O Level Revision : Combined Science - Ecosystems

An ecosystem is a self-contained system of interdependent organisms and their physical environment. An ecosystem has various components that may include air, water, soil, light and living organisms.

  • Ecosystems are either natural or artificial (man-made).
  • Components can be classified into two groups; physical components and biological components.
  • The soil is a key component of some ecosystems and soil properties relate to crop production.
  • The living organisms in an ecosystem are interdependent.
  • Living organisms can be classified according to their feeding levels. They can be producers, primary consumers, secondary consumers, tertiary consumers or decomposers.
  • Energy is lost in every ecosystem.
  • Nutrients must be recycled when organisms die, so that they are made available to future and present generations.
  • Artificial ecosystems lack species diversity.
  • Human activities affect ecosystems in a negative way.
  • Ecosystems should be managed to obtain maximum benefit from them.


Definition: A self-contained system of interdependent organisms and their physical environment.

Components of an ecosystem

    Components of an ecosystem

     The soil

    • Soil is the key component of an ecosystem.
    • Physical components of the soil are air, water, mineral salts and rock particles.
    • The properties of soil determine the kind of ecosystem it can support.

    Types of soil and their properties

    Types of Soil



    - large particles

    - holds very little water, it has a low holding capacity

    - good aeration

    - quick drainage

    - easy to cultivate


    - Very small particles

    - poor aeration, it does not allow much air to enter

    - bad drainage

    -difficult  to  cultivate  because  it  is  sticky  when wet and hard when dry


    - mixture of sand and clay particles (small and large)

    - good water holding capacity

    - good aeration

    - good drainage

    - relatively easy to cultivate

     Soil properties and crop cultivation



    Soil types and properties


    – many types of soil

    – does not require specific soil properties (loam soils)


    – dark, fine-textured soil (clay soils)


    – light soils

    – coarse grained soils (sandy soils)

    Biological components of soil

    • Various organisms are found living in and on the soil.
    • Organisms such as earthworms, nematodes, termites, fungi and bacteria are usually found in soil.
    • Litter and humus made from plant and animal remains also make up the biological components of the soil.

     Biological components of an ecosystem

    Role of biological components





    - Adds soil fertility

    -  Improves soil structure

    -  Improves water holding capacity

    -  Prevents soil erosion

    -  Raises soil temperature


    -  Cause decay of dead plants and animals causing the formation of humus

    -  May cause diseases


    -  Increase humus context and soil fertility by pulling leaves into their burrows

    -  Improve aeration and drainage through their tunnels


    -  Breakdown plant material

    - Tunnel wooden structures through


    -  Help in humus formation by carrying plant material and dead insects into

    their burrows

     Natural ecosystems

    • Organisms in an ecosystem are connected by feeding relationships.
    • Feeding relationships can be simple or complex.
    • A food chain shows simple feeding relationship.
    • A food web shows complex feeding relationship.
    • A food chain is the sequence of organisms whereby one organism feeds on the next in an ecosystem e.g.   grass            hare            cheetah            vulture.
    • A food chain starts with the primary energy source, usually the sun.
    • The next link in the chain is an organism that makes its own food from the primary energy source.

    An example is photosynthetic plants that make their own food from the sunlight through photosynthesis. There are called primary producers.

    • Next are the organisms that eat the primary producers (autotrographs); these are called herbivores They are the primary consumers, e.g. a hare or rabbit eats grass.
    • The next link in the chain are the animals that eat herbivores and these are called secondary consumers e.g. cheetah or snake that eats the hare/rabbits.

    –     In turn, these animals are eaten by tertiary consumer e.g. owl or vulture. These are predators.


    Examples of a food chain

    grass        →          hare      →    cheetah

    Food web

    • A graphical representation of feeding relationships in an ecosystem.
    • Consist of many food chains.
    • Illustrates that an organism may not depend on only one source of food.

    Loss of energy in food chains

    • Energy in lost from one trophic level to the next as heat and this heat will not be passed on to the next level.
    • As an organism feeds some of the food eaten is used to provide energy for life processes such as chemical reactions, locomotion, digestion and homeostasis.
    • The energy used for life processes will not move to the next trophic level. Energy is lost in excretory products.

    Pyramid of numbers

    • These are diagrams to show the number of organisms at each trophic level.
    • The length of the bar is proportioned to the number of organisms in the trophic level.

      A pyramid of numbers

    Pyramid of biomass

    A pyramid of  biomass  shows the total mass of organisms in each trophic level.

    Pyramids of biomass always have broad bases and taper as they go up.

    Nutrient cycling

    All important nutrients are recycled in the ecosystem. Among the important nutrients recycled are nitrogen and carbon.

    Artificial ecosystem

    • An artificial ecosystem is made by man e.g. a farmland.
    • This is a natural ecosystem altered by man through farming.
    • The major characteristic of an artificial ecosystem is limited species diversity.
    • This is because the farmer gives priority to one type of plant only.

    Problems caused by limited species (biodiversity)

    • Limited species diversity leads to soil infertility, pest problems and production for human consumption only.

      Management of ecosystems

    Ground cover

    • Ground cover reduces surface run-off, thereby preventing dislodging of top soil particles hence reduce soil erosion.

    Ground cover prevents heat from striking soil particles directly hence reduces evaporation of water from the soil thus helping to increases water retention by the soil.

    • Mulching provides artificial ground cover which helps to reduce evaporation, surface runoff and erosion.

    Effects of human activities on ecosystems

    • Agricultural activities lead to soil erosion and reduced species diversity.
    • Social activities like church gatherings etc lead to soil erosion as well as desertification.
    • Industrial activities lead to pollution that is both water and air.
    • Industrial air pollution results in the formation of acid rain as well as leading to global warming.


    Characteristics of Zimbabwean Savanna soils

    • These are characterized by high temperatures, rapid denitrification, few earthworms and low soil fertility.

    Problems of farming on marginal land

    • The main problems are low fertility, poor productivity, low rainfall and unreliable rainfall patterns.
    • Marginal land is land which is barely profitable to use due to poor soil and erratic rainfall e.g. Region 4 and Region 5 in Zimbabwe.

    Ways of utilising/ using marginal land

    Since there are not suitable for agricultural activities other ways of using them include:

    • Game ranching.
    • Growing of suitable crops e.g. rice in swamps and gum trees in water logged areas; drought tolerant crops.

    Carrying capacity

    Definition: The maximum number of organism an area can sustain for a given period of time without deterioration.

    • Carrying capacity should not be exceeded. Exceeding carrying capacity leads to overstocking, overgrazing and deterioration of veld.

    Limiting factors to carrying capacity are oxygen, food space, shelter and water.

    •  Animal populations can be maintained within carrying capacity of a habitat by culling, destocking and paddocking.