Estimates of the extinction rate
To publicize the rate of species loss, the United Nations organizes an International Day for Biological Diversity (www.cbd.int) which takes place on May 22 each year. The UN Secretary General said in a 2007 press statement that species are becoming extinct at an unprecedented rate.
Alarming figures have often been quoted, but how do scientists know how many species go extinct each day, week, month or year? It isn't possible to gather data non-stop in every location around the world, and the estimated extinction rate greatly exceeds the number of known species that have been declared extinct after thorough surveys failed to locate any survivors. The difference is due to the way the projections are calculated. Estimates for the number of species that will become extinct each year are extrapolated from the known rate of habitat loss.
When land is transformed for human use — agriculture, mining, dams, new residential developments, golf courses, etc. — the area of land consumed is invariably recorded by one government department or another. Even illegal land clearance comes to the attention of a government department sooner or later. Satellite images of each part of the world's surface are being updated constantly. Visit NASA's Visible Earth website to see examples.
How can scientists use this data? They construct estimates of the number of unique species in each locality where the natural habitat is destroyed. They already have estimates of the ratio of species that have been formally identified compared to the likely number which remain unclassified. Pygmy shrews were the smallest known mammal species until a tiny bat was discovered in the forests of Thailand in 1973. Kitti's hog-nosed bat is about the size of a bumblebee (IUCN: database entry). New species of plants and animals (insects included) are discovered every year. Species in vulnerable habitats may be going extinct before their existence is even known.
Unidentified species
For reptiles and amphibians, Robert MacArthur and E.O. Wilson demonstrated that there is a strong relationship between the size of an island and the number of identified species which inhabit it. The same approach can be used to estimate the number of species in habitat "islands" such as coral reefs or patches of rainforest. The graph below is adapted from their book, "The Theory of Island Biogeography" (Princeton University Press, 1967).
In 2006, a survey of the Nassau plateau in Suriname found 24 species previously unknown to science, including amphibians, fish and insects. One of them was a toad of the genus Atelopus, with fluorescent purple markings. Known members of the genus in other parts of the world have already been classified as extinct or critically endangered (read more at Science Daily).
Percentage of species assessed
Out of approximately 1.8 million known species more than 1.6 million are acknowledged in the database maintained by the International Union for the Conservation of Nature (IUCN), and 44,838 species have been assessed for a conservation status report. The vast bulk of species without an assessment are invertebrates and plants. The data in the following charts can be found in Table 1 of the 2008 Red List statistical summaries.
When guesses about the number of species that go extinct each day come up with high numbers they must be including estimates for unknown exotic insect species in patches of tropical rainforest. That's where the big numbers come from, because rainforest clearance is going on every day. The numbers are a lot smaller, but much less uncertain, for animals and plants which occupy habitats that expanding human populations want to use for agriculture, reservoirs or urban expansion. There are also changes in habitat boundaries due to global warming which show up year-by-year on satellite images.
Percentage of species threatened
All the mammal and bird species in the database have been assessed and almost 99% of amphibians. Over 20% of mammal species are endangered by habitat loss and poaching. The chart below shows the percentage of species that have been evaluated in each taxonomic group which are threatened with extinction.
Already there have been extinctions since the beginning of this century. The BBC interviewed Craig Hilton-Taylor, Programme Officer for the Red List unit of the IUCN, and asked "How are species classified as extinct?" The BBC article states that the Unit is based in Oxford. It's actually in Cambridge. There is concern among scientists that if species are classed as extinct too soon, conservation efforts may be abandoned even though a small number of individuals remain.
Genetic diversity in decline
Another threat to species is reduced genetic variability resulting from the obstruction of migration routes used by land animals. New highways, urban sprawl, and other man-made barriers, can shut off contact between species subpopulations.
