Extinction is a fact of life. Species have been evolving and dying out ever since the origin of life. One only has to look at the fossil record to appreciate this. (It has been estimated that surviving species constitute about 1% of the species that have ever lived.) However, species are now becoming extinct at an alarming rate, almost entirely as a direct result of human activities. Previous mass extinctions evident in the geological record are thought to have been brought about mainly by massive climatic or environmental shifts. Mass extinctions as a direct consequence of the activities of a single species are unprecedented in geological history.

The loss of species in tropical ecosystems such as the rain forests, is extremely well-publicised and of great concern. However, equally worrying is the loss of habitat and species closer to home in Britain. This is arguably on a comparable scale, given the much smaller area involved.

It must be emphasised that these are only predictions. Most predictions are based on computer models and as such, need to be taken with a very generous pinch of salt. For a start, we really have no idea how many species there are on which to base our initial premise. There are also so many variables involved that it is almost impossible to predict what will happen with any degree of accuracy. Some species actually benefit from human activities, while many others are adversely affected. Nevertheless, it is indisputable that if the human population continues to soar, then the ever increasing competition with wildlife for space and resources will ensure that habitats and their constituent species will lose out.

It is difficult to appreciate the scale of human population increases over the last two centuries. Despite the horrendous combined mortality rates of two World Wars, Hitler, Stalin, major flu pandemics and Aids, there has been no dampening effect on rising population levels. In 1950, the world population was 2.4 billion. Just over 50 years later, the world population has almost tripled, reaching 6.5 billion.

In the UK alone, the population increases by the equivalent of a new city every year. Corresponding demands for a higher standard of living for all, further exacerbates the problem. It has been estimated that if everyone in the world lived at the UK standard of living (and why should people elsewhere be denied this right) then we would either need another three worlds to supply the necessary resources or alternatively, would need to reduce the world population to 2 billion.

The only possible conclusion is that unless human populations are substantially reduced, it is inevitable that biodiversity will suffer further major losses.

Some species are more vulnerable to extinction than others. These include:

Species at the top of food chains, such as large carnivores. Large carnivores usually require fairly extensive territories in order to provide them with sufficient prey. As human populations increasingly encroach on wild areas and as habitats shrink in extent, the number of carnivores which can be accommodated in the area also decreases. These animals may also pose a threat to people, as populations expand into wilder areas inhabited by large carnivores. Protective measures, including elimination of offending animals in the area, further reduces numbers.

• Endemic local species (species found only in one geographical area) with a very limited distribution.
  These are very vulnerable to local habitat disturbance or human development.

• Species with chronically small populations.
  If populations become too small, then simply finding a mate, or interbreeding, can become serious problems.

• Migratory species
  Species which need suitable habitats to feed and rest in widely spaced locations (which are often traditional and 'wired' into behaviour patterns) are very vulnerable to loss of these 'way stations'.

• Species with exceptionally complex life cycles
  If completion of a particular lifecycle requires several different elements to be in place at very specific times, then the species is vulnerable if there is disruption of any single element in the cycle.

• Specialist species with very narrow requirements such as a single specific food source, e.g. a particular plant species.

Loss of an individual species can have various different effects on the remaining species in an ecosystem. These effects depend upon the how important the species is in the ecosystem. Some species can be removed without apparent effect, while removal of others may have enormous effects on the remaining species. Species such as these are termed "keystone" species.