Development and change of ecosystems

Topic 2

Community ecology

1. Main questions of the theory

Biocenosis is a historically established, stable system, consisting of biological components, occupying a habitat area.

Biogeocenosis is a stable system consisting of biocenosis and inorganic components . (V.N. Sukachev.)

An ecosystem is a functional system that includes communities of living organisms and their habitat . (A. Tensley.)

Connections between components arise on the basis of food connections and ways of obtaining energy.

In a biogeocenosis , it is necessary to have a plant community (phytocenosis) as the main link, which ensures the potential immortality of the biogeocenosis due to the energy produced by plants. Ecosystems may not contain phytocenosis. The ecosystem has no rank and dimension. It is applicable to both simple, artificial (aquarium, reservoir) and complex systems (biogeocenosis).

The main properties of the ecosystem:

1. Circulation of matter and energy.

2. Stability, stability, self-regulation.

3. Biological products.

The structure of the biocenosis

1. Species structure – the diversity of species, the ratio of their abundance and biomass.

Dominant species are the dominant species.

Edificators are environment-forming species.

Ashby’s law : the higher the species diversity, the more stable the biocenosis is.

2. Spatial structure . It is determined by phytocenosis, which is characterized by layering.

Vertical zonality – tiered distribution of plants by ↑, due to their requirements for the amount of light.

(Video clip “Tiers of forest biocenosis”.)

Light-loving 1st tier: oak, linden, elm. 2 tier: mountain ash, apple tree.
Shade-loving 3rd tier: hazel, buckthorn.
shade-tolerant 4 tier: nettle. 5th tier: sedge.

Animals are also confined to a specific plant layer:

pheasant, black grouse – earth;

thrush, bullfinch – shrub;

chaffinch, goldfinch – crown of trees.

Ground layering is determined by the root system (aeration, mineral nutrition, water supply).

Horizontal layering – the horizontal distribution of various biocenoses of all natural zones (tundra, forests, steppes, meadows, semi-deserts, deserts).

It is determined by the conditions of the habitat (illumination, t, soil composition).

3. Trophic structure – the relationship between the components of the eco-system, arising on the basis of food relations and ways of obtaining energy.

The trophic structure is represented by trophic chains and trophic networks . One link in the food chain is the trophic level .

(Video clip “Chains and power networks”.)

Food links go in the direction of:

The arrows indicate the energy flow at the trophic levels.

Food chains

Pasture (eating) Detrital (decompositions)
characteristic of herbaceous ecosystems. characteristic of ecosystems with a predominance of dead organic matter.
* plants → grasshopper → lizard → hawk; * plants → grasshopper → frog → snake → eagle; *phytoplankton → zooplankton → peaceful fish → predatory fish → birds of prey. *detritus → dung-eating insects → small carnivores; *leaf litter → earthworm → blackbird → hawk.

Only 10% of the energy passes into the body of the consumer organism along with the increasing mass. This is the 10% rule (energy transfer rule in power circuits ).

A food web is a collection of several interconnected food chains, each of which is a separate channel through which matter and energy are transferred.

The rule of the ecological pyramid is a pattern that reflects the progressive decrease in mass, energy or individuals of each subsequent link in the food chain.

(Video clip “Rule of the Ecological Pyramid”)

Indicators of biogeocenosis

1. Species diversity (number of species forming a biogeocenosis).

2. Density of species populations (number of individuals per unit S or V).

3. Number.

4. Biomass – the total amount of living organic matter in an ecosystem, expressed in units of mass, accumulated during the existence of an ecosystem.

5. Productivity – the formation of products per unit of time per unit S or V.

Primary Secondary
the total total production of photosynthesis (production of plants). products (biomass) accumulated by consumers and decomposers.

Properties of biogeocenoses

1. Stability – the ability to withstand changes created by external influences.

Factors that increase the stability of the biocenosis:

– species diversity (Ashby’s law);

– branched trophic chains and networks;

– a balanced cycle of substances.

2. Self-regulation – maintaining a certain number of populations at all levels of food chains.

Development and change of ecosystems

Any biogeocenosis develops and evolves.

(Video clip “Change of natural communities”.)

Succession is a change of biogeocenoses (ecosystems), a directed and continuous sequence of the appearance and disappearance of populations of different species in a given biotope (habitat).

Primary Secondary
– the formation of biogeocenosis on an initially lifeless substrate (sand quarry, lava). – self-restoration of some stable biogeocenoses after disturbance or their destruction (deforestation, forest fire).
1. Population by living organisms. 2. Increase in species diversity. 3. Increase in soil fertility. 4. Reducing the number of ecological niches. 5. Gradual formation of more complex ecosystems. It begins at the site of disturbed or destroyed ecosystems from intermediate stages against the background of richer soils and proceeds much faster. (Video clip “Example of secondary succession.”)

The succession series ends with little changing CLIMAX ecosystems.

Causes of successions

1. The existing ecosystem creates unfavorable conditions for the organisms that fill it (changes in the environment by the living organisms themselves) due to soil fatigue, incomplete circulation of substances, self-poisoning by decomposition products.

2. Influence of climatic factors, global catastrophes (fires, volcanoes, floods), anthropogenic factors (deforestation, plowing of steppes, drainage of swamps, recreation, chemical pollution, grazing, fires).

Digression is the simplification of ecosystems under anthropogenic influence.

Pasture ↓ Recreational ↓
catocenosis ↓
complete collapse of the ecosystem.

General patterns of the successional process

1. At the initial stages, the species diversity is insignificant, productivity and biomass are low. These figures increase as the succession develops.

2. With the development of the successional series, the interrelationships between organisms increase, food chains and networks become more complicated, and the habitat is more fully mastered.

3. The number of free ecological niches is decreasing.

4. The processes of the circulation of substances and the flow of energy are intensified.

5. The rate of succession depends on the life span of the organisms in the ecosystem. (The longer, the slower the succession, the largest is the forest compared to herbaceous communities and aquatic ecosystems).


Agrocenosis is a community of living organisms created to obtain agricultural products and regularly supported by humans.

Agrocenoses, like biogeocenoses, have the following characteristics:

– have a certain species composition: cultivated plants, weeds, pests of agricultural plants, earthworms, etc.;

– relationships develop at the level of trophic relationships, but under the influence of a person: tillage, sowing, etc.;

– food chain: producers (weeds, wheat) → consumers (insects, birds, voles) → decomposers (fungi, bacteria).

Differences between agrocenosis and biogeocenosis

signs Agrocenosis Biogeocenosis
1. Energy source. Solar E and man (mineral nutrition). solar E.
2. Species diversity. Few species, monoculture. Many different types.
3. Regulation. Man. Self-regulation.
4. Selection. Artificial. Natural.
5. Circulation of substances. Incomplete (non-closed) – part of the substances is seized by a person. Closed (balanced).
6. Sustainability. Unstable. Stable.
7. Productivity. High. Low.

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