Loss of Biodiversity

Loss of Biodiversity

While it is uncertain whether new species are still being added to Earth's total through speciation, what is certain is that species are being lost at an alarming rate. The biological wealth of the planet is declining rapidly, and the evidence points clearly to human activities as the cause.

When humans colonised the tropical Pacific Islands, it is believed that more than 2,000 species of native birds were driven to extinction. According to the IUCN Red List (2004), 784 species — including 338 vertebrates, 359 invertebrates, and 87 plants — have gone extinct in the last 500 years. Well-known examples of recent extinctions include the dodo (Mauritius), the quagga (Africa), the thylacine (Australia), Steller's sea cow (Russia), and three subspecies of tiger — the Bali, Javan, and Caspian. In just the last twenty years, 27 more species have disappeared.

Extinctions across different groups of organisms are not random. Some groups, such as amphibians, appear to be more vulnerable than others. Adding to this grim picture, more than 15,500 species worldwide are currently facing the threat of extinction. Presently, 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species, and 31 per cent of all gymnosperm species are threatened.

From fossil records, we know that large-scale losses of species have happened before, even before humans existed. In the more than 3 billion years since life originated and diversified on Earth, there have been five episodes of mass extinction. What makes the current 'Sixth Extinction' different is the rate at which it is happening. Current species extinction rates are estimated to be 100 to 1,000 times faster than in pre-human times, and human activities are responsible for this acceleration. Ecologists warn that if present trends continue, nearly half of all species on Earth could be wiped out within the next 100 years.

In general, the loss of biodiversity in a region can lead to:

• a decline in plant production
• lowered resistance to environmental disturbances such as drought
• increased variability in certain ecosystem processes, including plant productivity, water use, and pest and disease cycles

Causes of Biodiversity Losses

The accelerated rates of species extinction we see today are largely due to human activities. There are four major causes, often referred to as 'The Evil Quartet'.

1. Habitat Loss and Fragmentation

This is the most important cause driving animals and plants to extinction. The most dramatic examples come from tropical rain forests. Once covering more than 14 per cent of Earth's land surface, these forests now cover no more than 6 per cent, and they are being destroyed rapidly. By the time you finish reading this chapter, another 1,000 hectares of rain forest will have been lost. The Amazon rain forest — so huge it is called the 'lungs of the planet' and home to millions of species — is being cut and cleared for soya bean cultivation or for conversion to grasslands for raising beef cattle.

Besides total loss, the degradation of many habitats by pollution also threatens species survival. When large habitats are broken into small fragments due to human activities, mammals and birds that require large territories, as well as animals with migratory habits, are badly affected, leading to population declines.

2. Over-exploitation

Humans have always depended on nature for food and shelter, but when 'need' turns to 'greed', it leads to over-exploitation of natural resources. Many species extinctions in the last 500 years — such as Steller's sea cow and the passenger pigeon — were due to over-exploitation by humans. Today, many marine fish populations around the world are overharvested, endangering the continued existence of some commercially important species.

3. Alien Species Invasions …