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Part 1
Components of Biodiversity

|Components of Biodiversity | Biodiversity classification | Number of species globally|


The total biodiversity of an area can be broken down into two hierarchical components:

1) The number of functional types of organisms (e.g. carnivorous animals, Nitrogen (N)-fixing plants) or ecosystems (coniferous forest, prairie, tundra, marine intertidal).

2) The number of functionally equivalent organisms/ genotypes within each functional type (e.g. the number of species of wood-rotting fungi). These organisms perform the same role in an ecosystem (e.g. moose and caribou are both large herbivores in boreal ecosystems; mussels and tunicates are both sessile marine filter feeders). (Huston, Ch.1 )

The basis for this division is that mechanisms that drive the diversity of functional types are different from the mechanisms that drive diversity among functionally equivalent organisms. For example, competition plays a significant role in determining diversity of functionally equivalent organisms but has little influence on functional diversity in an ecosystem. (Huston, Ch.1)

Many other factors will influence the diversity of a system besides competition. These include evolutionary changes, geology, human history, environmental variability, disturbance and random population fluctuations. (Huston, p.76)

These factors are further discussed in Part 3: Processes and Patterns of biodiversity.


Ecologists have developed ways to characterize species diversity in a given area:

Within-habitat diversity or alpha-diversity: refers to a group of organisms interacting and competing for the same resources or sharing the same environment. Measured as # of species within a given area. (Huston p.72; Whittaker, 1960, 1967; Fisher et al., 1943)

Between-habitat diversity or beta-diversity: refers to the response of organisms to spatial heterogeneity. High beta-diversity implies low similarity between species composition of different habitats. It is usually expressed in terms of similarity index between communities (or species turnover rate) between different habitats in same geographical area (often expressed as some kind of gradient). (Whittaker 1960, 1967)

Geographical diversity or gamma-diversity (Whittaker 1960, 1972)

alpha, beta, gamma diversity.gif

Reprinted from figure 5.6 in Perlman, D.L. and Adelson, G., 1997. by permission of Blackwell Science, Inc.


Number of species globally

How many species are there on Earth? There is no definitive answer . Estimates fall between 1.5 and 30 million species of plants and animals. Another recent estimate claims that a more realistic number is 6 million. (Dobson, 1996). What we do know is that between 1.5 and 1.8 million species have been identified. The majority of species remain unidentified. Of the 34 known animal phyla, only one phylum lives exclusively on land while 33 are found in the ocean. Of those 33, 14 are found nowhere else on earth.

Of the species that have been described, approximately:

monarch.jpg750 000 of these are insects

41 000 are vertebrates coyote.jpg

wild columbine.jpg250 000 are plants

The remaining species are comprised of invertebrates, fungi, algae and other microorganisms. The biological diversity of many ecosystems remains poorly explored, even today. These ecosystems include the deep ocean and the tree canopy and soil of tropical forests.

For more information on marine biodiversity, visit the Conservation International website:




next section

Part 2: Importance of Biodiversity