Biodiversity and Extinction Rates

17 May 2004

Many people worry that human activity is causing large scale extinctions of plant and animal life. Some claim that human caused extinctions are on a similar scale to those that occurred 65 million years ago at the boundary between the Cretaceous and Tertiary eras when most species perished including the dinosaurs.

This causes two distinct worries.

1. The loss of species will harm humans.

2. Quite apart from any harm to humans, there is a duty to prevent "ecocide". Different people evaluate this duty differently. Since the purpose of these pages is establish the sustainability of material progress, I'll take the view that although biodiversity is an important amenity, we are mainly concerned with the extent to which losses of diversity are a threat to human progress.

Here's Peter Raven of the Missouri Botanical Gardens and former Home Secretary of the National Academy of Sciences (USA) and former President of the American Association for the Advancement of Science - in short a member of the scientific establishment.

We are confronting an episode of species extinction greater than anything the world has experienced for the past 65 million years. Of all the global problems that confront us, this is the one that is moving the most rapidly and the one that will have the most serious consequences. And, unlike other global ecological problems, it is completely irreversible.

One of the claims of large extinction rates concerns insects in the Amazon, and it based on the fact that many species are being discovered with extremely small ranges - as small as a few acres. Therefore, it is inferred that (a) there are very many insect species in the Amazon and (b) that when a relatively small area is developed, or even logged, species disappear. I suppose both of these contentions are true.

It is then claimed from the geological record that the average life of an insect species is 10 million years, and therefore we are killing of insect species at thousands of times the natural rate of extinction. I don't buy this, because I doubt that species with extremely small ranges have such long lifetimes on the average. I haven't seen this argument elsewhere, so I don't know how well it will stand up. [I asked a very well-known biologist about this at a National Academy of Sciences dinner, and he said that the average life of limited range species was unknown. I'll not give his name, since I believe people should not be held responsible for what they say across the dinner table.]

Opinions differ on the actual rate of extinctions.

The book Extinction Rates edited by John Lawton and Robert May (Oxford University Press, 1995) contains chapters on the extinction rates of various animals and plants. However, it concerns mainly the extinction rates for known species, whereas the very high estimates of extinction rates are based on estimates about unnamed species. The number of unclassified species is estimated by measuring how many new species turn up when a new area is explored, e.g. the forest canopy of some limited region.

Sir (now Lord) Robert May, FRS, is the head of population biology at Oxford University and is (or was) Chief Scientific Adviser to the British Government.

Here's the first paragraph of the preface.

Hardly a day passes without one being told that tropical deforestation is extinguishing roughly one species every hour, or maybe even one every minute. Such guesstimates are based on approximate species-area relations, along with assessments of current rates of deforestation and guesses at the global total number of species (which range from 5 to 80 million or more.) While such figures arguably have a purpose in capturing public attention, there is a clear and increasing need for better estimates of impending rates of extinction, based on a keener understanding of extinction rates in the recent and far past, and on the underlying ecological and evolutionary causes.

The articles in the book are are:

1. Assessing extinction rates

2. Extinctions in the fossil record

3. Constancy and change of life in the sea

4. Insect faunas in ice age environments: why so little extinction?

5. Bird extinctions in the Central Pacific

6. Extinctions in Mediterranean areas.

7. Recent past and future extinctions in birds.

8. Rates and patterns of extinctions among British invertebrates.

9. Assessing the risk of plant extinctions due to pollinator and disperser failure.

10. Population dynamic principles.

11. Estimating extinctions from molecular phylogenies.

12. Biological models for monitoring species decline: the construction and use of databases.

13. Classification of threatened species and its role in conservation planning.

14. The scale of the human enterprise and biodiversity loss.

The last chapter is by Paul Ehrlich. It has his usual I = PAT.

A chapter I found particularly interesting is Chapter 6 on the extinction of plants in Mediterranean climates.

The author is Werner Greuter, Botanischer Garten und Botanisches Museum Berlin-Dahlem, Koenigin-Luise Strasse, 6-8, D-14191 Berlin Germany and is entitled "Extinctions in Mediterranean Areas". (I include the address in case anyone wishes to write for a reprint). Greuter is evidently as specialist in this matter, because he refers to several previous papers.

The total number of Mediterranean higher plants presumed to be extinct is thus 37, of which 35 were species and 2, subspecies. This compares to a total of about 23,300 Mediterranean plant species, or 29 000 taxa (species plus subspecies). The present Mediterranean extinction rate for vascular plants is therefore 0.15% at the species level, or a mere 0.13% of the taxa.

He remarks that Mediterranean regions including the Mediterranean, California, central Chile, the Cape of Good Hope region and south-western Australia `total some 70 000 species and represent a major part of the higher plant diversity of the world, exceeding the combined floras of tropical Africa and Asia'. The reason for the large number of species is that many have very small ranges and the diversity in any given area is not so large. He also writes that the extinction rates are lowest in those areas where humanity has been active longest, e.g. the Mediterranean itself. I suppose that almost all the species incompatible with humanity died off long ago.

Let me refer to the chapter "Species Loss Revisited" by Julian Simon and Aaron Wildavsky in The State of Humanity edited by Julian Simon. They document the way species loss has been exaggerated by alarmist writers. I'll quote extensively from this article when I get around to it.

2003 September: The Specter of Species Extinction Will Global Warming Decimate Earth's Biosphere? by Sherwood, Keith and Craig Idso presents evidence against the widely believed theory that increases in temperature and CO2 will lead to widespread species extinctions. They discuss many studies that suggest the opposite - that increased temperature and CO2 will benefit most species of plants and extend their range. They discuss many articles offered as evidence by two groups of extinction advocates and show that the articles themselves lead to a different conclusion.

One interesting fact in the article concerns the effect of an increase in temperature on the north-south range of a plant species, especially of trees. It turns out that the northern limit of a species is determined by temperature. As that limit is approached the rate of growth goes to zero. However, the rate of growth of a species does not decline as it approaches the southern limit of its range but remains stable or even increases. What determines a species's southern boundary is competition from other species that require high temperatures. For this reason the southern boundary of a species is likely to change slowly as its territory is gradually invaded by species liking warm temperature. The invasion is likely to begin in gaps caused by logging and various kinds of die-off.

Scientists who worry about extinctions often agree that the world will reach a new equilibrium as temperature increases - assuming it does. However, they worry that the rate of increase of temperature is unprecedented and that species, especially of plants, will migrate northward too slowly and become extinct. None of the articles I have seen expressing this worry mention the possibility of replanting areas further north of the main present range of a species. It should work, because it's just agriculture. Failing to mention replanting is an example of the widespread inhibition against mentioning positive human activity to arrange the planet to obviate harmful changes or to make the planet more suitable for humanity.

Why do people believe these things? Explanations are offered about how ideology affects science in Ideology and sustainability, especially the section on Environmentalist ideology.

SLOGAN: The world is humanity's garden, to be arranged to suit humanity [Of course, it suits humanity to keep some part of the world wild.]

OPPOSITE SLOGAN: The world is a wild place, and humanity is an unwelcome intruder.