We explain what ecological succession is, its relationship with evolution and examples. Also, primary and secondary succession.
What is ecological succession?
ecological succession is the gradual change in the structure of a community over time. It can occur in an aquatic or terrestrial environment and always begins with a disturbance that modifies the availability of resources or creates opportunities for new individuals.
This natural process is driven by the colonization and replacement of species that occurs in a given place. Over time, This dynamic causes community composition and species richness to vary.
Generally, ecological successions begin with a simple community that becomes more complex over time until reaching a climax community. This mature community is usually made up of species more adapted to the specific conditions of the environment (specialists), compared to the initial species (generalists).
However, the climax community is not the one with the greatest diversity. When species arrive and colonize a place, they increase its diversity. But when these species replace those that were there to be better competitors or to tolerate environmental changes better, diversity decreases.
Therefore, the greatest species richness occurs in the intermediate stages of ecological succession. This can also occur when the frequency of disturbances is intermediate.
When disturbances are very frequent, the place is only inhabited by species that manage to invade, mature and reproduce before the next disturbance occurs. When they are infrequent, climax community may be reached.
Differences between ecological succession and evolutionary succession
Ecological succession and evolution are not exactly the same even though they occur over time.
Evolutionary succession is the replacement of one species by another better adapted to the dynamics of the environment. This occurs over thousands of years, which is how long it takes for a new species to appear.
On the other hand, in ecological succession, The replacement of one species by another can occur in a few hundred years. This process does not imply the emergence of new species.
Primary succession
Primary succession is that which occurs in a virgin place that is, it has not previously been occupied by living beings. It may be a recently exposed rock surface, a newly formed dune or body of water, a piece of land covered by volcanic lava, or one that was discovered by the retreat of a glacier.
In general, the erosion and the weathering of the soil prepare the ground for the arrival of the first colonizers which are usually simple organisms such as mosses and lichens. Its biological activity provides organic matter to the soil, allowing other organisms, such as grasses, to establish themselves.
The transformation of the physical environment continues as different forms of life succeed one another. Grasses are followed by other perennials and shrubs, and later trees are established.
These changes in the structure of the community influence, for example, the nutrients and humidity in the soil, the light that enters the environment, the shelters that are provided for fauna, and the availability of food.
Therefore, the successive change in vegetation also influences the fauna that can colonize the new environment. Bacteria, fungi and invertebrates are the first to establish themselves; Then come the small vertebrate animals, such as rodents and reptiles, and later the larger ones.
Secondary succession
Secondary succession is one that occurs in environments where there was a previous community that was eliminated totally or partially due to a disturbance. Fires, floods, pests, deforestation and livestock farming are examples of disturbances in natural environments.
After a disturbance, ecological succession restarts, but not from scratch as in virgin lands, but from the remains of the previous community. For example, it can give way to the growth of renewables (young trees) or the germination of seeds that were dormant.
In general, the replacement of communities occurs more quickly in a secondary succession than in a primary one.
Pioneer species and late species
In an ecological succession, the species that initially colonize an environment are usually differentiated from those that do so later.
Pioneer or early species are generally characterized by being small and for having a high dispersion capacity and a high population growth rate. This allows them to quickly colonize the available space and develop.
On the other hand, late species tend to be larger and longer-lived, with lower dispersal and growth rates (they grow slowly). This means that they need a more stable environment to develop and, many times, they require the facilitation of pioneer species.
Mechanisms of ecological succession
There are three mechanisms that explain the replacement of species that occurs throughout an ecological succession. These mechanisms are not mutually exclusive, but can occur simultaneously.
- Facilitation mechanism This model suggests that pioneer species modify the terrain and make it more suitable for the settlement of late species, that is, they facilitate their arrival.
- Inhibition mechanism This model proposes that the first species to colonize a land compete for resources with other species and make their establishment difficult. Replacement only occurs when the first species die or weaken.
- Tolerance mechanism This model proposes that the turnover of species throughout the succession only depends on how they tolerate the physical conditions and the availability of resources at each moment: the species that dominates the community at a given moment will be the one that best tolerates the conditions. of the environment.
Examples of ecological succession
Some examples of ecological successions are:
Succession on volcanic islands
After a volcanic eruption, lava, ash and other volcanic materials cover the ground and create a virgin substrate as it cools. Over there simple autotrophic organisms settle first that arrive dispersed by the wind, sea currents or birds.
As time passes, the pioneer community (made up of algae, mosses, lichens and ferns) is replaced by more complex vegetation, and the first animals (such as insects and different aquatic species) may also appear.
This occurs, for example, in the volcanic islands of the Pacific that gradually expand their size.
Succession in rocks that fall into water
In intertidal zones, it is common for rocks to break away and end up submerged. This can occur due to wave action. In the water, the surface of these rocks constitutes a virgin place ready to be colonized.
Algae are the first organisms to arrive at the site. Even within this large group of autotrophic organisms, a turnover of species occurs over time. In turn, they enable the arrival of zooplankton organisms, mollusks and other animals as the succession progresses.
Succession after a forest fire
Many fires (natural or caused by humans) destroy forests and jungles, destroy the plant community and cause the migration or death of fauna. This clears the ground for secondary succession to occur.
Many plants are pyrophilous and have the ability to grow after a fire, either because they sprout at the base (although their aerial parts have died) or because their seeds and fruits resist fire and grow on the ground covered by ashes.
At the same time, there are species that proliferate thanks to the fact that there are no longer trees to generate shade. Thus, they occupy places that would not have been possible otherwise.
Slowly, The recovery of flora and soil means that more and more animals arrive. Many forests and jungles depend on fire cycles for their renewal.
Succession on a fallen log
The fall of a trunk triggers a heterotrophic succession, that is, one that begins with decomposer organisms. In general, invertebrates that feed on the bark are the first to colonize the trunk.
These invertebrates are followed by fungi, bacteria and other detritivorous organisms that feed on the remains and continue the decomposition of organic matter. Over time, mosses and lichens colonize the softened trunk, and small animals (such as salamanders and mice) use the gaps for shelter and food.
References
- Smith, T.M., & Smith, R.L. (2007). Ecology. (6to). Pearson Education.
- Curtis, H., Barnes, N.S., Schnek, A., & Massarini, A. (2008). Biology. (7th ed.). Panamericana Medical Editorial.
- Thompson, J. N. (November 28, 2022). Ecological succession. Encyclopedia Britannica. Britannica
- of the Order, EA (2020). Ecological succession. Expected trends. University Scientific Publishing House. UNCA
- Cano Santana, Z and Meave, J. (1996). Primary succession in volcanic spills: the case of Xitle. Sciences, (41). MagazineCienciasUnam
- “Ecological succession” on Wikipedia.
- “Ecological succession” at Khan Academy.
- “Ecological Succession: Change is Good” (video) at CrashCourse.
- “Ecological succession” in The Encyclopaedia Britannica.
