May 19
Extinction rate calculations are overestimating the role that habitat loss has on species, according to a new study published in the journal Nature.   

The most widely-used method for calculating species extinction is apparently “fundamentally flawed” and can overestimate extinction rates by as much as 160 percent. However, the authors of the study add that habitat loss is still the main threat to biodiversity.

Photo of male golden toad

The golden toad has not been seen since 1989, and is believed to be extinct.

Predicting extinctions 

As there are few ways of directly predicting extinctions, scientists use a mathematical model called the ‘species-area curve’. This starts with the number of species in an area and estimates how many species there will be as the area is increased. The calculations can then be reversed to estimate how many fewer species there will be when the amount of land decreases due to habitat loss. 

However, the authors of this new study say that this method is too simplistic and fails to take into account the full complexity of what influences species numbers. 

Co-authors Professor Stephen Hubbell, from the University of California, Los Angeles, and Professor Fangliang He, from Sun Yat-sen University, China, said “estimates based on this method are almost always much higher than actually observed.” 

This may go some way to explain why past predictions of extinction rates – such as the 1980 US National Research Council report which predicted losses of millions of species by the year 2000 – have not been realised.

Photo of mounted skin of female Falkland Island wolf

First encountered in 1833 by Charles Darwin, the Falkland Island wolf became extinct 50 years later after the arrival of permanent settlers.

Situation not as dire as believed 

While the results of this study show that the problem of species extinction caused by habitat loss is not as dire as many conservationists and scientists had believed, some are concerned about the way the results could be interpreted. 

Jean Christophe Vie, the IUCN’s Species Programme deputy director, said that it was good that it was a clear effort to “get the science right”, but that he is “worried about how this report could be used by people who are reluctant to take environmental issues seriously.” 

What is the actual concern is the rate of decline in populations,” he went on to say. “You do not see that many extinctions, but you do see many more species that are ending up with very small populations. So, focusing purely on extinctions is – to me – a problem.”

Photo of quagga in enclosure

The quagga roamed the plains of South Africa until the late 19th Century, when persecution for sport and the leather trade drove it to extinction.

In their paper, Professors He and Hubbell warned that their study must not “lead to complacency about extinction [as a result of] habitat loss”, which was a “real and growing concern”. 

The good news is that we are not in quite as serious trouble right now as people had thought, but that is no reason for complacency. I don’t want this research to be misconstrued as saying we don’t have anything to worry about when nothing is further from the truth”, said Hubbell. 

We have bought a little more time with this discovery, but not a lot.” 

Read more about this at Nature – ‘Hidden assumption hypes species-loss predictions’.

Alex Royan, ARKive Species Text Author

  • Stuart Pimm (May 20th, 2011 at 7:23 pm):

    The paper is a sham: it does not report extinction rates or the numbers of species that are threatened. Despite its posturing, it deals with a different issue. The paper is riddled with false statements. For instance:

    The paper states: “Estimates of extinction rates based on (the species-area) method are almost always much higher than those actually observed.” It is unequivocally false. One reference used to support this (Pimm and Askins) uses a species-area relationship to predict 4.5 bird extinctions following deforestation in Eastern North America and then notices that four species went extinct and one is threatened.

    There are dozens of other studies of many taxa around the world that find equally compelling agreements between predicted and observed extinctions. A small selection of them follows.

    So what does the paper model — and why does it poorly address the issue of extinctions? Imagine destruction that wipes out 95% of the habitat in an area metaphorically “overnight”. How many species have disappeared “the following morning”? The paper tells you. It is not many, just those wholly restricted to the 95% (and absent from the 5% where they would survive). The important question is …
    How many of additional species living lonely lives in their isolated patches (the 5%) would become extinct eventually because their population sizes are too small to be viable? A different species-area curve applies — the one for islands, which are isolated. It is a much larger number of extinctions, of course, and the one used in the studies mentioned above that find such compelling agreement between predicted against observed extinctions.
    By all means, feel free to share this.
    A response will be submitted to Nature shortly.

    Pimm, S. L. & Askins, R. A. Forest losses predict bird extinctions in eastern North America. Proc. Natl Acad. Sci. USA 92, 9343–9347 (1995).

    Brooks, T. M. et al. Habitat loss and extinction in the hotspots of biodiversity. Conserv. Biol. 16, 909–923 (2002).

    Grelle, C. E. de V., Fonseca, G. A. B., Fonseca, M. T. & Costa, L. P. The question of scale in threat analysis: a case study with Brazilian mammals. Animal Conserv. 2, 149–152 (1999).

    Brooks, T. & Balmford, A. Atlantic forest extinctions. Nature 380, 115 (1996).

    Cowlishaw, G. Predicting the pattern of decline of African primate diversity: an extinction debt from historical deforestation. Conserv. Biol. 13, 1183–1193 (1999).

    Brook, B. W., Sodhi, N. S. & Ng, P. K. L. Catastrophic extinctions follow deforestation in Singapore. Nature 424, 420–423 (2001)

    Brooks, T. M., S. L. Pimm, V. Kapos and C. Ravilious 1999. Threat from deforestation to montane and lowland birds and mammals in insular Southeast Asia. J. Anim. Ecol. 68: 1061-1078

    Brooks, T. M., Pimm, S. L., & Oyugi, J. O. Time lag between deforestation
    and bird extinction in tropical forest fragments. Conserv. Biol. 13, 1140-1150

    A full discussion of species area curves appears in

    Rosenzweig, M.L. Species diversity in space and time. (Cambridge Univ.
    Press, 1995)