BIOCENOSIS: CHARACTERISTICS, COMPONENTS, TYPES AND EXAMPLES - BIOLOGY - 2025

A biocenosis, biological community, or ecological community is a grouping of populations of organisms that live in a common area. In this definition, we have implicitly that the area must be delimited in order to define the community. In most cases, the delimitation is purely arbitrary.

Communities are characterized by being extremely complex, since there is variation at each level of organization (individual, population, species, etc). This is in addition to the fact that individuals interact in multiple ways, including competition, mutualism, predation, or commensalism, among others.

Source: Key45

Furthermore, delimiting a community is (for some authors) a controversial issue, since the existence of the community as a biological unit is questioned.

The branch of biology that aims to study the community as a level of organization is called community ecology, seeking to specify aspects such as the composition and diversity of species in them. This includes the study of two or more species located in the same area, evaluating the interactions and competition.

Ecologists try to understand the functioning of communities, to deduce how to manage them and thus preserve biodiversity.

History

At the beginning of the 20th century there was an important debate related to the nature of communities.

At the time, there were two extreme and opposite visions: one of them considered communities as a super-organism, where the individuals that compose them established very deep relationships between them.

The relationship was believed to be so extreme that communities could be classified just as organisms are classified: using Linnaean taxonomy.

The opposite view was totally individualistic, arguing that each organism had specific traits that allowed it to inhabit a certain area.

Following this idea, a community was made up of a set of species that had similar traits or characters, and therefore coexisted in the same region. The proponent of this idea was HA Gleason. Currently, what is closest to the modern vision are the ideas of this author.

characteristics

In the area of ​​biology, a community is defined as a set of two or more populations that interact in a defined area. They are highly dynamic entities, which come in different sizes and with varying levels of interaction.

Populations are groups of organisms that belong to the same species, and different populations can be found in communities. Thus, in each of these environments we will find both animals, plants and microorganisms.

We will now describe the most salient aspects of a biological community, in terms of its structure and the multiple and complex relationships that occur within it.

Structure and components

There are four fundamental parameters that are used by biologists to describe the structure of a community. These are: their species richness, the interactions between them, the diversity of species, and their physical attributes.

Species richness

The first of these is the easiest to quantify, and consists of counting the number of species that exist within the community that you want to study.

The more species the community has, the richer it is. Generally, the richest communities are located in regions near the equator.

This high richness is probably due to the large amount of solar radiation (thus increasing the productivity of photosynthetic organisms), the high temperature, the few variations in temperature and the large amounts of rainfall in these areas.

In contrast, as we get closer to the poles, the species richness decreases, as the environment is considered less conducive to the development and establishment of life.

Interactions

The second factor is the sum of the interactions that exist between each of the species that make up the community. Peer interactions generally begin to be studied and then a network is formed. These networks can contain all the types of interactions that we will discuss later.

Species diversity

The diversity parameter is determined by relative abundance (how uniform the species are in terms of their characteristics) and by the number of species present in the community.

It has been proposed that the stability of the community is proportionally related to the diversity that we find in it. However, it is considered that this rule does not always apply.

Mathematically, there is a series of indices that allow quantifying the diversity of the species of a biocenosis. Among the most famous and most used in literature we have the Simpson index and the Shannon-Wiener index.

Physical attributes

Finally we have the physical attributes of the community, including biotic and abiotic factors.

As the community structure increases its complexity (either due to a high number of species or interactions between them), computer programs can be implemented to characterize it.

Are all the species in a community equally important?

Ecologically, not all species within a community have the same weight or importance.

Some are of much greater importance, disproportionately influencing the abundance and diversity of the rest of the species. These are called keystone species.

A famous experiment in community ecology was carried out using the sea star belonging to the species Pisaster ochraceus as the study organism. By removing the star from its natural community, the species of mussel it consumed began to increase disproportionately.

The mussel negatively affected a significant number of species, reducing the species richness of the community. For this reason, P. ochraceus is considered a key species in this biocenosis.

On the European continent, bats from the pteropod family are also key species, since they are responsible for pollination and seed dispersal of a significant number of plants.

Types

There are two main types of communities: major and minor. A larger community is defined as a community large enough in size to sustain and regulate itself independently. For example, the communities that we find in a pond or in a forest.

The larger communities are in turn made up of smaller communities, also known as societies. These are much smaller, in terms and size, and are not able to support themselves, as they depend on neighboring communities.

Relationships between individuals in a biocoenosis and examples

In communities, there are multiple ways in which its members can interact, which happens constantly. Many times the fate of a population is directly linked to its interaction with another group of species, either by exchanging nutrients, through competition, or by providing habitats for its companion.

Biologists classify interactions depending on the effect of the fitness of one species on the other and vice versa. Fitness, or biological attitude, is defined as the ability of an individual to produce viable and fertile offspring.

Commensalism

In commensalism, one species benefits (that is, has a positive effect on the fitness of the population) from the interaction, while the other species involved is not affected. In practice, a commensal relationship is extremely difficult to test, as few relationships translate into zero fitness change.

This type of relationship is found in plants called epiphytes. These organisms are located on the branches of some tall trees to receive sunlight, obtaining a direct benefit. The tree is not affected by the presence of the plant.

The relationship will continue as a "commensal" as long as the number of epiphytes is not extremely high. If the number increases in significant amounts that block sunlight to the tree, both species will begin to compete.

Competition

When two species pursue a common resource, which for some reason is limited, they will compete to acquire it. In ecology, it is known that two species cannot compete indefinitely: one will end up displacing the other. This is known as the competitive exclusion principle.

The other possible scenario for the two species to be in sympathy is that one of the two modifies a characteristic that reduces competition.

For example, if two species of birds use the same resource (say, a particular seed) they will compete for food. If both species are ecologically very similar, they must be separated along some axis of the niche in order to sustain coexistence.

As competition has negative consequences on the fitness of the species, natural selection will act strongly to avoid it. This evolutionary change in the use of resources caused by the existence of competition for several generations is called niche differentiation.

The reduction in fitness is not always of the same magnitude for competitors. If any of the species is superior, its fitness will decrease to a lesser extent than that of its companion.

Consumption

The consumption of one species by another can take the form of herbivory, predation or parasitism. In all these scenarios, the organism that acquires or absorbs the nutrients receives a benefit in its fitness, while the species that is consumed or that acts as a host is negatively affected.

Evolutionarily, the existence of these antagonistic relationships between species can lead to several scenarios. The first of them, and more intuitive, is that one of the species ends up extinguishing its prey or its host.

Evolutionary consequences: arms races

Second, mutual selective pressures translate into the emergence of new, better “weapons” in each of the species, generating an arms race. In it, each species involved in the interaction increases the effectiveness of their weapons.

For example, plants develop chemical defense mechanisms against herbivores, and these develop detoxification mechanisms. When a new toxin emerges in a plant population, consumers (in the event of an arms race) will improve their detoxification strategies.

The same is true of the relationships between predators and their prey: each time one improves their skills in movement, the counterpart also improves it.

Consumer applications

By knowing the network of interactions of a particular community, you can get the most out of that information. For example, when you want to eliminate a pest (from a crop or an area), the natural consumer of the pest can be introduced to eliminate it without the application of toxic chemicals to the ecosystem.

This modality of pest control is called biocontroller agents and has proven to be quite effective in the regions where it has been implemented.

Mutualism

The last type of interaction occurs when the two species involved gain fitness benefits.

The classic example is the relationship between plants and their pollinating agents. The former obtain an energy reward and the plants manage to disperse their gametes. Pollinators can be insects, birds, or bats.

Another example of mutualism occurs between nitrogen-fixing bacteria and the plants in which these bacteria grow. The plant that takes the role of host provides protection and nutrients (such as sugars) to the bacteria, and this provides the ammonium or nitrate it needs.

Historically, this type of relationship was called symbiosis where both species obtained a benefit from these together. Today the term symbiosis has a much broader meaning and is used to describe the close relationship between two species.

Not an example of altruism

Finally, it is important to note that in mutualistic relationships we do not find two species being altruistic with each other. During the interaction, each species tries to keep benefits to a maximum and costs to a minimum.

Therefore, when it comes to mutualistic relationships, it is typical to observe the development of characteristics that seek to deceive their partner.

For example, certain species of flowers produce bright, colorful structures that attract pollinators but do not contain nectar. There are multiple examples of complex structures - some even manage to simulate the shape of a female insect so that the male tries to copulate with the flower.

In the same way, some animals steal nectar from flowers and do not perform the pollination service, since they open a hole in the flower and do not come in contact with pollen.

References

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