Species richness
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Species richness is the simplest measure of biodiversity and is simply a count of the number of different species in a given area. It is referred to in equations as S.
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[edit] Reasons of being Commonly used
[edit] Practical Application
It is proven that species richness is measurable in practice. It is one of the two components of the ecosystem's species diversity.
[edit] Existing Information
A huge amount of information of pattern of species richness exists. This information can be found in academic literature. Further information on Species richness can be extracted from different kinds of database, such as museums.
[edit] Surrogary
Species richness has been found to be a good surrogate for other measures of biodiversity that would be more difficult to be measured directly as species richness is positively correlated with some measures.
[edit] Wide application
'Species richness' has been widely applied in practical management, legislation, and political discourse.
[edit] Factors affect Species Richness
There is a strong inverse correlation in many groups between species richness and latitude - the farther from the equator, the fewer species can be found, even when compensating for the reduced surface area of the globe in higher latitudes. Equally, as altitude increases, species richness decreases, indicating an effect of area, available energy, isolation and/or zonation (intermediate elevations can receive species from higher and lower).
[edit] Latitude
[edit] Latitudinal Gradient
See also: Rapoport's rule and Latitudinal gradients in species diversity
- The species richness increase from high latitudes to the low latitudes.
- The peak of the species richness is not at Equator, however. It is deducted that the peak is between 20-30°N.
- The gradient of species richness is asymmetrical about the equator. The level of species richness increase rapidly from the north region but decrease slowly from the equator to southern region.
[edit] Area effect
The latitudinal gradients of the species richness may result from the effect of area. The area at lower latitudes is larger than that at higher latitudes, leading to higher species richness at lower latitudes.
[edit] Productivity
The latitudinal gradients of species richness may be result from the energy available to the ecosystems. At lower latitudes, there are higher amounts of energy available because of more solar radiation, more resources (for example, minerals and water); as a result, higher levels of species richness can be allowed at lower latitudes.
[edit] Area
The relationship between species richness and area is commonly approximated as following equation: S = cAz or logS = logc + zlogA where S is the number of species (reflect the species richness), A is the Area, and z and c are constants.
The above relation explains the variation of species richness among different areas.
[edit] Sampling
Species Richness may not really relate to the area size but the statistical artefact. More species can be recorded maybe just because more samples are collected in larger area.
[edit] Habitat diversity
It is possible that larger area contain more habitats as it is said that larger area is more topographically and environmentally diverse. Therefore, there are more opportunities for more species to set up their populations due to higher habitat diversity.
[edit] Relationship between Endemism and Species Richness
The levels of endemism and that of species richness are frequently positive correlated; however, in some oceanic islands, there is high levels of endemism but the levels of species richness are quite low.
[edit] Other Methods for measuring Biodiversity
[edit] Adjusting the species richness
In order to account for the probability of missing some of the actual total number of species present in any count based on a sample population, the Jackknife estimate may be employed:
S=n+((n-1)/n)k where
- S=species richness
- n=total number of species present in sample population
- k=number of "unique" species (of which only one organism was found in sample population)
Similarly the equation may also be noted as:
S = E + k(n-1)/n where
- E = the summation of number of species in each sample
- k = number of rare/unique species
- n = number of sample
As well, when looking at local diversity the appropriate formula to use is:
S = cAz where
- c = a specific number for each taxa
- A = the area of study
- z = the slope perimeter
Other measures of biodiversity may also take into account the rarity of the taxa, and the amount of evolutionary novelty they embody.
[edit] Weakness
As a measure of biodiversity, species richness suffers from the lack of a good definition of "species." There are at least 7 definitions, with their own strength and weakness. Still, it is easy to measure, and is well studied.
Species richness fails to take into consideration species evenness. Other measures of biodiversity, such as the Simpson index, the Shannon-Wiener index, and the fundamental biodiversity parameter θ of the unified neutral theory of biodiversity take species evenness into consideration.
[edit] References
- Kevin J. Gaston & John I. Spicer. 2004. Biodiversity: an introduction, Blackwell Publishing. 2nd Ed., , ISBN 1-4051-1857-1(pbk.)
