Species richness

Species richness is the number of species in a given area. It is represented in equation form as $S$.

Typically, species richness is used in conservation studies to determine the sensitivity of ecosystems and their resident species. The actual number of species calculated alone is largely an arbitrary number. These studies, therefore, often develop a rubric or measure for valuing the species richness number(s) or adopt one from previous studies on similar ecosystems.

Factors affecting 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 in higher latitudes due to the spherical geometry of the earth. 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).

Latitude

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.fact|date=September 2007
* 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.

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.

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. However, there have been relevant studies showing that species richness and primary productivity are actually negatively correlated [http://www.physorg.com/news140269006.html Maths model helps to unravel relationship between nutrients and biodiversity] .

The Millennium Ecosystem Assessment, an international ecological effort initiated by the United Nations, states:

"In most ecosystems, changes in the number of species are the consequences of changes in major abiotic and disturbance factors, so that the ecosystem effects of species richness (number of species) per se is expected to be both comparatively small and very difficult to isolate. For example, variation in primary productivity depends strongly on temperature and precipitation at the global scale and on soil resources and disturbance regime at the region-to-landscape-to-local scales. Factors that increase productivity, such as nutrient addition, often lead to lower species richness because more productive species outcompete less productive ones. In nature, therefore, high species diversity and high productivity are often not positively correlated."

Area

The species-area relationship is commonly approximated as following equation: $S=cA^z$ or $log(S)=log(c)\; +\; z\; log(A)$ where S is the number of species, reflecting the species richness (sometimes also called species diversity), A is the area given in hectares, and c and z are constants. c is the species richness factor, usually between 20 and 2000; z is the species accumulation factor, usually between 0.2 and 0.5. This equation was first described by Arrhenius in 1921 [Arrhenius, O. 1921. " [http://www.jstor.org/pss/2255763 Species and Area] " J. Ecol. 9: 95-99] and explains the variation of species richness among different areas [http://www.pre.nl/download/EI99_methodology_v3.pdf M. Goedkoop, R. Spriensma, R. Müller-Wenk, P. Hofststter, T. Koellner, T. Mettier, A. Braunschweig, R. Frischknecht, R. Heijungs, E. Lindeijer et al. (2001) The Eco-indicator 99 – A damage-oriented method for Life Cycle Assessment – Methodology Report. Third Edition. Pré Consultants, Ammersfoort, Holland.] .

ampling

Species richness may not really relate to the area size but rather be a statistical artifact. More species can be recorded maybe just because more samples are collected in larger area.fact|date=September 2007

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.

Relationship between endemism and species richness

The levels of endemism and that of species richness are frequently positively correlated. However, on some oceanic islands, there are high levels of endemism but the levels of species richness are quite low.

Other methods for measuring biodiversity

Adjusting the species richness

The most common formula for working out Species Diversity is the Simpson's diversity index, which uses the following formula:

$D\; =\; frac\{N(N-1)\}\{sum\; n(n-1)\}$

Where:
* D = diversity index
* N = Total number of organisms of all species found
* n = number of individuals of a particular species

A high D value suggests a stable and ancient site, while a low D value could suggest a polluted site, recent colonisation or agricultural management.

Usually used in studies of vegetation but can also be applied to animals.

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:

$S\; =\; n\; +\; Big(frac\{n-1\}\{n\}Big)^k$

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:

$S\; =\; E\; +\; kBig(frac\{n-1\}\{n\}Big)$

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:

$S\; =\; c\; Big(ABig)^z$

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.

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 index, and the fundamental biodiversity parameter $heta$ of the unified neutral theory of biodiversity take species evenness into consideration.

References

Further reading

* Kevin J. Gaston & John I. Spicer. 2004. Biodiversity: an introduction, Blackwell Publishing. 2nd Ed., , ISBN 1-4051-1857-1(pbk.)
* Diaz, et al Ecosystems and Human Well-being: Current State and Trends, Volume 1. Millennium Ecosystem Assessment. 2005. Island Press.

ee also

*Abundance (ecology)
*Scaling pattern of occupancy
*Species-area curve
*Storage effect