Functions in Ecosystem – SS1 Biology Lesson Note (Second Term)

An ecosystem functions through the feeding relationships among its organisms: producers make food, consumers feed on producers or other consumers, and decomposers break down dead matter into simple compounds that can be used again.

Behavioural objectives

1. Name the three major groups of the biotic community and give examples of each.
2. Explain what a trophic level is and describe a feeding pathway.
3. Distinguish between a food chain and a food web.
4. Write a food chain and state the trophic level of each organism in it.
5. Describe the pyramid of numbers, the pyramid of energy and the pyramid of biomass.

What are feeding relationships in an ecosystem?

A feeding relationship is the way one organism obtains energy and nutrients by feeding on another.

• Why they exist: all living organisms must get energy and nutrients from the environment to stay alive.
• Result: the organisms depend on one another, which makes the ecosystem a functional unit.
• Groups involved: every organism belongs to one of three major groups of the biotic community.

What are the major groups of the biotic community?

There are three major groups of the biotic community: producers (autotrophs), consumers (heterotrophs) and decomposers.

Producers (autotrophs)

Producers are organisms that make their own food and provide food for other organisms in the habitat.

• Terrestrial producers: grasses, trees and shrubs.
• Aquatic producers: phytoplankton and seaweeds.

Consumers (heterotrophs)

Consumers are organisms that cannot make their own food and so feed on other organisms.

• Herbivores (primary consumers): feed on plants, as in cows and goats.
• Carnivores (secondary consumers): feed on primary consumers, as in lions.
• Tertiary consumers: feed on secondary consumers.
• Omnivores: feed on both plants and animals, as in man.
• Terrestrial examples: cows, dogs, lions and man.
• Aquatic examples: water fleas, tadpoles, and larvae of insects and fishes.

Decomposers

Decomposers are organisms that break down dead organic matter into simple chemical compounds.

• Examples: termites, larvae of houseflies (maggots), bacteria and fungi.
• Function: the simple compounds they release can be absorbed and used again.

What are trophic levels?

A trophic (feeding) level is each step along a feeding pathway in an ecosystem.

What is the feeding pathway in an ecosystem?

Energy and nutrients pass step by step among organisms along a feeding pathway, which follows a similar pattern in all ecosystems.

1. It begins with a producer, for example a green plant such as grass.
2. The producer is eaten by a primary consumer, for example a zebra or goat.
3. The primary consumer is eaten by a secondary consumer, for example a lion.
4. Decomposers convert the remains of dead producers and consumers into simple inorganic substances, which return to the non-living environment.

• Where pathways end: some end at the primary consumer, while others continue to the secondary consumer, the tertiary consumer and beyond.
• Comparison: pathways within one ecosystem, or between different ecosystems, can end at different steps.

How are trophic levels numbered?

• Numbering: levels are numbered in ascending order, starting from one, to show the path of energy flow.
• Trophic level 1: always producers (autotrophs).
• Trophic level 2: always primary consumers.
• Above level 2: consumers may be carnivores, parasitic organisms and scavengers.
• Final consumers: they eventually die and are fed upon by decomposers.

What are a food chain and a food web?

A food chain is a single line of feeding, while a food web is many food chains joined together.

What is a food chain?

A food chain is the feeding relationship in which energy passes through food from producers to consumers in a linear form.

• Usual start: most food chains begin with a producer.
• Other start: a few begin with dead plants or animals.
• Example from a producer: grass → zebra → lion.
• Example from dead matter: humus → earthworm → domestic fowl → man.

What is a food web?

A food web is a complex feeding relationship among organisms in the same environment, made up of two or more interrelated food chains.

• How it forms: the many food chains present in an ecosystem join to produce a food web.
• Number of organisms: a food web contains more organisms than a single food chain.
• Shared food: a single plant can be fed upon by more than one or two kinds of organism.
• Survival: a consumer has a better chance of survival in a food web because it feeds on different types of plants or animals.

What is the difference between a food chain and a food web?

What are ecological pyramids?

Ecological pyramids are diagrams used in ecology to show the relationship between organisms at different trophic levels.

There are three types of ecological pyramid: the pyramid of numbers, the pyramid of energy and the pyramid of biomass.

What is the pyramid of numbers?

The pyramid of numbers shows the number of individual organisms at each trophic level of a food chain.

• Pattern: numbers decrease from the first level to the last level in a food chain.
• Weakness: all individuals are given the same status, though they vary greatly in size.
• Example of the weakness: grass and trees are both counted as producers and are not drawn to scale.

What is the pyramid of energy?

The pyramid of energy shows the amount of energy present in the living organisms at each trophic level of a food chain.

• Pattern: energy decreases from the first level to the last, from the base of the pyramid to the apex.

What is the pyramid of biomass?

The pyramid of biomass shows the total mass of living organisms at each trophic level.

• Biomass: the mass of living organisms.
• Measure used: total wet mass or dry mass at each level.
• Advantage: it gives a more accurate picture of the relationship between trophic levels than the pyramid of numbers.

Evaluation

1. Name the three major groups of the biotic community.
2. What is a trophic level?
3. State the difference between a food chain and a food web.
4. Write a food chain of four organisms.
5. Name the three types of ecological pyramid.

Assignment

1. Draw a food web of at least six organisms from a pond or a grassland near you. Label the producers, consumers and decomposers. Then write one food chain from the web and state the trophic level of each organism in it.