O Level Biology Ecology

Topics: Ecology, Food chain, Trophic level Pages: 8 (1560 words) Published: March 4, 2013
Biology Notes
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● Ecology is the study of interactions among organisms,
and between organisms and the physical and chemical
factors making up their external environment
● Ecologists study both the non-living (abiotic) or physical environment, and the living (biotic) environment.

Physical environment
● Amount of light
○ Light intensity affects the distribution and growth of both plants and animals

● Amount of water
○ Xerophytes are plants which can live in conditions of prolonged drought in their habitat
○ Plants which live in water or very wet places are called hydrophytes ○ Pneumatophores arise from the root system and project above the surface

● Temperature
○ Within a certain range, a fall in temperature usually results in a decrease in the metabolic activities of the organisms

● Salinity and pH of the soil or water
○ The salt concentration of the cytoplasmic contents of freshwater organisms is usually higher than that of the surrounding water so that water tends to enter these organisms by osmosis
○ Aquatic organisms are sensitive to the effects of the pH of the water and may be killed if the pH changes appreciably

Biotic environment
● Habitat: the place where an organism lives
● An organism in any habitat is never an independent unit. ○ Its daily existence depends on and is influenced by other organisms around it

● Ecological niche: the function of an organism or the role it plays in the habitat is known as the
● Population: a group of individuals of the same species occupying a given area – expressed in terms of density (number of individuals per unit area)
○ Population increases when the number of new individuals added to it exceeds the number lost through death or migration and vice versa

● Ecological unit: when different populations of plants and animals live together and interact within the same environment
● Ecosystem: an ecological system formed by the interaction of living organisms and their non-living environment (both biotic and abiotic factors)
○ A functioning ecosystem uses both energy and materials (inorganic


Organization levels in ecology

● In any ecosystem, the living organisms can be identified as producers, consumers or decomposers
● Producers: mainly green plants that manufacture complex organic food substances from raw materials
○ They are the only organisms that can convert radiant energy into chemical energy and store it in their food molecules during
○ Producers affect the lives of other organisms because they are the beginning of a food chain

● Consumers: obtain their energy from other organisms on which they feed. Animals are consumers
○ herbivores feed directly on plants (primary consumers
○ Carnivores feed on herbivores (secondary consumers)

● Decomposers: act on dead organisms and enable the materials locked up in them to be returned to the physical environment and be used again by green plants

Food chain and food web

Food chain and food web
● Each stage in a food chain is known as a trophic level
● A predator is an animal that feeds on another animal
called prey
● Food chains are linked up together to form a food
web or food cycle

Ecological pyramids

Ecological pyramids: pyramid
of numbers
The number of
organisms at each
trophic level can be
used to construct a
pyramid of numbers

Ecological pyramids: pyramid
of numbers
● Variation in pyramid of numbers
○ Pyramid of numbers may be inverted if organisms of one
trophic level are parasitic on organisms of another trophic

Ecological pyramids: pyramid
of biomass

Biomass is the total mass
of living matter in each
trophic level at any one

Ecological pyramids: pyramid
of energy
● A lot of energy is lost to the
environment as food is transferred
from one trophic level to the next
● The total energy level decreases
progressively along the food chain
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