Biodiversity generally refers to the variety of life forms within an area. Thereby including the different plants, animals and microorganisms, the genes they contain, and the ecosystem they form. Biological diversity is usually considered at three different levels: genetic diversity, species diversity and ecosystem diversity. We will concentrate in this lesson on the species diversity.
Biodiversity is largely affected by the size of the area that is considered. As the sample area increases, more species present in the habitat are likely to be included in the sample and as the sample area further increases, a greater variety habitats, each with new species, will be included in the sample. The species/area relationship therefore increasingly reflects habitat heterogenity on a larger scale. To take this fact into account, various kinds of biological diversity are differentiated:

• The diversity within a site or habitat is generally referred to as α-diversity
   
• The diversity between habitats are referred to as β-diversity

Thus an area with a high variability of habitats will have a high β-diversity, even if each of the habitats included, itself has a low α-diversity.

To assess the biological value of a region, not only the number of species is important, but also the range size of the occurring species. Species with a restricted range that occur only in a particular area, are called "endemits". Thus, it should be clear that the term endemism without a declaration of the reference area makes no sense. If we speak of endemism in this lesson, we are considering the alpine zones of the particular mountains under discussion as reference areas.

Where do we find high biodiversity and endemism?

1 - Comparison between species richness and endemism of birds and mammals in Africa. Mountain regions are important for endemism, but rarely for species richness. (Figure after Rahbeck & Balmford)

Biodiversity follows some general rules: The number of species increases with the temperature and precipitation of the area, and decreases with increasing altitude and geographically latitude. Thus the regions with the highest biodiversity lay around the equator in the tropical and subtropical forests.

Quite another distribution is found when looking at endemic species. Here we can see that mountain regions play an important role in endemism. Endemits reflects the long geological and climatological history of an area. Factors that favor endemic species are isolation and diversity of habitats.

29 August 2011
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