WHAT IS ECOLOGICAL DENSITY? - BIOLOGY - 2024

The ecological density is the number of individuals per unit habitat. This is an important characteristic in the study of populations.

In addition to ecological density, there is also the so-called raw density, which is defined as the number of individuals per unit of total area (or space).

Figure 1. Ecological density Vs. raw density of a fish population. Kahl's experiment (1964). Image edited by: Katherine Briceño

It is important to recognize the subtle difference between the two estimates of population density.

While in the raw density the area (or volume) is defined arbitrarily, in the ecological density the area (or volume) that can actually be colonized by the population in question is considered, that is, its habitat.

For this reason, raw densities tend to be consistently lower than ecological densities.

Ecological density Vs. raw density

In nature, organisms generally associate in groups and are rarely evenly distributed in a given environment.

For example, in plant species such as Cassia tora or Oplismemis burmanni, the organisms are more clustered in some areas, forming patches in certain areas, while in other areas these associations are not found.

In cases like this, the density calculated considering the total area or volume would be the raw density, instead the density that considers only the area where the plants actually grow, would be the ecological density.

Other examples of ecological density

We can find that in an oak forest, the raw density of black oak is 200 trees per hectare. This measure is obtained by sampling at various locations in the forest, regardless of whether the site is a typical forest site or a lake area.

Since raw density measures the number of organisms per unit area or space, then if you want to know the population density of black oak in those areas where the species normally inhabits, you would measure the number or biomass of black oak trees by unit area only in those areas.

Therefore, other spaces or areas where oak does not live should be excluded, that would be lakes and river beds for example.

Thus, the figure in numbers of black oaks per hectare (of usable space) would be a somewhat higher number, corresponding to their ecological density.

Kahl's experiment

Kahl's (1964) experiment is a very useful example to distinguish between raw density and ecological density. The study was based on the density of fish in a variable environment.

Figure 1 shows that the raw density of small fish in the area generally decreases as the water level decreases during the dry winter season.

However, the ecological density increases, since in the dry season the water masses are reduced to puddles where fish accumulate while the habitat is reduced more and more.

Therefore, with the passage of time and the variation of the estimated area, the two densities (ecological and raw) are different.

Population density can remain constant, it can fluctuate, or it can constantly increase or decrease. Density is the result of the dynamic interaction between processes that add individuals to a population and those that remove individuals from it.

Additions to a population occur through birth (natality) and immigration. The factors that eliminate individuals from a population are death (mortality) and emigration.

Immigration and emigration can represent biologically significant exchanges between populations.

Factors to consider

The methodology for estimating population densities is very varied and depends on the type of organism and the habitat in question.

There are a wide variety of methods available that must be carefully evaluated before use. In some cases, various methods are adopted to provide comparative data.

It is recommended that before attempting to determine the densities of a population in the field, the specialized works on the methodology for each type of organism of interest be taken as a reference.

References

1. Gaston, K. (2012). Rarity Vol 13 of Population and Community Biology Series. Illustrated ed. Springer Science & Business Media.
2. Osborne, P. (2012). Tropical Ecosystems and Ecological Concepts. 2nd ed. Cambridge University Press.
3. Sharma, P. (2005). Ecology and Environment. Rastogi Publications.
4. Sharma, P. (2014). Environmental Biology and Toxicology. Rastogi Publications.
5. Sridhara, S. (2016). Vertebrate pests in Agriculture. Scientific Publishers.
6. Ward, D. (2012). Biological Environmental Impact Studies: Theory and Methods. Elsevier.