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Lesson Twenty One: Energy Flow in Ecosystems

Outcome

Teaching Strategy: Group, lecture method, field visit (school compound).

Starter Activity (5-7 minutes)

Let the students in small group to construct a typical food chain showing each trophic level (producers as well as primary, secondary, and tertiary consumers). 

The functioning of our planet depends on a constant input of energy from the sun. The Earth is an open systemwith respect to energy. Energy from the sun enters the environment through the making of food by green plants, a process called photosynthesis.  From here, animals eat the plants and some of the energy moves from the plants to the animals. And as these animals are eaten by other animals, energy moves through the ecosystem.  Eventually, it is lost entirely to the environment.

The flow of energy through an ecosystem is one of the most important factors that determine the   system’s capacity to sustain life.

  1. Autotrophs: These are organisms that use energy of the sun to synthesize complex molecules from simple inorganic molecules. Plants, some algae, and certain bacteria are organisms that synthesize their own food. Organisms that make their own food are also called producers or autotrophs. Autotrophs may be photo autotrophs or chemo autotrophs.

    Chemo-autotrophs: These are organisms that use energy from chemical compounds to synthesize food from inorganic molecules. E.g Some bacteria.
    Photo- autotrophs: These are organisms that use the energy of the sun to synthesize food.
    E.g green plants and algae.
    Consumers: These are organisms that cannot make their food and they depend on producers

  2. Heterotrophy:  These are organisms that rely on other organisms for their energy and food supply. These include animals, fungi and some bacteria. Heterotrophies are also called consumers.

    Herbivores:  obtain energy by eating plants. E.g. cows, sheep, caterpillars, and deer,
    Carnivores: eat animals. E.g. snakes, dogs, lion, tiger and owls etc.
    Omnivores: eat both plants and animals. E.g.  Humans and rat
    Detritivores:  depend on plant and animal remains and other dead matter.
    e.g mites, earthworms, snails, and crabs
    Decomposers:  break down organic matter. E.g.  Bacteria and fungi.

Feeding Relationships

Energy flows through an ecosystem in one direction, from the sun or inorganic compounds to autotrophy (producers) and then to various heterotrophy (consumers). 

Food Chain:- A food chain is a simple feeding relationship in which energy is transferred from one organism to another along the trophic level.

Food Web: A food web is a feeding relationship among various organisms in an ecosystem that form a network of complex interactions.  A food web links all the food chains in an ecosystem together. 

Trophic Levels

A trophic level is a position of organisms in a food chain or food web. Producers make up the first trophic level. Consumers make up the second, third, or higher trophic levels. Each consumer depends on the trophic level below it for energy.

Ecological Pyramids: The amount of energy or matter in an ecosystem can be represented by an ecological pyramid.  An ecological pyramid shows the relative amounts of energy or matter contained within each trophic level in a food chain or food web.

Pyramid Energy

It shows the amount of the energy available to each trophic level.  Only about 10 % of the energy available within one trophic level is transferred to organisms at the next trophic level.  E.g. 100% energy is available to producers, 10% energy is available to the first-level consumers, 1% energy is available to second-level consumers, 0.1% to the third level consumers. The more levels that exist between a producer and a top-level consumer, the less energy that remains from the original amount.

Pyramid of Biomass

Biomass is the total amount of living tissue within a given trophic level. Biomass is expressed in terms of grams of organic matter per unit area. A pyramid of biomass represents the amount of potential food available for each trophic level.

Pyramid of Numbers

Pyramid of numbers is based on the number of individual organisms at each trophic level.

Often the trophic structure of an ecosystem is represented as an ecological pyramid in which producers form a base for tiers of consumers above them. Some pyramids are based on biomass (the weight of all the members at each trophic level), others on energy flow. In general, there is roughly a 10-fold loss of energy for each level up in trophic level.


Fig.  21.1 Trophc Levels.


Fig. 21.2:  Pyramid of Numbers ( in grass lands)


Fig. 21.3:   Pyramid of Dry mass ( Biomass)

Activity

Allow students to construct an ecological pyramid consisting of at least four trophic levels. The students should indicate the position of producers, primary, secondary and tertiary consumers. Using the numbers 1, 10, 100 and 1,000, the students are expected to indicate the levels of energy (biomass or Calories) stored at each trophic level.  Ask students to compose and record a group consensus as to the efficiency of energy transfer from one level to the next, moving up the pyramid.

Fig.21.4:   Pyramid of Energy

Biological Magnification

In food chains, it is not only nutrients that get transferred but also toxic substances are transferred from one trophic level to another too. In some cases, the concentration of toxic substance increases with an increase in the trophic level. This increase in concentration of a toxic substance, with every link in the food chain, is called biological magnification or biomagnifications. One good example of biomagnifications is the buildup of DDT across various trophic levels in a food chain.

Do you think biomagnifications is a problem in our country?        

Evaluation/Assessment Questions

  1. What is the initial source of energy in ecosystem?
  2. Is the amount available energy increasing or decreasing as one move from one trophic level to next higher trophics?
  3. Distinguish between detrivores and decomposers?