The number of species within a region. (A term commonly used as a measure of species diversity, but technically only one aspect of diversity.) Details... 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 represented in equation form as S. Species richness is also one level of biodiversity known as alpha-diversity. Species richness is commonly used, along with other factors, as a measure for determining the overall health of different biological ecosystems. High species richness for a given area denotes a high level of redundancy in ecosystem function, which further denotes the ecosystems ability to withstand natural disturbances by the natural world or man (i.e. fires, floods, disease, deforestation, etc.). Therefore, high levels of species richness in ecosystems typically characterize these ecosystems as healthy and robust. How it is used 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.[citation needed]
- 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. 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 outcompetete less productive ones. In nature, therefore, high species diversity and high productivity are often not positively correlated."
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.
Sampling Species richness may not really relate to the area size but the statistical artifact. More species can be recorded maybe just because more samples are collected in larger area.[citation needed]
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 positive correlated; however, in some oceanic islands, there is 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=N(N-1)/Σn(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+((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.
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 θ of the unified neutral theory of biodiversity take species evenness into consideration. | 
