A community is a group of populations who live in the same territory and who interact together.
Community dynamics, therefore, studies the interactions between various populations of different species living in the same habitat.
A habitat provides the populations who live there with the resources necessary for their survival (food, shelter, etc.). Each of the populations that make up the community occupies a different ecological niche. In this way, the different species interact and several types of relationships are possible. On the other hand, the environment is usually stable; however, it can be modified by certain events called disturbances, which can stem from human or natural causes.
The biodiversity of a community refers to the variety of species, that is, the number of different species making up a community, whether they are plants, bacteria, fungi, algae, or animals.
Biodiversity can be assessed using two criteria:
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Species richness: this is the total number of species present in the community.
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Relative abundance: this is rather the number of individuals of a species in relation to the total number of individuals making up the community. This information is always given as a percentage.
For the biodiversity of a community to be high, the species richness must be high and the relative abundance of species must be similar.
Thus, in the presence of two communities with the same species richness, we must look at the relative abundance percentages to determine which has the greatest biodiversity. The community where the relative abundance percentages are close together is the one with the greatest biodiversity.
Let us compare the following two communities by calculating their species richness and relative abundance.
In the first case, it can be said that the species richness is 3 species. As for the relative abundance of each species, the ratio of the number of individuals of each species to the total number of individuals, all species combined, is to be determined.
Total number of individuals: 7 + 4 + 3 = 14 individuals
Species A = 7 / 14 * 100 = 50%
Species B = 4 / 14 * 100 = 28.6%
Species C = 3 / 14 * 100 = 21.4%
Let's move on to the second community.
In this case, we can see that the species richness is the same as for the first community since it is also composed of 3 species. Now, let us look at the relative abundance of species.
Total number of individuals: 4 + 8 + 5 = 17 individuals
Species A = 4 / 17 * 100 = 23.5%
Species B = 8 / 17 * 100 = 47.1%
Species C = 5 / 17 * 100 = 29.4%
We can, therefore, conclude that even if the species richness is the same for the two communities, the relative abundance of each species differs from one community to another.