very skeptical of the theory of evolution as it now stands. How does the theory of evolution -- or we could also say that as evolutionary thought -- explain and predict what's going on with the current cycle of extinction, better than if we had to just rely on what we can observe? In other words, what context and what value does evolutionary thought provide to this discussion that we're having?
Several other questions relate to this: Loss of biodiversity, water and air pollution, and the greenhouse effect are all impacting the biosphere, and if these are indeed combining to produce a mass extinction, do you think our species will survive it? Peter's already said that he thinks yes. How about some other perspectives?
Let's move onto some more specific questions. Why do bigger species go extinct faster than smaller ones?
Do you feel that extinctions of large Ice Age mammals had any human causes?
Another Web user wrote: I know there is a gene databank under construction right now. Could this be used as a modern Noah's Ark for the future?
We'll wrap up with a different kind of question. There was one that came from a younger visitor to the Web site who asked, What can kids do to help save the rain forest and animals?
   

 

Q: Some Web visitors wrote to say they have heard it said, especially of tropical and marine organisms, that we are losing species before we even know of them. And the question is, if we haven't discovered a species yet, how do we know that we've lost it?
Panelist Responses: < back to intro page
Tundi Agardi
It's an excellent question. Certainly, we cannot be sure that we're losing species unless they are species that have been scientifically described and for which we are certain that we have lost every individual or, at least, that we have a kind of ecological extinction where there's no likelihood whatsoever that that species will be able to propagate into the future.

The estimates of how many species we are losing that are unknown or undescribed in science come largely from models. In the marine realm this is particularly true, because so much of the ocean ecosystem is unexplored and so many of the marine habitats really haven't been inventoried for species. And even those that have been inventoried, we're way behind in terms of taxonomic description -- in other words, scientific descriptions -- of new species that we're finding.

But the models have been based on samples that have been taken of different kinds of habitats, including some really intriguing studies that were done and continue to be done on the deep sea, where we used to think that species diversity must actually be very low. We always thought of the deep sea as a dark, very homogenous environment, unchanging, and therefore it was assumed, about 25 years ago, until we started exploring the deep sea, that the species diversity was quite low.

And much to our surprise, when we developed the technology to be able to go down to the deep sea, either in submersibles or with remote vehicles, we found that in fact the species diversity, and also the diversity at the level of genera or families, was quite high, so that we have many, many more species [than we expected]. On every single dive that was done, on every trip, new species [were] being discovered, and also entirely new life forms that were not dependent on solar energy, like much of the life that we're familiar with, but [forms that] were actually dependent on chemical energy coming from vents in the sea floor.

So that's just an example of one situation where we used to assume that the species diversity was quite low, but in fact the species diversity is much higher. And there are unique kinds of sets of organisms that exist in different parts of the ocean floor, so it's not at all homogenous and not at all uniform.

There's also been some really interesting work that has been done with very small organisms known as microbes, and those, again, seem to have a much higher diversity. Many of these microbes live on the sea surface micro-layer, so they're living right where the ocean touches the sky, and they're not at all uniform in distribution. So different parts of the ocean have very different microbes in them.

And the interesting thing is, where we used to think that there was kind of a uniformity to the oceans, even among very well studied kinds of habitats, like coral reefs, we're finding, actually, that endemism, or uniqueness of species to a particular place, is quite high. So even as recently as a couple of months ago, when scientists went into a new part of Irian Jaya, which is the eastern end of Indonesia, in about a week of surveys they were able to come up with a handful of new species.

So we're quite confident in saying, at least on the marine side, that as we know we are losing habitats all around the world, we're losing these species, even though those species haven't yet been recorded scientifically.
Daniel Simberloff
Very similar methods have been used for insects. Probably at least half of all the species we know are insects that live on plants or that feed on other insects that live on plants, and a rather small part of the world has been examined fully to find those species -- especially in the tropics, where most of them live. But it's possible, from the data that have been gathered on sites that have been examined very closely, to look at the degree of overlap between the specie sets in the different areas. And the general picture is that most of these species are very restricted in their geographic range -- they're found in only one region.

From the actual statistics on how many species are found in which regions, it's possible to extrapolate to how many new ones will be found in areas that simply haven't been studied very carefully. This is generally the basis for extrapolations of the numbers of species that aren't known.
Ariel Lugo
I think certainly we are grossly underestimating the number of species on Earth, and the more we look, obviously, we're going to find a lot more than we think we have. And the destruction of habitat is why we are so scared about the potential for huge extinctions. The problem is that we also need to rely on models to relate the extinction rate to the destruction of habitat, and we don't have, I don't think, sufficient information to define the relationship between habitat loss and species loss, mostly because the complexity of nature is just absolutely huge, and our understanding is very primitive.

And most of the controversy, I think, comes because we're naturally cautious -- it's obviously better to err on the side of caution -- but nevertheless, we don't seem to give credit to nature's resilience and to the ability of organisms to cope with change. And to the degree that the models do not incorporate resilience mechanisms -- I'm referring to the trick of relating, for example, rate of deforestation to rate of species loss -- to the degree that those models don't take into consideration the possibility that species recover from changes in the habitat, to that same degree, our estimates of the numbers of extinctions may be too high. So I would assume that we're grossly underestimating the diversity of the world, but we're probably overestimating the rates of extinction.
Peter Ward
I would hearken back to the theory of island biogeography, and also the sense of looking at how many species can fit into a given landmass. There is a sense that the larger an area of land, a continent, the more species it will have. I guess what worries me most is that, when you look at islands, compared to large continents, islands clearly have fewer numbers of species.

We are, at least in North America, making large numbers of islands, and we are doing this not simply by changing habitat and destroying habitat, but also we're doing a physical "islandification" -- if I can make up a word -- by putting in highways, by putting in barbed wire fences, and putting in farmland, in areas where we did not have farmland in the past. And by doing this we are making many tiny islands in a continent, and once you make islands you reduce the number of species.

It is my fear that the simple changeover of the world from one of natural habitats to one of human-induced fields or cities is, in itself, reducing species simply because we are changing from big habitats to little ones.
Daniel Simberloff
I'd like to respond to Ariel Lugo's argument that we're underestimating resilience. I guess we usually call this the Lazarus effect: species that are thought to have disappeared and then reappear. And it happens occasionally, so it's certainly true that we have occasionally been overly pessimistic when we say a species is gone. We may be overly pessimistic when we say a species is doomed. But these are a very small minority of cases. And, if we'd been conservative, it's by erring on the side of not calling a species extinct when they probably are.

I can give you a numerical example. Until very recently, we had a little over 300 species of freshwater mussels, clams, in the United States. There's 19 that everyone agrees are already extinct within the last century and a half. So that's about six or seven percent. That's a huge number. None of them have ever been seen again since we began to recognize them as extinct.

There's another 17 that haven't been seen again for a very long time. Most people think they're extinct, but they're not listed as extinct in any official list. Of those -- I know this literature quite well -- there's only one case where one of these was seen again after at least one expert felt it probably had disappeared.

So, if we add those two together, that's 36 species. That's about 12 percent of all the species of freshwater mussels. There are about 100 other freshwater mussels in the United States that are believed to be critically imperiled. Most of them have status under the Federal Endangered Species Act, and they're monitored very carefully. We know of no case where their status has improved, and about 80 of them, the populations are declining rapidly.

So, there is no Lazarus effect here, and we would know it if there was. They've been studied carefully enough that we would see such changes. So it is true that occasionally we underestimate what a species can do, but these underestimates are a very small fraction of the problem.
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