RELATIVE ABUNDANCE: WHAT IT IS AND HOW IT IS STUDIED - BIOLOGY - 2025

The relative abundance in community ecology is a component of the diversity that is responsible for measuring how common - or rare - it is a species compared to other species that are part of the community. In macroecology, it is one of the best defined and most studied parameters.

Seen from another point of view, it is the percentage that a certain species represents with respect to other organisms in the area. Knowing the abundance of each of the species in the community can be very useful to understand how the community works.

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Collecting data on the abundance of species is relatively easy, compared to other ecological parameters, such as competition or predation.

There are several ways to quantify it, the first and most intuitive would be to count the number of animals, the second is according to the number of organisms found per unit area (absolute density) or finally as the density of the population, related to another - or with itself in another time (relative density).

For example, if we observe that two species coexist in various places, but never do so at high densities, we can speculate that both species compete for the same resources.

Knowledge of this phenomenon will allow us to formulate hypotheses about the possible niche of each of the species involved in the process.

How are communities studied?

The study of communities - a set of organisms of different species that coexist in time and space - is a branch of ecology that seeks to understand, identify and describe the structure of the community.

In community ecology, comparisons between these systems can be made using attributes or parameters such as species richness, species diversity, and uniformity.

Species richness is defined as the number of species found in the community. However, species diversity is a much more complex parameter and involves measuring the number of species and their abundance. It is generally expressed as an index, like the Shannon index.

Uniformity, on the other hand, expresses the distribution of abundance across species in the community.

This parameter reaches its maximum when all the species in a sample have the same abundance, while it approaches zero when the relative abundance of the species is variable. Likewise, as in the case of species diversity, an index is used to measure it.

General patterns of distribution and abundance

In communities we can assess the distribution patterns of organisms. For example, we call a typical pattern to two species that are never found together, living in the same place. When we find A, B is absent and vice versa.

One possible explanation is that both share a significant number of resources, which leads to a niche overlap and one ends up excluding the other. Alternatively, the tolerance ranges of the species may not overlap.

Although some patterns are easy to explain - at least in theory. However, it has been very difficult to propose general rules about the interactions and abundances of communities.

Species abundance patterns

One of the patterns that has been described is that few species always make up the majority of species - and this is called the species abundance distribution.

In almost all the communities studied where species have been counted and identified, there are many rare species and only a few common species.

Although this pattern has been identified in a significant number of empirical studies, it appears with greater accentuation in some ecosystems than in others, as in marshes, for example. In contrast, in the swamps the pattern is not as intense.

How is abundance studied?

The most parsimonious way to examine the number of species in a community is by constructing a frequency distribution.

As mentioned, the patterns of abundance in a community are somewhat predictive: most species have intermediate abundances, a few are extremely common, and a few are extremely rare.

Thus, the shape of the distribution that fits the predictive model increases with the number of samples taken. The distribution of abundance in the communities is described as a logarithmic curve.

Graphs to study relative abundance

Generally, relative abundance is plotted on a histogram called a Preston plot. In this case, the logarithm of the abundances is plotted on the x-axis and the number of species at that abundance is plotted on the y-axis.

Preston's theory allows to calculate the true richness of species in a community, using the log normal distribution of the community.

Another way to visualize the parameter is by making a Whittaker graph. In this case, the list of species is ordered in descending order and plotted on the x-axis, and the log of% relative abundance is located on the y-axis.

Comparisons between communities

Making comparisons of community attributes is not as straightforward as it appears to be. The result obtained when we evaluate the number of species in a community may depend on the amount of species collected in the sample.

Similarly, comparing abundances within a community is not a trivial task. In some communities there could be completely different patterns, making it difficult to match the parameter. Therefore, alternative tools for comparison have been proposed.

One of these methods is the elaboration of a graph known as the "species abundance curve", where the number of species is plotted against abundance, eliminating the problems of comparing communities that differ in complexity.

In addition, the diversity of the species tends to increase in proportion to the heterogeneity of the habitat. Thus, the communities that present a significant variation, have a greater number of available niches.

In addition to this, the number of niches also varies depending on the type of organism, a niche for an animal species is not the same as for a plant species, for example.

References

1. Cleland, EE (2011) Biodiversity and Ecosystem Stability. Nature Education Knowledge 3 (10): 14.
2. González, AR (2006). Ecology: Methods of sampling and analysis of populations and communities. Pontifical Javeriana University.
3. May, R., & McLean, AR (Eds.). (2007). Theoretical ecology: principles and applications. Oxford University Press on Demand.
4. Pyron, M. (2010) Characterizing Communities. Nature Education Knowledge 3 (10): 39.
5. Smith, RL (1980). Ecology and field biology. Addison Wesley Longman
6. Verberk, W. (2011) Explaining General Patterns in Species Abundance and Distributions. Nature Education Knowledge 3 (10): 38.