Species richness
Encyclopedia
Species richness is the number of different species in a given area. It is represented in equation
form as
.
Species richness is the fundamental unit in which to assess the homogeneity
of an environment. Typically, species richness is used in conservation
studies to determine the sensitivity of ecosystem
s 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.
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).
and Latitudinal gradients in species diversity
are actually negatively correlated.
The Millennium Ecosystem Assessment, an international ecological effort initiated by the United Nations, states:
is commonly approximated as following equation:
or 
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 and explains the variation of species richness among different areas .
, which uses the following formula:
Where:
As predicted by the intermediate hypothesis, a high D value suggests a site that has undergone intermediate disturbance, while a low D value could suggest either recent disturbance or protection from disturbance.
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 Jackoff estimate may be employed:
where
Similarly the equation may also be noted as:
where
As well, when looking at local diversity the appropriate formula to use is:
where
Other measures of biodiversity may also take into account the rarity
of the taxa
, and the amount of evolutionary novelty they embody.
. Other measures of biodiversity, such as the Simpson index
, the Shannon index
, and the fundamental biodiversity parameter
of the unified neutral theory of biodiversity
take species evenness into consideration.
Equation
An equation is a mathematical statement that asserts the equality of two expressions. In modern notation, this is written by placing the expressions on either side of an equals sign , for examplex + 3 = 5\,asserts that x+3 is equal to 5...
form as
.Species richness is the fundamental unit in which to assess the homogeneity
Species homogeneity
Homogeneous means consisting of parts or people which are similar to each other or are of the same type. Homogeneity means all of the same or similar kind or nature. In ecological terms it can also be expressed as a lack of, or reduction in biodiversity. Species richness is the fundamental unit...
of an environment. Typically, species richness is used in conservation
Habitat conservation
Habitat conservation is a land management practice that seeks to conserve, protect and restore, habitat areas for wild plants and animals, especially conservation reliant species, and prevent their extinction, fragmentation or reduction in range...
studies to determine the sensitivity of ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s 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 correlationCorrelation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
in many groups between species richness and latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
: the farther from the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, the fewer species can be found, even when compensating for the reduced surface area
Surface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
in higher latitudes due to the spherical geometry of the earth. Equally, as altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
increases, species richness decreases, indicating an effect of area, available energy, isolation and/or zonation (intermediate elevations can receive species from higher and lower).
Latitudinal gradient
See also: Rapoport's ruleRapoport's rule
Rapoport’s rule is an ecological hypothesis that states that latitudinal ranges of plants and animals are generally smaller at lower than at high latitudes.-Background:...
and Latitudinal gradients in species diversity
Latitudinal gradients in species diversity
The increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal diversity gradient , is one of the most widely recognized patterns in ecology. Put another way, in the present day localities at lower latitudes generally have more...
- Species richness increases from high latitudes to low latitudes.
- The peak of the species richness is not at EquatorEquatorAn equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. It is deduced that the peak is between 20-30°N. The tropics fall within this range which is about 24.3 degrees north and south and this attests to the fact that species richness and biodiversity is highest here. - 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 than normal.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, even higher levels of species richness can be allowed at lower latitudes. However, there have been relevant studies showing that species richness and primary productivityProductivity (ecology)
In ecology, productivity or production refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface per unit time, for instance grams per square metre per day. The mass unit may relate to dry matter or to the mass of carbon generated...
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 outcompete less productive ones. In nature, therefore, high species diversity and high productivity are often not positively correlated."
Area
The species-area relationshipSpecies-area curve
In ecology, a species-area curve is a relationship between the area of a habitat, or of part of a habitat, and the number of species found within that area. Larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to follow systematic mathematical...
is commonly approximated as following equation:
or 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 and explains the variation of species richness among different areas .
Sampling
Measurements of simple species richness are related to area size, but are also affected by sampling intensity. When relatively small numbers of samples are taken from a given area, the numbers of species identified will be lower than if greater numbers of samples are collected. The more samples that are collected, the greater the number of species will be found. Thus in some cases, measurements of species richness can be considered a statistical artifact.Habitat diversity
A larger geographic area will likely encompass more habitats than a smaller one, especially if it contains more variations of environmental factors such as topography, soil, and climate. In other words, the more physical variations in the environment, the more opportunities there are for different biological communities to develop, which means there will be more variation in the floral and faunal populations. Therefore, if an area being sampled has a diversity of habitats, it is expected to have a diversity of ecosystems.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.Adjusting the species richness
The most common formula for working out Species Diversity is the Simpson's diversity indexSimpson index
Simpson's diversity index is one of a number of diversity indices, used to measure diversity. In ecology, it is often used to quantify the biodiversity of a habitat. It takes into account the number of species present, as well as the relative abundance of each species...
, which uses the following formula:
Where:
-
= diversity index -
= number of individuals of species 
-
= total number of organisms of all species found, i.e.
As predicted by the intermediate hypothesis, a high D value suggests a site that has undergone intermediate disturbance, while a low D value could suggest either recent disturbance or protection from disturbance.
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 Jackoff estimate may be employed:
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:
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:
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
Rare
-In music:* Rare , a band from Northern Ireland* Rare , a Serbian alternative rock band from Belgrade* Rare , an album by the progressive rock band Asia* Rare , a compilation album by David Bowie...
of the taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, and the amount of evolutionary novelty they embody.
Weakness
Species richness fails to take into consideration species evennessSpecies evenness
Species evenness refers to how close in numbers each species in an environment are. Mathematically it is defined as a diversity index, a measure of biodiversity which quantifies how equal the community is numerically. So if there are 40 foxes, and 1000 dogs, the community is not very even. But if...
. Other measures of biodiversity, such as the Simpson index
Simpson index
Simpson's diversity index is one of a number of diversity indices, used to measure diversity. In ecology, it is often used to quantify the biodiversity of a habitat. It takes into account the number of species present, as well as the relative abundance of each species...
, the Shannon index
Shannon index
The Shannon index, sometimes referred to as the Shannon-Wiener Index or the Shannon-Weaver Index, is one of several diversity indices used to measure diversity in categorical data. It is simply the Information entropy of the distribution, treating species as symbols and their relative population...
, and the fundamental biodiversity parameter
of the unified neutral theory of biodiversityUnified neutral theory of biodiversity
The unified neutral theory of biodiversity and biogeography is a hypothesis and the title of a monograph by ecologist Stephen Hubbell...
take species evenness into consideration.
See also
- Abundance (ecology)Abundance (ecology)Abundance is an ecological concept referring to the relative representation of a species in a particular ecosystem. It is usually measured as the large number of individuals found per sample...
- Scaling pattern of occupancyScaling pattern of occupancyIn spatial ecology and macroecology, scaling pattern of occupancy , also known as the area-of-occupancy is the way in which species distribution changes across spatial scales. In physical geography and image analysis, it is similar to the modifiable areal unit problem. Simon A...
- Species-area curveSpecies-area curveIn ecology, a species-area curve is a relationship between the area of a habitat, or of part of a habitat, and the number of species found within that area. Larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to follow systematic mathematical...
- Species diversitySpecies diversitySpecies diversity is an index that incorporates the number of species in an area and also their relative abundance. It is a more comprehensive value than species richness....
- Storage effectStorage effectThe storage effect is a coexistence mechanism proposed in the ecological theory of species coexistence, which tries to explain how such a wide variety of similar species are able to coexist within the same ecological community or guild...
- DegeneracyDegeneracy (biology)Within biological systems, degeneracy refers to circumstances where structurally dissimilar components/modules/pathways can perform similar functions under certain conditions, but perform distinct functions in other conditions. Degeneracy is thus a relational property that requires comparing the...
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.