The relationships between organisms in a given ecosystem is primarily a feeding one. Organisms in a particular habitat have different feeding levels referred to as trophic levels.

There are two main trophic levels: Producers:

These organisms that occupy the first trophic level. They manufacture their own food hence are autotrophic. Consumers: These are the organisms that feed on organic substances manufactured by green plants.

They occupy different trophic levels as follows:

  1. Primary consumers:
  2. These are herbivores and feed on green plants.
  3. Secondary consumers:
  4. These are carnivores and feed on flesh.
  5. First order carnivores feed on herbivores while second order carnivores feed on other carnivores, i.e., tertiary consumers.


These are animals that feed on both plant and animal material. They can be primary, secondary or tertiary consumers.


  1. This describes the situation where two or more organisms in the same habitat require or depend on the same resources.
  2. Organisms in an ecosystem compete for resources like food, space, light, water and mineral nutrients.
  3. Competition takes place when the environmental resource is not adequate for all. Intraspecific competition.
  4. This is competition between organisms of the same species.
  5. For example, maize plants in a field compete for water and nutrients among themselves.


Interspecific competition.

This refers to competition between organisms of different species, e.g., different species of predators can compete for water and prey among themselves.


It is a relationship whereby one animal (the predator) feeds on another (the prey).


Saprophytism is the mode of nutrition common in certain species of fungi and bacteria. Such organisms feed on dead organic material and release nutrients through the process of decomposition or decay. Saprophytes produce enzymes, which digest the substrates externally. The simpler substances are then absorbed.

Saprophytes help in reducing the accumulation of dead bodies of plants and animals. Harmful saprophytes cause rapid decay of foods such as fruits, vegetables, milk and meat. Others damage buildings by causing wood rot.

Some fungi produce poisonous substances called aflatoxins. These substances are associated with cereal crops which are stored under warm, moist conditions. If the infected grain is eaten, it may cause serious illness, and death.


This is an association between members of different species. The parasite lives on or in the body of another organism, the host. The parasite derives benefits such as food and shelter from the host but the heist suffers harm as a result.


This is an association in which organisms of different species derive mutual benefit from one another. Some symbiotic associations are loose and the two partners gain very little from each other. Other symbiotic associations are more intimate and the organisms show a high degree of interdependence.

Nitrogen cycle

Is the interdependence of organisms on one another and the physical environment as nitrogen is traced from and back into the atmosphere Although nitrogen is abundant in the atmosphere, most organisms are not able to utilise it directly. Some bacteria are capable of converting atmospheric nitrogen into forms which can be used by other living organisms.

These bacteria are referred to as nitrogen fIxing bacteria. Symbiotic nitrogen fixing bacteria live in the root nodules of leguminous plants such as beans and peas. Non-symbiotic nitrogen fixing bacteria live in the soil.

Nitrifying’ bacteria convert ammonia into nitrites and nitrates. Denitrifying bacteria convert nitrates into atmospheric nitrogen.

Energy Flow in an Ecosystem

Most of the energy used in an ecosystem is derived from the sun. Solar energy is trapped by photosynthetic plants. It flows through different trophic levels.

At each level energy is lost as heat to space and also through respiration. Besides animals lose energy through excretion and defecation. The amount of energy passed on as food from one trophic level to another decreases progressively. The energy in the organisms is recycled back to plants through the various nutrient or material cycles.


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