Scientists estimate that over the past 150 years humans have altered nearly 47 per cent of the Earth's land surface. The World Atlas of Biodiversity, published by the United Nations yesterday, suggests that the complex but delicate interaction between terrestrial plants and animals will be threatened on almost three quarters of the total land surface within the next 30 years.

The terrestrial environment extends over little more than one quarter of the Earth's surface, or nearly 150 million square kilometers (58 million square miles). It is the most accessible to humans and has therefore suffered the most in terms of environmental degradation and loss of species.

Dry land is characterized by its extensive collection of complex plants harboring a rich and diverse array of animals. Forests and woodlands are the natural cover for much of the world's land. Photosynthesis – the process used by plants to convert sunlight into chemical energy – is most intense in tropical forests, which are among the richest places in the world in terms of biodiversity.

The Atlas says that half of the area of forest that had developed since the last ice age has since been cleared or degraded by man. The decline is especially prevalent today in the tropical rainforests of South-East Asia, the Congo and parts of the Amazon. About 22 per cent of these areas are used for farming, towns and other kinds of human development. By 2032, scientists estimate this exploitation may more than double to 48 per cent of the total land area that they now cover.

Areas of the land that are not covered by trees – such as shrubland, deserts and tundra – tend to be poorer in terms of biodiversity, with the exception of Mediterranean-type shrub, which can be among the richest places on Earth for plants and animals. Humans have degraded many of the natural shrubland regions by burning and overgrazing, activities that have also led to the loss of valuable plant species.

The extinction of land plants is contributing to the loss of important genes for crops as well as new sources of medicines and pharmaceutical products. The Atlas suggests that we are losing one important new drug every two years because of the extinction of plants and animals and yet less than 1 per cent of the world's estimated 250,000 tropical plants has been screened for potential pharmaceuticals.

Klaus Toepfer, executive director of the United Nations Environment Programme, said this loss of new sources of drugs could be arrested if local people were encouraged to take part in their preservation. "We must address the issue of genetic resource sharing by giving developing countries, where the majority of biodiversity remains, an economic incentive to protect wildlife by paying them properly for the plants and animals whose genes get used in new drugs or crops," Mr Toepfer said.

Oceans

Human activities, directly or indirectly, are now the primary cause of changes to the makeup of the animals and plants in the oceans.

About one third of the human population lives within the coastal zone – within 60km (37.3 miles) of the sea – and this proportion is forecast to rise this century. "Pressures exerted by the human population on the marine biosphere are substantial and increasing," the Atlas says.

In many ways the Earth is a water world, with oceans covering 71 per cent of the surface. Furthermore, the average depth of the oceans is about four times the average elevation of the land, making it even bigger in terms of total volume.

Despite its size, however, the marine environment produces about the same amount of organic matter as the land because photosynthesis – the process on which all life ultimately depends for energy – is limited to the upper layers of the ocean.

Some parts of the oceans are highly productive whereas others are virtually barren. The diversity of marine species is not as great as the diversity of terrestrial species, perhaps because the marine environment is more uniform and continuous.

The seas are the richest source of wild protein in the human diet. It comes in the form of fish, mollusks (e.g. mussels) and crustaceans (e.g. shrimps), but over the past decade this supply has begun to dwindle. Over the past 50 years the world catch has grown fivefold but since the 1990s it has declined substantially despite increased efforts to find fresh stocks of fish. More than half of the world's main fisheries are suffering from overfishing.

The Atlas identifies 18 marine "hotspots" in the world that are rich in endemic species. Many of these are coral reefs – often described as the tropical rainforests of the sea – which harbor some of the most diverse wildlife communities on Earth.

Although marine algae, microscopic plants, provide most of the energy needed to support life in the sea, other plants also play a role. Among the most important of these are the sea grasses. As with the coral reefs, some of the most important sea grass beds are being lost. Since the 1930s, sea grass beds along the Atlantic coasts of America and Europe have been lost to a marine slime mould possibly linked with toxic pollution and trawling.

In addition to fish, the oceans also support other important groups of animals, such as sea mammals and sea birds, which have all suffered through hunting and modern fishing methods.

Lakes and rivers

Inland waters constitute a tiny fraction of the Earth's total surface area yet they are vital for human settlements and have consequently borne the brunt of the worst excesses of man's activity, often by becoming a dumping ground for human waste. Only 2.5 per cent of the world's water is freshwater and a large proportion of this, some 69 per cent, is locked up in the form of ice and permanent snow. Much of the rest is in subterranean aquifers, which means that only 0.3 per cent of the world's freshwater resides in lakes and rivers.

While the amount of available freshwater is limited, demands on it continue to grow as the human population expands. Agriculture consumes about 70 per cent of all water drawn from the world's lakes, rivers and aquifers, but much of this soaks away or evaporates before it reaches the intended crop.

Freshwater, and the lakes and rivers that provide it, being a vital resource for human survival, come under conflicting demands with increasingly adverse consequences for their biodiversity, according to the World Atlas on Biodiversity. "There are indications that, overall, a higher proportion of inland water species are in decline than marine or terrestrial forms," it says.

Animals are particularly diverse in inland waters. Freshwater fish, for instance, are more diverse than their marine cousins, probably because of the isolated nature of rivers and lakes. Scientists estimate there is one fish species for every 15 cubic kilometers of freshwater compared with one fish species for every 100,000 cubic kilometers of seawater.

About 40 per cent of known fish species occur in freshwater, with about 10,000 species confined solely to lakes and rivers. Inland waters support a large number of wildlife species, wading birds, freshwater reptiles and mammals, despite accounting for such a small proportion of the Earth's surface area.

Freshwater, which links the land with the sea, typifies the complex interactions that characterize the study of biodiversity. Affecting the land can have an impact on lakes and rivers, which in turn can change the oceans.

Mark Collins, the director of the United Nations Environment Programme World Conservation Monitoring Center in Cambridge, emphasized at yesterday's launch of the Atlas that life on Earth cannot be considered in isolation. He cited the case of land degradation on the slopes of Mount Kenya, which is not just affecting the species on the mountain. People downstream from the river formed by Mount Kenya now have to contend with flash floods and droughts caused by a sporadic flow of water resulting from deforestation.

"Biodiversity is not just about species. It's about how they interact with one another," Dr Collins said. "The conservation of the resources of the natural world is not a luxury any more but essential to the quality of human life."

© 2002 Independent Digital (UK) Ltd

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