Extinction is simply defined as "when the last existing member of a given species dies". But extinction is not so simple, as there are various types of extinctions that occur on our planet, caused by a variety of reasons and conditions, some natural while others are the result of human activity.

The goal of this article is to raise awareness of the different forms of extinction.

The first type of extinction is functional extinction. This type of extinction occurs when only a few individuals of a species or population are left. Therefore the odds of reproduction are slim. These individuals may have not reached reproductive age or are too old or too sick so that any offspring produced may not survive to reproduce offspring of their own.

Some persons might have the belief that "one cannot cry over spilt milk" when it comes to extinction. Since we cannot do anything about those species that are already extinct but we can try to save the species that are alive from extinction.

What we may not realise, is that some species are so interconnected or dependent on each other that without one species, others become extinct, thus causing a "domino effect" through the trophic levels of the food chain.

This is what is known as coextinction. Examples of coextinction occur in parasite species when their host species becomes extinct. Another example is the Haast's Eagle which is a top predator in New Zealand which became extinct after its prey the Moa became extinct.

Another type of extinction is mass extinction. However in order to be classified as a mass extinction event it must be a rapid event involving a significant part of the species on the planet and resulting in species from different groups and habitats from around the world becoming extinct. Also the process, by which new species arise - speciation, decreases. At least five mass extinctions have occurred, the most recent occurring about 65 million years ago. There are two main causes for mass extinctions. External causes, which can be subdivided into terrestrial causes where the change to the habitat is so great and rapid that it does not allow the species present to migrate to another area or adapt to the change. An example of a terrestrial cause of mass extinction is global low sea-level, which spelt disaster for a majority of organisms that lived in depths less than about 1,000 metres.

Another form of an external cause of extinction is by extraterrestrial causes like meteors.

Internal causes which have also been suggested as a cause of a mass extinction has not been shown. The cause of which are bacterial and viruses, the only problem is that a wide variety of organisms in different habitats will have to be susceptible. However, this scenario as a cause of mass extinction although possible has not been shown to actually cause a mass extinction event but with human manufacture and dispersal it is possible.

A form of extinction which humans have total involvement is planned extinction which is carried out in order to eradicate many species of bacteria and viruses. A good example of this type of extinction is the eradication of the smallpox virus. However, one has to consider that if a disease carrying species is eradicated, if another will rise in abundance to take its place. For example, by eradicating the species of mosquitoes that cause yellow fever, malaria and dengue, millions of human lives will be saved. This is also beneficial to a country from an economic standpoint as well since more of the population is able to work and bring in an income.

Here in Tobago, overhunting has caused several mammalian species and the manatee, that previously inhabited the Richmond River to disappear from overhunting. We may well be on the way to many other species disappearing , including sea birds (Booby) and marine turtles.

Finally, there is pseudoextinction which is also considered a form of extinction which literally means "false extinction".

In this form of extinction, species that have evolved from the extinct or parent species are termed daughter species. The existence of these daughter species mean that most of the genetic material of the parent species are still around.

However, there must be strong evidence linking the living daughter species to the parent species to make a case for pseudoextinction.

So we can see that although extinction takes many forms, it still ends in a loss of species and biodiversity.