Ecology of Population – SS2 Biology Lesson Note (Third Term)

Ecological succession is the orderly, step-by-step change in the inhabitants of an area over time, as one community of organisms replaces another until a stable climax community is reached. Overcrowding happens when a population grows beyond the carrying capacity of its habitat, so that food and space cannot support every individual.

Behavioural objectives

By the end of the lesson, students should be able to:

• define ecological succession and climax community;
• describe succession in a pond;
• distinguish between primary and secondary succession;
• state the characteristics and outcome of succession;
• explain overcrowding, its causes and its effects;
• describe the adaptations that help organisms avoid overcrowding.

What is ecological succession?

The orderly change in the inhabitants of an area over time is called succession. It can also be defined as the step-by-step orderly and gradual replacement of communities of organisms that leads to a climax community.

Succession in a pond

An illustration of succession is that of a pond. Rain carries sediment from surrounding land into the pond, filling it and making it shallower. Algae that live in the pond die and eventually fall to the bottom, adding organic materials to the sediment.

Some plants such as pond weeds grow at the bottom. These plants make up the pioneer community. The pioneer plants are the first to inhabit the changing environment. The roots of these underwater plants hold much silt, quickly building up the bottom cover of the pond. As they die, their organic matter accumulates at the bottom. The water along the edges becomes so shallow that water lilies and other floating plants replace the pioneer plants.

What is a climax community?

The final stage of succession in a particular area is called a climax community. The species that constitute the climax community differ from biome to biome. A climax community is also a stable community because its appearance and species composition are stable. To become the climax, the community would have gone through a sequence of species.

Types of succession

There are two types of succession:

1. Primary succession
2. Secondary succession

Primary succession

This is a type of succession that begins from bare ground, bare rock or bare body of water.

Primary succession on land may be studied on a building site where a heap of subsoil, stones or cement block is left over after construction. In an aquatic habitat, primary succession may be observed in a new artificial pond.

The first in any succession are called primary colonisers and are usually autotrophic plants. These have simple requirements for life and can withstand exposure. By the second year of the primary succession, in addition to more algae and lichens, mosses may begin to grow. As they grow, they wear out some soil and some of them die and decay, creating more soil for their successors.

By the third year, small herbaceous plants may be present. These in turn help to change the habitat by overshadowing the smaller plants, causing them to die out and dropping their leaves, thus making the soil suitable for other organisms. As years pass, more species come into the habitat, while some fade out. A succession of species continues till the climax is reached. Then, bigger life forms like shrubs and trees are found growing.

Secondary succession

Secondary succession is a succession that occurs when an area has been stripped of its vegetation but not of its soil. It occurs more rapidly than primary succession because soil has already been formed. It also occurs when a farmer abandons an old field. Secondary succession begins from an existing community which has been interfered with by man and other factors. Fire, drought and floods can cause secondary succession.

What are the differences between primary and secondary succession?

Characteristics of succession

1. Plants form pioneer organisms, being the producers.
2. The number of organisms usually increases from year to year until a climax is reached.
3. The diversity of organism species increases from year to year.
4. Succession is orderly and progressive, starting with microscopic green plants and ending with big trees.
5. Each generation of species alters the habitat by making more soil, and when they die the soil becomes more fertile.
6. There is competition among organisms, in that the various species present compete for the available resources such as water, CO₂, O₂, light and space. The plants that are more able to compete displace others.
7. Species composition changes as the fittest survive and the unfit fade out.

Outcome of succession

1. Changes in the physical environment due to structural changes in the species and the activities in the community.
2. Simple organisms which start the succession are usually replaced by more complex ones in an evolutionary trend.
3. Equilibrium point is attained through colonisation of abandoned farmland by a wide variety of organisms.
4. The outcome of succession is the climax or stable community.

What is overcrowding?

Overcrowding is a situation in which a population increases beyond a point called the carrying capacity, where the resources (e.g. food and space) are not enough to support all the individuals in the population. Therefore, overcrowding reduces the food and space available for individual species in the population.

Factors causing overcrowding

1. Natality: An increase in the rate at which a particular species gives birth in a restricted area results in overcrowding.
2. Increase in food supply.
3. Decrease in mortality: Overcrowding results when the rate of death of organisms in a habitat is lower than the rate of birth.
4. Immigration: The inflow of individuals into a habitat increases the population, which later causes overcrowding.
5. Lack of dispersal of fruits or seeds of plants.
6. Social habits of animals like termites, ants and bees lead to their multiplication in the colony.
7. Inadequate space.

Effects of overcrowding

1. Shortage of food, since the available food in the habitat is rapidly eaten up.
2. Shortage of space due to the increasing population of species.
3. Competition, as the organisms struggle for scarce resources such as food and space. The stronger ones get the resources while the weaker ones are deprived, i.e. survival of the fittest.
4. Anti-social behaviours like fighting or cannibalism can result from the stress of overcrowding.
5. Easy spread of diseases, e.g. tuberculosis in humans (air-borne).
6. Preying or feeding on each other when food is in short supply.
7. Death of organisms, as weaker organisms in overcrowded areas easily die off due to lack of food and space.

How do organisms adapt to avoid overcrowding?

To survive overcrowding, plants and animals have developed the following adaptive features.

Territorial behaviour

Animals like mammals, lizards and birds establish territories of which they are possessive. They fight for and defend the territories against any intruders. A successful claim of the territory ensures sufficient food, space, mating partners and parental care for the organisms.

Swarming

This is exhibited by some social animals such as termites and bees when some of them move out from an old colony to a new one. Hence, overcrowding is avoided.

Emigration

This is the outward movement of animals out of their locality to another place of settlement. This prevents overcrowding.

Dispersal of seeds and fruits

This could be by water, wind, animals, insects and explosive mechanisms. The seeds and fruits are carried far away from the parent plants. Hence, overcrowding is prevented.

Formation of canopies

This is an attribute of forest trees. It helps the plants to trap enough sunlight and also prevents the shorter plants below from getting the light. The lower plants eventually die off and overcrowding is avoided.

Production of chemicals

Roots of some plants produce chemicals which prevent the growth of other plants close to them. Therefore, overcrowding is avoided.

Evaluation

1. Define ecological succession and climax community.
2. State three differences between primary and secondary succession.
3. List four characteristics of succession.
4. What is overcrowding? State four of its effects.
5. Describe three ways in which plants or animals avoid overcrowding.

Assignment

1. With the example of a pond, describe how succession takes place from the pioneer community to the floating plants.
2. List four factors that cause overcrowding and explain any two of them.