Chapter I
Silent Spring
People had protected forests and game for centuries, and the first national parks were created in the nineteenth century. But these efforts were driven by hunters, foresters and lovers of scenery, not by a science of extinction. That changed in 1962, when Rachel Carson, a marine biologist who had written best-selling books about the sea, published Silent Spring. She showed that DDT and other pesticides did not stay where they were sprayed. They accumulated up food chains, becoming more concentrated at each level, and killed birds and fish far from any field.
The chemical industry attacked her, but the evidence mounted. British ornithologists found that the eggshells of peregrine falcons and sparrowhawks had become thinner since the late 1940s, when DDT came into use, and the eggs were breaking under their parents. The United States banned DDT for farm use in 1972. Peregrines, bald eagles and ospreys recovered. The episode showed that ecology could detect damage that no one was looking for, and that it had a place in public policy.
Chapter II
A Crisis Discipline
In the 1970s biologists began to apply their theories to the problem of extinction. Biogeography had shown that smaller areas hold fewer species. Nature reserves were becoming islands in a sea of farmland, and would lose species as islands do. Jared Diamond, a physiologist and bird ecologist, drew rules for designing reserves in 1975: one large reserve rather than several small, linked by corridors. Daniel Simberloff replied that the theory did not justify the rules, and the SLOSS debate, over whether a single large or several small reserves are better, ran for a decade.
Population genetics supplied the other half of the science. In small populations, genetic drift erodes variation and inbreeding exposes harmful recessive genes. In 1980 Ian Franklin proposed rules of thumb for how small is too small, the 50/500 rule. The next year Mark Shaffer defined the minimum viable population, and models of extinction risk followed.
These strands came together in 1985, when Michael Soulé asked "What is conservation biology?". He described it as a crisis discipline, like surgery or cancer biology. Its practitioners must often act before they are sure, because by the time the evidence is complete the species may be gone. The Society for Conservation Biology was founded the same year.
Chapter III
A Closer Look: How Fast Small Populations Forget
Genetic variation is a population's raw material for adapting to change. A standard measure of it is heterozygosity, , the chance that an individual carries two different versions of a gene. In a population of effective size , genetic drift removes a fraction of the heterozygosity every generation:
The fraction remaining after generations is:
| Effective size | After 10 generations | After 100 generations |
|---|---|---|
| 10 | 0.599 | 0.006 |
| 50 | 0.904 | 0.366 |
| 500 | 0.990 | 0.905 |
| 5,000 | 0.999 | 0.990 |
A population of effective size 10 loses 40% of its variation in ten generations and almost all of it in a hundred. At 50 the short-term loss is modest, which is the reasoning behind the first half of the 50/500 rule. But even at 50, half the variation is gone after about 69 generations. At 500 the population keeps about 90% for a hundred generations, and new mutations roughly make up the loss, which is the reasoning behind the second half.
The catch is that is usually much smaller than the number of animals. In a species where a few males father all the young, the effective size is
A herd of 200 in which only 10 males breed with 190 females has , not 200. Fluctuations in numbers matter too. The effective size over several generations is the harmonic mean of the sizes, dominated by the smallest. A population that numbers 1,000, then crashes to 10, then recovers to 1,000 has an effective size over those three generations of
One bad year leaves a mark that decades of recovery do not erase. This is why conservation biologists count breeding animals, not just animals, and why a species that passed through a bottleneck, however abundant now, may carry little variation.
Chapter IV
Counting the Losses
To know how much is being lost, conservation needs a list. The International Union for Conservation of Nature began its Red List of threatened species in 1964. In 1991 Georgina Mace and Russell Lande proposed quantitative criteria for placing species in categories such as Endangered and Critically Endangered, and they were adopted in 1994. Today the Red List assesses well over a hundred thousand species, and more than a quarter of them are threatened.
How fast species are going extinct is harder to say. Early estimates, starting in 1979, projected enormous losses from the destruction of tropical forests, and were criticised when recorded extinctions stayed far lower. In 2014 Stuart Pimm and colleagues compared recorded extinctions with the background rate in the fossil record and estimated that species are now vanishing about a thousand times faster than normal. The exact figure remains disputed. Underneath it lies an older and simpler question that no one can yet answer: how many species there are to lose.