Ecological succession: definition, stages and examples

The ecological succession is a natural process that takes a sequence of changes in the ecological community that are observable in time and space. It is due to local colonizations and extinctions of species.

During ecological succession, the level of complexity of ecosystems evolves. The first stages represent communities with simple trophic chains and little biodiversity, but which over time are transformed into complex ecosystems where there are more interactions and a greater richness and diversity of living beings.

The ecosystem formed at the end of the succession is called the climax or climax community. This stage represents the maturity of the community, that is, when it remains stable and well developed for many years. In relation to these two concepts, climax and maturity, a certain controversy has arisen since there are immature communities (in primary stages) in climax, that is, without changes, such as the vegetation of the dunes or the deserts.

Depending on the starting point of the ecological community , ecologists differentiate two types of ecological succession :

• Primary succession.
• Secondary succession.

The primary succession is one that takes place in a virgin biotype , ie an area lacking a preexisting community, as new volcanic islands, glacier retreat, occurs in the dunes etc. It is rare.

The process begins with the formation of the soil, understood as the biologically active layer of the earth’s surface that contains roots, microorganisms, invertebrate communities and nutrients. Normally, primary succession occurs in areas where the bedrock is exposed to the surface, but it cannot be directly colonized. It first suffers the action of erosion and weathering, a set of physical and chemical processes that fragment, degrade and dissolve the rock. In this way, the colonization of the soil by lichens and mosses begins., as they can grow in substrates with little depth and little amount of organic matter. The biological action of these organisms contributes to the degradation of the rock and the release of nutrients, they also provide organic matter when they die. Thus, the arrival of other species is favored, such as annual plants, herbs that have a year of life, and later the perennial annuals that last longer. If the community continues to evolve, the seeds of shrub species could germinate forming thickets and eventually trees would grow, giving rise to forests.

The same happens with animals, because the first to settle will be insects and other invertebrates and small reptiles that can hide among the rocks and feed on invertebrates. Small birds could also arrive whose diet is based on seeds, or even invertebrates and reptiles. As the plant community becomes more complex, small mammals appear, such as rodents and other birds. Finally, large mammals and other predators would reach the community, as the ecosystem will provide them with enough food and a den.

Secondary succession appears when there has been a regressive succession in the ecosystem . This means that the existing ecosystem has been destroyed and, therefore, the process of ecological succession must begin . This ecosystem has been eliminated by fires , floods, diseases, logging , crops, etc.

The secondary succession occurs in places whose characteristics depend on the previous communities or previous disturbance conditions. The residues or legacies are those organisms, sometimes alive, that come from the previous community. The greater the amount of waste, the greater the speed of succession or recovery.

The presence and abundance of residues buffer the difference between the conditions before and after the disturbance. They function as a source of new species , increase the heterogeneity of the environment and also reduce the loss of soil and nutrients.

Ecologists have long studied how ecological succession occurs and have found a number of mechanisms that favor the succession of species in communities.

One of these mechanisms is facilitation , whereby species with a high colonization capacity favor the arrival and survival of species that appear in the later stages of succession. It is especially important in primary succession . If stress increases in the physical environment, facilitation increases. Although, if the environmental conditions become extreme, competition is imposed against facilitation, that is, the species become selfish and fight for the available resources to survive.

Ecological succession occurs thanks to pioneer species , those that reach inhospitable places first. They have a great capacity for colonization, but a low capacity to compete when resources are scarce. Some pioneer species are the lichens and mosses mentioned earlier.

In addition, pioneer plant species have tissues with high concentrations of nitrogen and phosphorus, making them the preferred food of herbivores. In this way, ecological succession is accelerated, as these plants will suffer high mortality and will be replaced by later-stage plants.

These are some clear examples of ecological succession .

The dunes of Lake Michigan

After the end of the last ice age, the glaciers that covered the Great Lakes gradually retreated, revealing large dunes. These dunes are large sand formations that accumulate on the shores of lakes.

Over the years, plant species have followed one another. First, drought-tolerant species were established that also allowed the dunes to be fixed, preventing the wind from eroding and transporting them. After a score of years, grasses appeared, shrubs such as sands’ cherry trees and trees such as willows and poplars, which continued to stabilize the substrate. After another 50 or 100 years, the pine forests began to grow rapidly, which were finally replaced by oak groves, and could be maintained for millennia.

Currently, this sequence of plants can be observed, since the lakes have a lower level and the process continues over time.

The clogging of a lake

An oligotrophic lake (with little amount of nutrients) begins to receive nutrients and sediment by the streams and rivers that flow into it. Thanks to the increase in nutrients, the algae begin to proliferate. If the nutrients increase, floating aquatic plants appear and others begin to take root. The death and decomposition of the organisms cause remains to accumulate at the bottom of the lake and the peat is created, at the same time it loses depth. In this way the lake begins to transform into a swamp. The soil becomes acidic and typical plants from the banks such as reeds begin to proliferate. Terrestrial animals such as earthworms and some insects can appear. Trees that tolerate high humidity such as alders or birches grow on top of the peat bogs. Over time they will be replaced by other trees that will form a more mature forest. The fauna of the place also evolves, amphibians disappearing due to the lack of humidity and birds and typical mammals of the forests appear. In the case of a lot of peat, a lot of moss would grow that would acidify the soils so much that the trees would die.

This process has originated the current peatlands of the planet after the glaciations and could happen within thousands of years in lakes such as Geneva or Leman, the largest lake in Western Europe and located in the Alps, and in Lake Constance, which borders with Germany, Switzerland and Austria.

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