Ecological successions

Ecosystem development trends

The development of any biocenosis aims to increase its stability and maximize the efficiency of available resource utilization. This process is accompanied by a series of regular changes:

1. Species diversity is continuously increasing.
2. The number of food chains increases, and they become more complex and longer.
3. The total energy flow through the system increases.
4. The total biomass of organisms increases.
5. The ratio between produced organic matter (production, $P$) and respiration costs ($R$) changes.

It is important to understand that as biomass grows, the energy costs required to maintain it inevitably increase as well. The culmination of ecosystem development is the climax community (from Lat. climax — resting stage). This is the most perfect form of biocenosis, in which production is practically equal to respiration costs ($P/R = 1$). Consequently, biomass in mature ecosystems stops increasing and remains at a relatively constant level.

Classification of successions

Successional changes are generally divided into several groups depending on starting conditions, causes, and the direction of development.

By the presence of organic matter in the environment:

• Primary (autotrophic): occur on an initially vacant substrate that has not been previously colonized by living organisms and contains no organic matter.
• Secondary (heterotrophic): represent the restoration of a biocenosis in an area where an ecosystem already existed but was destroyed (while soil and basic organic components were preserved).

By causes of occurrence:

• Autogenic (from Lat. auto — self): triggered by internal mechanisms and processes occurring within the ecosystem itself.
• Allogenic (from Lat. allos — other): provoked exclusively by external causes, including various anthropogenic impacts.

By the direction of the process:

• Progressive: develop towards structural complexity and strive for the climax phase.
• Regressive: proceed in the opposite direction, leading to simplification and degradation of the ecosystem.

Primary and secondary successions with examples

Primary succession starts under extreme conditions: on bare rocks, fresh sand dunes, or recently formed volcanic islands. First, the lifeless territory is colonized by pioneer species (bacteria, lichens, algae, fungi). Their main task is to create primary soil, which will become the basis for the introduction of more complex organisms. As soon as the substrate is ready, new species displace the pioneers. The general scheme looks as follows: Substrate → Pioneers → Mosses → Grasses → Trees. Following the plants, animals (consumers) are inevitably introduced into the system. At each stage, species richness steadily grows, but at the very final stage, it may slightly decrease due to fierce competition.

Example: Overgrowth of a standing water body (lake, pond). First, sediments accumulate at the bottom, then littoral plants take root. Soon, trees and shrubs appear, which eventually turns the aquatic ecosystem into a fully terrestrial one.

Secondary succession is a restorative process following fires, hurricanes, or human activity (e.g., logging). Since organic substances are already present in the ground, development proceeds manifold faster.

Example: Forest development on an abandoned field takes about 80–120 years. First, the territory is taken over by grasses, then birches grow, forming a mixed forest, which is eventually replaced by a stable spruce forest.

Mechanisms and Driving Forces

The replacement of one biocoenosis by another does not happen spontaneously. The driving forces of succession include abiotic factors, human activity, and internal (biotic) changes. In the course of their vital activity, living organisms inevitably transform the environment: they alter illumination levels, environmental humidity, and soil acidity. Thus, they make conditions unsuitable for themselves, yet ideal for their successors.

The entire successional series of organisms can be arranged into a clear hierarchy: pioneer species → early successional species → late successional species → climax species.

The climax communities formed at the final stage possess a unique property: they no longer produce significant changes in their habitat. This allows them to exist for an extremely long time. Only global events can dislodge such a system from its stagnant state and trigger a new species replacement: a sharp climate change, large-scale natural disasters, the introduction of aggressive alien species, or intensive anthropogenic impact.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Energy balance of biocoenoses: a detailed analysis of the production-to-respiration ratio
• Pioneer organisms and their physiological role in primary soil formation
• Mechanisms of intraspecific and interspecific competition at late stages of succession
• Impact of anthropogenic factors on the initiation of allogenic and regressive successions

Other topics in the "General Ecology" section

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