Loss of Biodiversity
While it is doubtful if any new species are being added through the process of speciation in the present times, there is no doubt that existing species are continuously being lost. The biological richness, or wealth, of our planet has been declining rapidly. The responsibility for this loss is largely attributed to human activities.
- The colonisation of tropical Pacific Islands by humans is said to have led to the extinction of over 2,000 species of native birds. According to the IUCN Red List (2004), 784 species have gone extinct in the last 500 years. These include 338 vertebrates, 359 invertebrates and 87 plant species. Some well-known examples of recent extinctions include the dodo (from Mauritius), quagga (Africa), thylacine (Australia), Steller’s Sea Cow (Russia), and the Bali, Javan and Caspian subspecies of tiger. Even in the last two decades alone, 27 species have disappeared.
- A careful study of extinction records shows that the loss of species is not uniform across all groups of organisms. Some groups, such as amphibians, appear to be more vulnerable. Currently, more than 15,500 species worldwide are facing the threat of extinction. Among these, 12% of bird species, 23% of mammal species, 32% of amphibian species, and 31% of gymnosperm species are threatened.
- A study of fossil records reveals that the Earth has experienced five major episodes of mass extinctions during its evolutionary history, even before the appearance of humans. These occurred over the course of more than three billion years since life originated and diversified on Earth. The current wave of extinction, often referred to as the “Sixth Extinction”, is different from the previous events in one significant way: the rate of extinction today is much higher. Estimates suggest that species are now going extinct 100 to 1,000 times faster than they did before human activity became widespread. If current trends continue, ecologists warn that nearly half of all species on Earth may be lost within the next 100 years.
- Loss of biodiversity has serious consequences. It can lead to a decline in plant productivity, reduce the ecosystem’s ability to withstand environmental disturbances such as drought, and increase the unpredictability in natural processes like plant growth, water use, and disease and pest control cycles.
Causes of Biodiversity Loss
The rapid rate of species extinction in modern times is largely the result of human activities. Ecologists refer to the four major causes as the “Evil Quartet”.
Habitat Loss and Fragmentation
- This is the most significant cause driving plant and animal species towards extinction. The most dramatic examples of habitat loss come from the destruction of tropical rainforests. These forests once covered more than 14 per cent of the Earth’s land surface, but now they occupy less than 6 per cent. This reduction continues at an alarming pace. By the time one finishes reading this chapter, approximately 1,000 hectares of rainforest will have been lost.
- The Amazon rainforest, often referred to as the “lungs of the planet”, is home to millions of species. It is being cleared to cultivate soya beans or converted into grasslands for raising beef cattle. In addition to outright destruction, many habitats are degraded by pollution, making them unsuitable for many species. When large habitats are broken into smaller, isolated patches (a process called fragmentation), it adversely affects animals that need large areas or that migrate seasonally, leading to population declines.
Over-Exploitation
- Humans have always depended on nature for basic needs such as food and shelter. However, when consumption exceeds nature’s capacity to regenerate resources, it leads to over-exploitation. Many species that have gone extinct in the last 500 years, such as Steller’s Sea Cow and the passenger pigeon, were lost due to over-hunting or over-harvesting. Currently, over-exploitation is threatening many marine fish populations, risking the collapse of entire commercial fisheries.
Alien Species Invasions
- The introduction of alien (non-native) species, either accidentally or deliberately, can lead to severe damage to local biodiversity. Some alien species become invasive, meaning they grow quickly and overpower native species, leading to their decline or extinction.
- For instance, the introduction of the Nile perch in Lake Victoria (East Africa) caused the extinction of over 200 species of cichlid fish that were unique to that ecosystem. In India, weeds like Parthenium (carrot grass), Lantana, and Eichhornia (water hyacinth) have invaded large areas and threatened native plant species. The illegal introduction of the African catfish (Clarias gariepinus) for aquaculture is now endangering the native freshwater catfish species in Indian rivers.
Co-Extinctions
- In ecosystems, many species are interdependent. When one species becomes extinct, other species that rely on it may also become extinct. This phenomenon is called co-extinction. For example, when a host fish species disappears, its parasites, which may be completely dependent on it, also vanish. In another example, when a plant and its pollinator are closely dependent on each other, the extinction of one inevitably leads to the extinction of the other.
