Biodiversity is most often thought of as the variety of organisms on earth. Yet it also includes two other factors: ecological diversity - the variety of ecosystems and ecological communities. And genetic diversity - the range of genetic differences found within and between species.
All three aspects are crucial for the success and development of life on earth. Since environmental conditions at every level are constantly changing, only diversity can ensure that some individuals and species will be able to adapt to the changes.
And all three have profound value for human beings. The value of biodiversity lies not just in our direct use of nature's vast range of products, from foods and medicines to fibres and materials.
Rosy Periwinkle, used for anti-leukaemia drugs
© Royal Botanic Gardens, Kew/WWF
Biodiversity also guarantees a permanent source of new genetic materials for future breeding programmes. Through ecosystems it delivers a vast range of environmental services from regulation of local climate to prevention of flooding and erosion. Not least biodiversity is one of the main sources of nature's perennial aesthetic appeal.
The rise of diversity
The history of life on earth has, to a large extent, been a story of increasing complexity. Though the fossil record may have obliterated traces of the earliest life forms, the number of families seems to have risen sharply in the Cambrian and Ordovician ages (about 543-443 million years ago). It then levelled out until the end of the Permian (about 248 million years ago), when the largest ever mass extinction wiped out more than 90 per cent of species on earth. There have been five massive episodes of extinction, but extinction is a normal facet of evolution: the average species in the fossil record lasts no more than five to ten million years. [See BBC pages on mass extinctions]
Diversity reached a peak in modern times. Its full extent is still largely unknown. Some 1.7 million species have been described, but mid-range estimates put the probable total at around 13 million - though this is bound to remain a guess for many decades to come. Although we are most familiar with land animals and plants big enough to be seen, plants and vertebrates make up less than three per cent of the probable total. Three-fifths of all known species are insects.
Diversity is not evenly distributed. It tends to be much higher in tropical regions, especially in rainforests and coral reefs. Equatorial countries have the highest level of biodiversity. For every 10,000 km² of territory, where Britain, for example, has 80 species of breeding bird, the Gambia has 269, Indonesia 271, and Costa Rica 350.
The sixth extinction
Life on earth is currently undergoing a sixth mass extinction event. Its extent is only vaguely known - species are only recorded as extinct after there is no reasonable doubt that the last individual has died.
The known figures are alarming enough. In the past 500 years 816 species have become extinct or extinct in the wild. Some 103 of these are known to have occurred since 1800 - an extinction rate 50 times greater than the natural background rate. Estimates of losses expected over the next 25 years vary from 2 to 25 per cent - but even the low end of this range is a thousand times the background rate of extinction.
The latest year 2004 assessment by the International Union for the Conservation of Nature (IUCN) (The Red List) records a worrying rise in the number of species threatened with extinction (falling into categories listed as vulnerable, endangered and critically endangered) compared to its first Red List in 1996. For example, the threatened mammals rose from 1096 to 1101, and of birds even more steeply from 1107 to 1213.
Some categories are especially at risk. The number of threatened primates rose from 96 to 114 between the two assessments. All of the planet's 21 species of albatross are now threatened to some degree because of the spread of long-line fishing. And the number of threatened fish (mainly freshwater fish) rose from 734 in 1996 to to 800 in 2004.
Wandering Albatross. Significant numbers are being drowned after being caught accidentally on baited hooks set by longline fisheries
© Tony Palliser/IUCN
In all, nearly one in five mammal species were threatened to some degree by 2004, along with one in 8 birds. The status of plants has been much less well researched, but cydads, the oldest seed plants on earth, are now the most threatened plants. Two species have already gone extinct in the wild and more are likely to join them. In 2004, 288 cydads were evaluated and 151 of them (more than 50 per cent) are threatened. This makes cydads one of the most threatened groups of species currently on the Red List.
The IUCN Red List now includes 15,503 species threatened with extinction. A total of 697 fauna and 87 plant species are now recorded as Extinct with a further 60 known only in cultivation or captivity.
These figures certainly underestimate the threat. The record is far better for Western countries with more biologists - the countries with the richest number of species are often those with the fewest scientists. Most extinctions must be occurring unseen among species that have not yet even been identified - among the countless insects, spiders, nematodes, molluscs and bacteria of cleared rainforest, or the largely unrecorded biota of the ocean floor.
The extinction of a species is of course the ultimate loss - but the process of local extinction has very serious consequences on local ecosystems, and effectively reduces people's chances of enjoying a glimpse of species - as the continued decline of songbirds in Britain illustrates.
Rainforest cleared for ranching, Brazilian Amazonia
© Mauri Rautkar/WWF
Climate change will be an increasing factor in loss of habitat. As sea levels rise, many marshlands will be flooded and will take many centuries to replace. Rising temperatures will push species that prefer cooler temperatures uphill or poleward - yet human barriers now make these migrations much harder than before. Those species that prefer the coolest habitats may find themselves with literally nowhere on earth to go. One entire biome - the tundra - may virtually disappear as higher latitudes are expected to warm much more than the average (see Climate Change and Biodiversity).
The second main source of threat is direct exploitation - in tropical areas many endangered mammals are a source of meat, while species such as tigers, rhino and turtle fall victim to exotic tastes in food or traditional medicine.
Finally there is the introduction of alien species, such as the introduction of rats and cats to small islands. Alien invasions have been at least partly responsible for the plight of 30 per cent of all threatened birds and 15 per cent of threatened plants.
These threats are the direct causes of extinction and endangerment - but driving these are the key underlying factors of human population, consumption and technology.
Street scene, Cambodia. The human development needs of Cambodia’s population are overwhelming.
© G. Bizzarri/FAO
It is not just total numbers, but the distribution of population that matters. Migration into environmentally sensitive areas has a big impact, while concentration of human activities in towns and cities focuses pollution onto small areas.
The technology used to meet demand also has a strong impact: some fishing methods, for example dynamiting of coral reefs, are extremely destructive of all species and habitats they touch, even though only a few species may be the actual target. Clear-felling of forest in large swathes is far more damaging to biodiversity than selective felling in smaller patches. Removal of hedgerows, killing of weeds and insect pests, and other practices of intense commercial agriculture, have had a massive impact on biodiversity in more developed countries.
What needs to be done
The transboundary Bentuang Karimun National Park shared by Indonesia and Malaysia revealed that the bay cat, thought to be extinct in that part of the region, is alive and living in the park.
Protecting habitats is an important part of the task (see Hotspots and threatened habitats). In 2003, some 12 per cent of the world's land area enjoyed protected status. This was made up of more than 100,000 reserves covering over 18 million kilometeres square today - an area larger than India and China put together. This is nowhere near enough to protect a major share of the world's biodiversity. Most of these areas are small - only 4 per cent are larger than 100,000 hectares, and a mere 0.5 per cent bigger than 1 million hectares.
Protecting the reserves that exist is not simply a matter of putting up fences and employing guards. The best results are obtained when local people are educated about the value of wildlife, and actually gain a share of the benefits from eco-tourism or bio-prospecting. Only then do they have a stake in seeing the reserves work.
Banning of trade in key species is crucial, along with effective enforcement of bans. Poaching for trade in wildlife products is a major pressure on many species. But even authorised trade is frighteningly high. In 1997 CITES reported a net legal world trade of 25,733 live primates, 235,000 parrots, 948,000 lizards, and 344,000 wild orchids, along with 1.6 million lizard skins, 1.5 million snake skins and 850,000 crocodile skins. These figures are only the tip of a vast iceberg.
Many areas have already been destroyed, and many species reduced to danger levels, so captive breeding and careful restorative conservation are increasingly needed.
But changes are needed in the major forces that cause our pressure on wildlife: consumption, pollution, and population growth. Perverse subsidies should be removed, for example for fishing and fossil fuel consumption, and the burden of taxation should be shifted away from employment onto environmentally destructive activities. Policies that help to slow human population growth, from improved female education and mother and child health to provision of a wide choice of contraceptives, will also help to reduce the growth in pressure on biodiversity.
© People & the Planet 2000 - 2007