Distribution of Two Species across a 1Hectare Qrid

XXXXXXXXXXX XXXXXXXXXXX XXXXXXXXXXX

OOO OOO

XXXXXXXXXXX XXXXXXXXXXX XXXXXXXXXXX

OO OO OO

OOO OOO

OOO OOO

OOO OOO

OOO OOO

OOO OOO

OOO OOO

OOO OOO

OOO OOO

X represents one specimen of species X O represents one specimen of species O

X represents one specimen of species X O represents one specimen of species O

individuals grouped as a single population in one location, whereas species O is fragmented into several isolated populations. If these populations are reasonably isolated, with relatively little gene flow between them, the fragmented populations of species O will be more genetically diverse than a single population.

Thus, even when using elements of species richness as surrogates for overall biodiversity, we should still carefully consider the following:

the number of individual organisms present that form the populations the number of populations present the number of species present the taxonomy (or evolutionary relatedness) of the species

Phylogenetic diversity is another important surrogate for evaluating biodiversity in some instances. Some regions may be the home of a burst of phylogenetic (or evolutionary) diversity, producing many closely related species. Various authors (for example, Seehausen, 2002) use as an example Lake Victoria, which has 500 to 1,000 closely related species of cichlids that evolved rapidly, perhaps within the last 14,600 years. Such areas are interesting not just because of their species richness but also because of our interest in understanding what conditions led to such a high rate of speciation. However, there are other reasons for using phylogenetic diversity as a surrogate. Stiassny (1997) explains that some regions may be very low in species richness but are the home to the basal (primitive) members of some groups of species. For example, Madagascar has very few species of cichlids, but those that are present appear to be the most primitive representatives of the group. Stiassny has shown that Madagascar is also home to basal representatives of other groups of fishes. The basal representatives are very important because they can tell us a great deal about the way in which certain features evolved in the group. In other words, we can look at the way that a particular feature (or character) is expressed by different representatives of the group; we can look at how the character is expressed in the most primitive member of the group, and from this, we can raise some hypotheses about the way that character has changed during the course of evolution of the group. Thus, although Madagascar may not be as species rich as other areas of comparable size, there is a special value to the diversity of the species found there, based on their phylogenetic or evolutionary history.

Species that are endemic to a certain region—that is, those that are found in one region of the world and nowhere else—are often used as a surrogate measure of the biodiversity value of a region. For example, Madagascar is often rated as one of the highest conservation priorities in the world, because a large majority of the species found there are endemic. One hundred percent of the primates, 80 percent of the flowering plants, and 95 percent of the plants found in the southern spiny forest are endemic.

Some areas may be home not just to individual endemic species but also to the only known representatives of entire groups of species. For example, the aye-aye, also found in Madagascar, is the only living representative of the primate family, Daubentoniidae. This is another aspect of using taxonomic or phylogenetic diversity as a particular measure of biodiversity.

The tuatara (Sphenodon) is a large, lizardlike animal that occurs only on islands off the coast of New Zealand. It is the only surviving representative of an entire order of reptiles, and it is also a phylogenetically basal representative of living reptiles. Therefore it confers special importance to the reptile biodiversity of these islands.

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