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Biodiversity
07 December 2002
NewScientist.com news service
Ann Fullick

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There was great news for sea horses, basking sharks and whale sharks last month. The big-leaf mahogany tree may also have been celebrating in its own woody way. The good news was that, against all the odds, a meeting in Chile of the UN Convention on Trade in Endangered Species (CITES) had voted to control international trade in these creatures. Decisions like this give hope that the tide of opinion is turning in favour of tough international legislation to preserve biodiversity. They make it less likely that endangered trees will end up as coffee tables, that the only surviving sea horses will be swimming in aquaria, and that the remains of the last shark's fin will be floating menacingly in a bowl of Chinese soup.

It seems biodiversity has become a buzzword beloved of politicians, conservationists, protesters and scientists alike. But what exactly is it? The Convention on Biological Diversity, an international agreement to conserve and share the planet's biological riches, provides a good working definition: biodiversity comprises every form of life, from the smallest microbe to the largest animal or plant, the genes that give them their specific characteristics and the ecosystems of which they are a part.

In October, the World Conservation Union (also known as the IUCN) published its updated Red List of Threatened Species, a roll call of 11,167 creatures facing extinction - 121 more than when the list was last published in 2000. But the new figures almost certainly underestimate the crisis. Some 1.2 million species of animal and 270,000 species of plant have been classified, but the well-being of only a fraction has been assessed. The resources are simply not available. The IUCN reports that 5714 plants are threatened, for example, but admits that only 4 per cent of known plants have been assessed. And, of course, there are thousands of species that we have yet to discover (see Figure). Many of these could also be facing extinction.

Why the fuss? Does it really matter if there are fewer species of snail or beetle in the world, if some unknown plant species ceases to exist or if the gene pool of a rare species is shrinking? In short, yes. Biodiversity is the basis of a healthy, balanced global ecology capable of sustaining life on Earth. A diverse ecosystem is a stable ecosystem because it is complex and flexible enough to be self-regulating. Earth's air and water, for example, are kept pure through the action of a wide range of organisms. Even the humblest creatures play their part. Through decomposition, dead matter is recycled and often detoxified in the process. For instance, microorganisms in soil and water convert toxic ammonia to nitrate ions, which are then taken up and used by plants. The atmosphere and the world's climate are stabilised by plants through photosynthesis, absorbing carbon dioxide and producing oxygen.

A wide variety of plant life reduces the chances of flooding and drought. Roots hold the soil together and absorb vast amounts of water that is then evaporated into the atmosphere through transpiration. Conversely, plants that are tolerant of low water levels help to prevent desertification by maintaining a micro-environment under their canopy that reduces evaporation and run-off, and maintains fertility.

Plant pollination, seed dispersal and nutrient recycling in systems such as the nitrogen cycle all maintain healthy ecosystems, and all depend on high levels of biodiversity. Some of these systems are so efficient we have harnessed them to improve our own personal environment. Sewage-treatment works are one of the best examples. Microbial decomposers are kept in huge quantities and given ideal conditions to break down our waste into relatively harmless substances that can safely be discharged into rivers or the sea, or even be used as fertiliser.

Healthy, varied ecosystems deliver many direct and indirect benefits. In agriculture, the wild relatives of livestock and crops provide a reservoir of genetic diversity that can be drawn upon to develop improved breeds and varieties. This natural resource will become all the more important as the world comes to terms with climate change, providing genetic characteristics that will allow crops to thrive despite changes in temperature and rainfall.

Of course, plants are more than just food. For thousands of years the forest and savannah have been our medicine chest. And now the pharmaceuticals industry draws upon this huge natural resource to develop new drugs. Some 56 per cent of the top 150 prescribed drugs in the US are based on chemicals derived from plants, but only 1 per cent of the 250,000 known species of tropical plant have been screened for potential pharmaceutical use. Until we have a better idea of the diversity of plant life on Earth, we cannot know how many more life-saving drugs are waiting to be discovered.

On a geological timescale, more species have become extinct than have survived on Earth to this day. Most, if not all, species become extinct eventually. However, there has to be a balance between the rate of extinction and the creation of new species. According to Donald Levin, a botanist at the University of Texas, Austin, the rate of extinction now runs at between a hundred and a thousand times the natural rate. He estimates that on average a species of animal or plant becomes extinct every 20 minutes, making our times one of the six great periods of mass extinction in the history of the Earth (see "Mass extinctions", Inside Science No. 126).

Much of the observed loss of biodiversity in recent years has been either directly or indirectly the result of human activities. Across the globe, large areas of land are now devoted to growing food crops, and in these areas biodiversity has been virtually wiped out as acre upon acre of land is covered with the same plant species. When clones of plants are used, genetic diversity is reduced even further. Herbicides and pest control can mean that the number of species in some areas is more or less down to one.

Razed to the ground

Another major impact of farming practices is the destruction of forests and the desertification that often accompanies it. Slash-and-burn farming continues to wipe out huge tracts of tropical rainforest. As agriculture and habitation spread into the remaining areas of wilderness, many other species will lose their habitats. Even when the habitats themselves are not destroyed, pollution can upset the ecological balance or introduce toxins. Sometimes the loss of a single food plant due to farming practices, habitat destruction or pollution can lead to widespread loss of biodiversity if that plant is a key component of several food chains and webs.

At sea, overfishing on a massive scale threatens the complete loss of certain species in particular areas. Recent research on New Zealand snappers has shown that overfishing delivers a double whammy to biodiversity. As fish populations get smaller, only a few fish seem to breed successfully in each generation. Most of the surviving offspring are genetically related to a small number of parents, so not only is the overall number reduced, genetic diversity is too.

The long-term prognosis isn't good. Many scientists now believe that the world's climate is changing as a result of the burning of fossil fuels, forest destruction and intensive livestock farming (pigs, sheep and cattle produce vast quantities of the greenhouse gas methane). The UN Intergovernmental Panel on Climate Change estimates that global surface temperature will rise on average by between 1.4 and 5.8 °C by the end of the century. Rising temperatures have a major effect on many ecosystems and inevitably lead to loss of diversity, particularly as the changes seem to be happening so fast that evolution is unlikely to keep up.

Some species are more sensitive to temperature change than others. A rise of just 1 °C could lead to the extinction of New Zealand's tuatara, a reptile that has been described as a "living fossil" because it first appeared at the same time as the dinosaurs. Recent research at Victoria University in Wellington showed that at 21 °C, 96 per cent of all tuatara eggs are female, whereas at 22 °C they are all male. So it seems likely that the tuatara, an animal that survived the cataclysm that led to the extinction of the dinosaurs, and doubtless many other species, will fall victim to global warming.

The risks to biodiversity are not evenly spread around the world (see Figure). Certain areas are much more vulnerable - particularly small, isolated populations such as islands, rainforest fragments, coral reefs, bogs and wetlands. Many of these areas also have particularly rich diversity of flora and fauna, so if the ecosystem is damaged many species will be lost. Aside from rising sea levels as a result of global warming, islands are vulnerable to industrial exploitation for resources such as oil and minerals. Tourism also takes its toll. Coral reefs - some of the most species-rich environments on Earth - are very sensitive to pollution and changes in water temperature and depth. Bogs and wetlands are often drained for building projects or farming, to tap into oil and gas reserves or extract peat. They are also susceptible to changes in climate.

We may not know what we're losing until it's too late. To avoid falling into that trap, we need to measure biodiversity. The number of different species in a particular area is a useful basic index of biodiversity, but the concept is much more far-reaching. The differences between individuals in a species, between populations of the same type of organism, between communities of different organisms and between ecosystems are all features of biodiversity (see Figure). So a particular habitat's overall health can be gauged from the diversity of species living there, both in terms of the total number of species and the range of animals, plants and microorganisms. But some habitats are more precious than others.

Suppose the number of species in a 10 metre by 10 metre "transect" of a garden were compared with a similar transect of woodland. The garden would almost certainly show a far higher number of species - it has greater biodiversity per square metre. But which area is more biologically important? Clearly, not all biodiversity has equal value from a conservation point of view.

And there are times when the presence of a single species makes all the difference. Imagine there were two areas of Scottish woodland threatened by a road-building project, each with identical levels of biodiversity and almost identical species, except that one is home to the capercaillie - a turkey-sized bird that is not uncommon in northern Europe but is rapidly heading for extinction in Scotland. Which should be protected from the bulldozers? The one that's home to the capercaillie would win hands down.

An assessment of biodiversity doesn't end there. Even supposing an endangered species were sighted in an area, does it live there or is it just passing through? A single individual does nothing to preserve biodiversity - a breeding population is needed for that.

The way in which observations are made also has a major effect. Each technique has its advantages and disadvantages. For example, light traps are often used to capture nocturnal flying insects, but the geographical range of the insects is unknown. We don't know where they've flown from. Beating the branches of trees and collecting what falls out is a common way of sampling insects, but this can mean those living at the tops of trees are overlooked. A weevil population in Richmond Park in London was ignored for years because the researchers used this collecting technique. Another increasingly common and effective strategy is to spray the entire tree with an anaesthetic gas then collect everything that falls to the ground. But even this technique has its limitations, because organisms that live in the bark are unlikely to be dislodged.

Another major hurdle is recognising and categorising the thousands of different species (see "The species enigma", Inside Science No. 111). Fortunately, new technologies are playing an increasingly important role in classification. For many years, species have been identified by careful observation of their physical features. Now DNA technology means that species can be compared at a genetic level, greatly increasing our understanding of biodiversity both within and between species. Information technology is also proving invaluable. Vast and cumbersome paper filing systems are being replaced by easily searched databases, making identification of new specimens easier, and new software is helping to reconcile different classification systems (see "Geeks to the rescue").

Not all techniques for measuring biodiversity rely on such high-tech methods. Scientists trying to gauge the size of populations of elusive creatures often resort to counting dead or trapped animals, and extrapolate this figure to estimate the size of the total population. For example, in rural Britain one effective if gruesome way of doing this is to check on "road kill" numbers - the more squashed badgers or hedgehogs counted along a particular stretch of road, the higher the numbers will be in the local population.

It is important to develop a picture of the diversity of life on Earth now, so that comparisons can be made in the future and trends identified. But it isn't necessary to observe every single type of organism in an area to get a snapshot of the health of the ecosystem. In many habitats there are species that are particularly susceptible to shifting conditions, and these can be used as indicator species (see "How are we feeling today?").

In the media, it is usually large, charismatic animals such as pandas, elephants, tigers and whales that get all the attention when loss of biodiversity is discussed. However, animals or plants far lower down the food chain are often the ones vital for preserving habitats - in the process saving the skins of those more glamorous species. These are known as keystone species.

By studying the complex feeding relationships within habitats, species can be identified that have a particularly important impact on the environment. For example, the members of the fig family are the staple food for hundreds of different species in many different countries, so important that scientists sometimes call figs "jungle burgers". A whole range of animals, from tiny insects to birds and large mammals, feed on everything from the tree's bark and leaves to its flowers and fruits. Many fig species have very specific pollinators. There are several dozen species of fig tree in Costa Rica, and a different type of wasp has evolved to pollinate each one. Chris Lyle of the Natural History Museum in London - who is also involved in the Global Taxonomy Initiative of the Convention on Biological Diversity - points out that if fig trees are affected by global warming, pollution, disease or any other catastrophe, the loss of biodiversity will be enormous.

A sea otter's garden

Similarly, sea otters play a major role in the survival of giant kelp forests along the coasts of California and Alaska. These "marine rainforests" provide a home for a wide range of other species. The kelp itself is the main food of purple and red sea urchins and in turn the urchins are eaten by predators, particularly sea otters. They detach an urchin from the seabed then float to the surface and lie on their backs with the urchin shell on their tummy, smashing it open with a stone before eating the contents. Urchins that are not eaten tend to spend their time in rock crevices to avoid the predators. This allows the kelp to grow - and it can grow many centimetres in a day. As the forests form, bits of kelp break off and fall to the bottom to provide food for the urchins in their crevices. The sea otters thrive hunting for sea urchins in the kelp, and many other fish and invertebrates live among the fronds.

The problems start when the sea otter population declines. As large predators they are vulnerable - their numbers are relatively small so disease or human hunters can wipe them out. The result is that the sea urchin population grows unchecked and they roam the sea floor eating young kelp fronds. This tends to keep the kelp very short and stops forests developing, which has a huge impact on biodiversity.

Conversely, keystone species can also make dangerous alien species: they can wreak havoc if they end up in the wrong ecosystem. The cactus moth, whose caterpillar is a voracious eater of prickly pear, was introduced to Australia to control the rampant cacti. It was so successful that someone thought it would be a good idea to introduce it to Caribbean islands that had the same problem. It solved the cactus menace, but unfortunately some of the moths have now reached the US mainland - borne on winds and in tourists' luggage - where they are devastating the native cactus populations of Florida.

Internationally, the problems of loss of biodiversity have been aired and recognised, but that's the easy part. Agreeing on a course of action is fraught with difficulty because so many of the threats result from the way of life the developed world enjoys and the developing world wants to emulate - a way of life based around burning fossil fuels and eating a diet rich in meat. Despite these problems, steps are being taken in a number of ways to conserve biodiversity.

Organisations like the Convention on Biological Diversity work with groups such as the UN and with governments and scientists to raise awareness and fund research. A number of major international meetings - including the World Summit on Sustainable Development in Johannesburg this year - have set targets for governments around the world to slow the loss of biodiversity. And the CITES meeting in Santiago last month added several more names to its list of endangered species for which trade is controlled. Of course, these agreements will prove of limited value if some countries refuse to implement them.

There is cause for optimism, however. There seems to be a growing understanding of the need for sustainable agriculture and sustainable tourism to conserve biodiversity. Problems such as illegal logging are being tackled through sustainable forestry programmes, with the emphasis on minimising the use of rainforest hardwoods in the developed world and on rigorous replanting of whatever trees are harvested. CITES is playing its part by controlling trade in wood from endangered tree species. In the same way, sustainable farming techniques that minimise environmental damage and avoid monoculture are becoming increasingly popular.

Action at a national level often means investing in public education and awareness. Getting people like you and me involved can be very effective. Australia and many European countries are becoming increasingly efficient at recycling much of their domestic waste, for example, preserving natural resources and reducing the use of fossil fuels. This in turn has a direct effect on biodiversity by minimising pollution, and an indirect effect by reducing the amount of greenhouse gases emitted from incinerators and landfill sites.

Preserving ecosystems intact for future generations to enjoy is obviously important, but biodiversity is not some kind of optional extra. Variety may be "the spice of life", but biological variety is also our life-support system.

From issue 2372 of New Scientist magazine, 07 December 2002

Geeks to the rescue The Natural History Museum in London is home to millions of specimens from all over the world, collected over several hundred years. At present, each specimen in the vast collection is classified on a handwritten or typewritten index card. Every time a new specimen is sent to the museum for identification, the index system has to be laboriously searched by hand. So scientists at the museum and at the University of Essex are developing a system called VIADOCS that will scan and interpret the cards and transfer the information they contain to an Internet-based database and paper catalogue. Another problem with identifying organisms is that a number of different classification systems have developed over the years. Prometheus is a computer program that compares different classification systems. It not only allows scientists to use the best aspects of several systems, it also helps to avoid an overestimation of biodiversity, by ensuring organisms don't appear more than once under different names.

How are we feeling today? Some creatures are particularly sensitive to changes in their environment. If their numbers are easily monitored, they can be used as "indicator species" to carry out regular health checks on a habitat. Many of the organisms chosen as indicator species are microbes or small invertebrates. For example, the enormous number of types of beetle and their immense range of habitats makes them useful as indicator species. However, larger organisms can sometimes give useful indications of the state of an ecosystem. Lizards are very common in sub-Saharan Africa and in many other tropical, sub-tropical and warm temperate regions. They eat insects and are in turn eaten by birds and small mammals, so they form an important part of many food chains and webs. They have a slow metabolic rate and are sensitive to many pesticides, making them useful indicators of the effect of pesticide spraying. By keeping an eye on lizards, researchers can infer how spraying is affecting insects, birds and mammals in related food chains, and predict any knock-on effect on the pollination rates of plants.

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