On a global basis, the biodiversity is so comprehensive that no one has a full overview of all living species. So far, between one and two million species have been described, and the number of undiscovered species is considered to be between five and 15 millions.
Biodiversity, however, is more than just species. At least three levels are commonly distinguished: ecosystem diversity , species diversity, and genetic diversity.
Ecosystem diversity s and habitats
An ecosystem consists of all species living together within an area, as well as environmental factors like climate, topography and geology. Environmental factors play a major role in deciding which species are able to live in an area. Such species are described as adapted to the area. A high diversity of ecosystems provides a variety of different habitats, and is therefore important for the diversity of species.
Species diversity
The concept of species diversity normally includes all species of animals, plants and fungi, and also bacteria and protists.
The most numerous species groups generally consist of small organisms, e.g. protists that comprise a large number of heterogeneous organisms: protozoans, algae, fungus-like organisms and parasites. A more familiar group are the insects, including approximately 850 000 species around the world. Fungi, also a species rich group, includes approximately 70 000 species worldwide.
Genetic diversity
The genetic variation within a species is also an important part of the biodiversity. Within multi-cellular organisms, each individual is genetically unique unless they are natural clones. The result is variation in appearance, anatomy, physiological properties and behaviour.
As a result of genetic variation, each species has an individual reservoir of different qualities, making it adaptable to the environment. Without a minimum of genetic diversity, the species is vulnerable to threats like environmental changes, diseases or competition from other species. The genetic variation is small if the population size is small.