Importance of Ecological Succession

Process

The Earth was not always a planet full of life. In all the ecosystems we see today, full of diversity, at some point in geological history, there was nothing. Only barren land and rocks. Once upon a time, there was a wave of “pioneer” organisms that colonized those wastelands. From that moment on, an ecological process known as ecological succession begins; it is responsible for establishing, or restoring, the conditions for other organisms to live there.

We are very used to thinking of human interventions as agents of ecosystem degradation, but a natural catastrophe, such as a fire, a flood; or even a climate change such as a glaciation can also “reset” the biological community of the site to zero and return the ecosystem to an initial state, where there is almost nothing alive left. These external events are known as disturbances .

The final result of succession, which is a mature ecosystem , is known in ecology as climax and is the moment when the biological community reaches its maximum complexity and stability. Climax ecosystems are fully functional , ensuring the provision of ecosystem services.

This state of equilibrium can persist for long periods if there are no significant disturbances.

Types of ecological succession

There are two main types of ecological succession: primary and secondary .

Primary succession

Primary succession occurs in areas where there was previously no established ecosystem and is the process of colonization of new areas by living beings .

Newly formed volcanic islands are good examples of primary succession. So is the bare ground left behind by melting glaciers or the ground that emerges after a drop in sea level.

The Galapagos Islands are an archipelago of volcanic islands located off the coast of Ecuador in South America.

These islands were formed about 5 million years ago, with no life present. Geologically, they are very young islands, as 5 million in geological time is almost the blink of an eye.

After the islands formed, organisms from the mainland began to colonize them, starting with lichens and mosses . Pioneer plants such as grasses and shrubs later became established. Over time, birds and other animals arrived on the islands, carrying seeds with them and contributing to the diversification of flora and fauna.

This process of complexification of the community of living beings, or biota , continued throughout this time, giving rise to the formation of the current ecosystems of the islands, which are unique and therefore legally protected.

Secondary succession

Secondary succession occurs in areas where there was a previous ecosystem but which has suffered such a great disturbance that it has been completely degraded or destroyed.

Unlike primary succession, where very slow soil formation processes must occur, here the soil and other resources necessary for plant growth are still present ; but the community has been destroyed or severely decimated .

The result of secondary succession is a new climax state, which is not necessarily the same as the first . This is an important point, because it must be kept in mind that a secondary succession will lead the ecosystem to a new state of equilibrium or climax, and that state may or may not be similar to the one that existed before the disturbance.

For example, in the Amazon rainforests, after deforestation caused by agricultural or logging activities or after a fire, the forest is often replaced by a savannah, which is a landscape dominated by grasslands and shrubs, which form a secondary forest.

However, over time, other plant species begin to establish themselves, as their seeds are dispersed by wind or animals from adjacent areas of original forest. The secondary forest becomes increasingly similar to the original primary forest (rainforest) but is never identical : there may be species that have been lost forever, or new species may appear; the disturbance, having erased everything that had come before, has created new environments.

This secondary forest is an ecosystem as valuable as the rainforest itself, and is a climax state. Some areas may remain secondary forests forever (or at least forever on the human time scale).

The pioneers

The different phases of succession , both primary and secondary, are characterized by similar biological communities.

At each stage, different groups of organisms play a key role.

During the early stages of primary succession, lichens and fungi are the pioneers . They are able to colonize bare rocks and begin the soil-forming process, preparing the ground for other organisms.

Next to arrive are usually mosses , which settle in the cavities of the rocks made by lichens and where there is a thin layer of soil. As organic matter accumulates and the soil becomes richer, herbaceous plants such as Poaceae (the grass family) become established. Shrubs and trees follow. Animals become established as soon as there is enough food, and they usually do so after the first plants, but this is not a general rule.

In secondary succession, the difference is that you don’t start from scratch: the soil is already ready to welcome new life, and in these areas succession usually begins with grasses. The rest of the sequence remains similar.