Sexta extinção em massa: onde estão os cadáveres???

terça-feira, março 08, 2011

Where Are The Corpses?

Guest Post by Willis Eschenbach


The record of continental (as opposed to island) bird and mammal extinctions in the last five centuries was analyzed to determine if the “species-area” relationship actually works to predict extinctions. Very few continental birds or mammals are recorded as having gone extinct, and none have gone extinct from habitat reduction alone. No continental forest bird or mammal is recorded as having gone extinct from any cause. Since the species-area relationship predicts that there should have been a very large number of recorded bird and mammal extinctions from habitat reduction over the last half millennium, I show that the species-area relationship gives erroneous answers to the question of extinction rates.

Figure 1. The Object of My Quest — The Corpse of an Extinct Bird


A recent study in Nature [Thomas 2004] stated that 37% of all species might soon go extinct because of habitat reduction due to global warming. This same prediction of impending mass extinctions from habitat reduction due to global warming has been made a number of times recently, for example in a book by Professor Michael Benton of Bristol University (Benton 2003), as well as in studies by Parmesan and Yohe (Parmesan 2003) and Root et al. (Root 2003).

Habitat reduction has also been cited as being responsible for the continuing extinction of species which is said to have already happened due to the cutting down and fragmentation of tropical forests (Wilson 1995, 2001). For example, a recent study in Conservation Biology (Harris 2004) opens by saying that “Intense deforestation causes massive species losses.” Wilson says that due to habitat reduction we are in the “sixth great wave of extinctions”, comparable in size to the five previous great waves of extinctions in geological time. (Wilson 1992).

Reading these kinds of claims over and over again made me think, “Well, if there’s been all of those claimed extinctions of birds and mammals … where are all the corpses? What are the names of all the extinct birds and mammals”? This research paper investigates these claims that habitat reduction from temperature change and deforestation has led and will continue to lead to the extinction of a large number of species.

A few clarifications are in order.

This study is not about estimated, predicted, or calculated extinctions. It is an analysis of the actual historical record of extinctions, with the purpose of understanding the nature and size of extinctions from historical habitat reduction.

By extinction I never mean local extinction. I have analyzed total extinctions of species (not subspecies). Local extinction is a separate and valuable study, not covered by this work.

I am not referring to “almost extinct,” “on the brink of extinction,” or “reportedly extinct.” I am discussing the actual extinction of species as confirmed by the relevant authorities.


In their seminal work, “The Theory of Island Biogeography”, Macarthur and Wilson further explored the “species-area” relationship (Macarthur 1963). This relationship, first stated mathematically by Arrhenius in 1920, relates the number of species found to the area surveyed as a power law of the form S = C * a ^ z , where “S” is species count, “C” is a constant, “a” is habitat area, and “z” is the power variable (typically .15 to .3 for forests). In other words, the number of species found in a given area is seen to increase as some power of the area examined.

By surveys both on and off islands, this relationship has been generally verified. It also passes the reasonability test — for example, we would expect to find more species in a state than we find in any one county in that state.

Does this species-area relationship work in reverse? That is to say, if the area of a forest is reduced, does the number of species in the forest decrease as well? And in particular, does this predicted reduction in species represent species actually going extinct? One of the authors of “Island Biogeography” thinks so. In 1992, E. O. Wilson wrote that because of the 1% annual area loss of forest habitat worldwide, using what he called “maximally optimistic” species/ area calculations, “The number of species doomed [to extinction] each year is 27,000. Each day it is 74, and each hour 3.” (Wilson 1992).

If we have lost 27,000 species per year since 1992, that’s over 300,000 species gone extinct. In addition, Wilson said that this rate of forest loss had been going on since 1980, so that gives us a claim of over well over half a million species lost forever in 24 years, a very large number.

Wilson also wrote, “Some groups, like the larger birds and mammals, are more susceptible to extinction than most.” (Wilson 1995) So, following Wilson’s lead to see if the extinction claims are true, I have investigated the timing and number of mammal and bird extinctions in modern times (the last 500 years) which are due to habitat reduction.

There are many different estimates of species loss, varying by orders of magnitude. I have seen extinction claims as high as “one species per minute” (over half a million extinctions per year, 10 million species extinct in 20 years) quoted in a number of places. However, I wanted facts, not estimates.

There are two main lists used by scientists to keep track of the facts of extinction. One is the “Red List”, maintained by the International Union for Conservation of Nature and Natural Resources (IUCN), which lists species which are either extinct or at risk of extinction. The Red List database can be searched online at

The other is the CREO list, from the Committee on Recently Extinct Organisms at the American Museum of Natural History. Their database is online at The CREO has established very clear criteria for declaring a species extinct, not extinct, or unresolved. The criteria include precise definitions for such things as adequate taxonomy (including DNA comparisons if available), sufficient hypodigm (actual specimens of the species), and adequate surveying of the species’ habitat to verify extinction. Starting afresh, they have then uniformly applied these criteria to the historical record of purported extinctions of mammals and fish in the last 500 years.

The conclusions of the CREO list are noted in the Red List, and vice versa. Although the two lists are very similar, I find the CREO list to be more thoroughly investigated and more uniformly and scientifically based than the Red List, so I have used it for mammal extinctions (it does not yet cover birds).