Theory of Species Diversity
Species diversity is an expression of community structure and is a characteristic unique to the community level of organization. A community demonstrates a high species diversity if many equally or nearly equally abundant species are present. If a community is composed of only a few species, or if only a few species are abundant, then species diversity is low.
High species diversity indicates a complex community in which a high degree of species interaction is possible. For this reason, communities with higher diversities typically have higher levels of energy transfer (food webs), predation, competition and niche availability. In several ecological community studies, species diversity has been construed as a measure of community stability in which low or changing species diversity may indicate a stressed or unstable environment. However, many ecologists argue that there is no direct correlation between species diversity and community stability or community stress.
The theory of species diversity takes into account three different ecological phenomena (i.e., species richness, relative abundance and community evenness). When viewed separately, each of these parameters can reveal valuable ecological insight. However, when viewed cumulatively through studies of species diversity indices, these parameters combine to give an excellent overall picture of community structure.
SPECIES RICHNESS
The first of the three parameters contained within the theory of species diversity is the total number of species present in a community, or its species richness. A high species richness indicates a complex community in which a high degree of species interaction is possible. For this reason, communities that possess a high species richness can be said to have higher levels of energy transfer (food webs), predation, competition and niche availability, than other similar communities that exhibit low species richness. However, species richness does not take into account the number of individuals per species or the evenness of the individuals within each species. For this reason, this method is generally not used singularly to describe detailed community characteristics rather it is sometimes loosely used to make initial inferences on the condition of a given community. By qualitatively determining the number of species present in a community, limited assumptions as to the "health" and community structure of an environment can be made.
RELATIVE ABUNDANCE
The second parameter contained within the theory of species diversity relates to the number of individuals within each species, or the relative abundance of individuals within a given community. If one accepts the theory that when species richness is high that higher levels of energy transfer are possible, then the idea that if within each species the number of individuals is also high then higher levels of energy transfer within the community should be higher still.
Relative abundance calculations show the percentage of individuals within each species present in a community and how that species relates numerically to the abundance of any other species present in that community. Calculations of this type are valuable to ecologists because they reveal ecological patterns that indicate which species is dominant or least dominant, or if there is an even distribution of individuals within the community. The following is an example of relative abundance calculations from theoretical data.
|
Species |
Number of Individuals per Species (Ti) |
PERCENT TOTAL Ti/T*100=(A) |
DOMINANCE RANK |
|
Species A |
5 |
5/76*100=6.58% |
4 |
|
Species B |
10 |
10/76*100=13.16% |
3 |
|
Species C |
5 |
5/76*100=6.58% |
4 |
|
Species D |
30 |
30/76*100=39.47% |
1 |
|
Species E |
20 |
20/76*100=26.32% |
2 |
|
Species F |
5 |
5/76*100=6.58% |
4 |
|
Species G |
1 |
1/76*100=1.32% |
5 |
|
TOTAL(T) |
76 |
100% |
COMMUNITY EVENNESS
The third parameter contained within the theory of species diversity relates to the evenness of the number of individuals within each species or its community evenness. Communities that exhibit more even numbers of individuals within the total number of species present are thought to be closer to a state of equilibrium than those in which the numbers of individuals is less even. Because the energy flow within ecological systems is constantly changing, consistent patterns of evenness within a given community can be equated with community stability. Communities that historically exhibit good community stability can, over time, become less stable through the introduction of natural/unnatural alterations to the system and/or introductions of highly competitive species.
SPECIES DIVERSITY INDICES
A large number of species diversity indices have been proposed, and many are in contemporary use. It has been observed that there are two basic groups of species diversity indices: those that are affected most by the occurrence of rare species in the community and those that are most sensitive to the relative abundance of the species within the community. The first group is best utilized when observations of slight changes in community interactions within rare species are desired. This first group is also most highly affected by variations in sample size. Measures in the second group tend to be more accurate for examining the affects of one or more changing parameters on the study community as a whole.
The Shannon and Simpson indices are the two most widely used species diversity indices for examining overall community characteristics. Both are derived from a function used in the field of information (mainly insurance companies) and have been adapted by ecologists to describe the average degree of uncertainty of predicting the species of an individual picked at random from the community. The uncertainty of occurrence increases both as the number of species increases and as the individuals are distributed more and more evenly among the species already present.
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