A study in Nature suggested the without some form of radical change 25% of the world's land animals will become extinct within the next 50 years.

When considering the reasons why so many species are becoming endangered it is important to realise that this is very closely linked to the need to conserve the biodiversity of the planet.

Habitat loss is by far the most widespread cause of species endangerment. Usually this is due to some form of human activity. Forests are cut down to create more land for agriculture or building and coastal marshlands are drained for the same reason. Agricultural activity such as removal of hedgerows and pesticide spraying have removed both habitat and food supply for many species.

It was only after a noticeable decline in numbers of the bald eagle and the peregrine falcon that the use of DDT and other persistent pesticides began to be questioned. When the ecosystem of a species is not maintained, such as the removal of a food supply, the species is forced to adapt to new surroundings or perish.

Pollution is a major disrupter and destroyer of ecosystems and this was graphically illustrated following the April 2010 Deepwater Horizon oil disaster in the Gulf of Mexico. This devastated many marine ecosystems and caused the death of countless seabirds and marine creatures.

Overexploitation, such as deepwater trawling has put a number of species of fish at serious risk. This can also have a knock-on effect of removing the food supply of other marine creatures, putting them at risk as a consequence.

Climate change can alter the delicate balance of an ecosystem. Relatively minor changes in temperature can allow some species to thrive, while others perish. More dramatic climate changes can lead to the melting of ice caps and glaciers, with the consequent disruption to the local ecosystems. On a worldwide basis, the resulting rise in sea levels can disrupt the ecosystems of many species, including humans.

Habitat loss can also occur when alien species are introduced into ecosystems, either by chance or by design. In 1918 a ship ran aground on a Pacific island. While the ship was being repaired a number of Black Rats escaped and set up a thriving colony on the island. Within a short time they had wiped out several of the island's native birds and other fauna. The islanders introduced masked owls in an effort to control the rats, but this simple led to the loss of many of the remaining sea birds.

As habitat loss combines with other ecological disruptions, many species find it increasingly difficult to breed. This leads to a gradual decline in numbers until the point is reached where the species is no longer sustainable.

The International Union for Conservation of Nature (IUCN) maintains a Red List of Threatened Species. This is the most comprehensive inventory of the global status of plant and animal species. IUCN calculates that around 40% of the world's organisms are endangered.

The IUCN has a classification system to enable criteria to be set with respect to rate of decline, population size, area of geographic distribution and the degree of population and distribution fragmentation.

NatureServe is a conservation organisation that has its headquarters in the United States, but operates throughout North America, Latin America and the Caribbean. Its prime aim is to make biodiversity a mainstream consideration in all significant conservation and natural resource management decisions. NatureServe is actively engaged with a number of leading international initiatives that promote the development, distribution, and sharing of biodiversity information.

It uses a slightly different scale to define risk of endangerment.

A prefix letter, G, N or S is inserted to indicate whether the risk is global, national or more localised within a particular country. For instance, G3 would indicate that a species was globally vulnerable, but N3 would indicate that it was vulnerable within a particular country, although more secure elsewhere.

It has been claimed in some quarters that maintaining biodiversity in today's world is both costly and time consuming and in short is a waste of time and money. The counter argument is that not only should species be saved for aesthetic and moral reasons, but wild species are also important as providers of products and services essential to human welfare. Many scientific breakthroughs have come from the study of wild organisms.

25% of western pharmaceuticals are derived from ingredients discovered in the tropical rainforests. Vincristine, a drug that has been particularly successful in treating childhood leukaemia, is extracted from the rainforest plant periwinkle.

As the population of the world increases, further demands are placed on its resources. In 2001 it was estimated that world demand for timber would double by 2050. Most of this timber would come from tropical rainforests and already more than 20% of the Amazon rainforest has been lost, along with the ecosystems of the species that it supported.

The goal of conservationists is to find ways to preserve endangered species and maintain biodiversity, especially for those species that are approaching extinction.

Where the reasons for population decline are not immediately evident it is possible to undertake detailed research to investigate the causes. In other cases it is possible to create more awareness of environmental ethics that can help to encourage species preservation, such as the importance of not removing coral.

Where species are very rare and in danger of extinction a programme of captive breeding can be successful. However, it is important to guard against inbreeding since this may result in loss of immunity to disease.

Legal private farming has also proved to be successful in Africa. Where previously poaching was a serious problem, now significant numbers of southern black and southern white rhinoceros are privately owned and there have been substantial increases in the populations of both groups. This is applauded by IUCN since there is now a strong economic incentive to look after the rhinos, whether for reasons of ecotourism or selling for profit.

Salmon farming has already proved to be very successful and there has been some success farming of other species of fish. Over a third of all fish consumed in the United States is now farm raised.

On the one hand this might be seen as a good thing because it takes pressure off the oceans, but there are some unfortunate downsides. Salmon farming is a case in point. Salmon are carnivores and farmed salmon commonly need about three pounds of processed wild fish for every pound of salmon raised. This amounts to about 2 to 3.5 million tons of wild fish to be processed annually.

In addition, an average salmon farm has around 200,000 salmon. Each day these fish produce as much faeces as a town of 62,000 people. This accumul sinks to the seabed generating killer bacteria that consume the oxygen vital to the wild fish that live there. Farmed fish also often escape and can spread diseases that did not previously exist in wild populations.

In short, such farms can be resource-intensive, can cause damage to the environment and bring further danger to already endangered species.

The natural world and the species that inhabit it are vital to us all. Either directly or indirectly our world provides us with clean air, food, water, shelter, energy, soil, medicines, protection from natural disasters, as well as recreation, inspiration, diversity and beauty.

But this is a fragile world. Many its diverse species are in danger of being lost of ever; basically the choice is ours.