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Form 3 Form 3 Biology Notes

Published on December 9th 2023 | 6 mins , 1162 words




Ecology is the study of the interrelationships of organisms to each other and to their environment (biotic and Abiotic factors).

Autecology; study of single species within a community and how it relates with both the biotic and Abiotic factors.

Synecology. This is the study of many different species of organisms’ interacting among themselves within an ecosystem.
Ecology helps to address the following issues.

  • Sustainable food production
  • Pollution control
  • Natural resources conservation 
  • Pest and disease control 
  • Population control 
  • Eco-tourism
  • Prediction of adverse weather conditions


Concepts of ecology

Biosphere/ecosphere. This is the part of the earth and atmosphere inhabited by living organisms.
Habitat. This is a specific locality with a particular set of conditions where an organism lives. Habitats can be terrestrial or aquatic.
Ecological niche. This is the position occupied by an organism in a habitat. It includes the physical space where an organism is found and its role in the habitat.

Population. This refers to all members of a given species in particular habitat.
Community. This refers to all organisms belonging to different species interacting in the same habitat. Many populations make up a community.
Ecosystem. This is a natural unit made of biotic and Abiotic factors whose interactions lead to a self sustaining system. E.g. a tropical rain forest, a small pond etc.
Biomass. This is the total dry weight of living organisms at a particular Trophic (feeding) level or per unit area. 

Carrying capacity. This is the maximum number of organisms an area can comfortably support without depletion of the available resources. E.g. the maximum number of cows a pasture land can comfortably hold without overgrazing.
Study Question 1

Factors in an Ecosystem
They are divided into two:
1. Abiotic factors or the non living factors
2. Biotic or the living factors

Abiotic Factors

Light. This is required by plants and photosynthetic bacteria to manufacture food. The sun is the source of light energy. Light intensity and quality (wavelength) affects the rate of photosynthesis, flowering and germination in plants, while in animals it affects migration, hibernation and reproduction. Light intensity is measured using a Photographic Light meter while a Seechi disc measures light penetration in water.

Atmospheric pressure. Variation in atmospheric pressure affects the availability of oxygen and carbon (IV) dioxide in the atmosphere. These two gases in turn affect the distribution of living organisms. Low atmospheric pressure increases the rate of transpiration. Barometer is used to measure it.
Humidity. This is the amount of water vapour in the atmosphere. It affects the rate of water loss from plants and animals surfaces through transpiration and sweating respectively. The higher the humidity the lower the rate of loss and vice versa. It is measured using the hygrometer.

Salinity. This refers to the salt concentration of the water. This divides the aquatic environment into marine, estuarine and fresh water. Only organisms with adaptable osmoregulatory features can comfortably occupy such habitats. In estuaries, there are fluctuations of salt concentrations at different times. When the sea tide is low, the salt concentrations are low due to the greater diluting effect of the fresh water being discharged. High tide raises the salt level. Estuarine organisms must therefore be adapted to cope with such wide salt variations.

Wind. This is moving air. It increases the rate of water loss from organisms affecting their distribution. It also influences rain formation. It helps in formation of sand dunes in deserts which become habitats for the growth of deserts plants. Its an agent of seed and fruit dispersal
Temperature. This affects the distribution of organisms in any habitat. Very low temperature may inactivate enzymes while very high temperatures denature them. Temperature varies due to seasons, altitude, and latitude and diurnally in hot deserts.
pH (hydrogen ion concentration.)

This is the measure of acidity or alkalinity of water in aquatic habitats or soil solution. This influences the distribution of plants and animals in soil and aquatic habitats. Different organisms have different pH requirements. pH is determined using the pH meter.

Biotic Inter-Relationships

Living organisms compete for resources such as nutrients, space, light and mates. There are two types of competition.

i.) Inter-specific competition. This is the competition between individuals of different species for the same resources. For example. An experiment6 was carried out on two closely related species of paramecia- Paramecium caudatum and Paramecium aurelia. It was observed that when each species is grown separately in controlled cultures with constant food supply, they show normal population growth. When they are grown together in the same culture, thre is competition and Paramecium caudatum is eliminated. See graphs.

competition.png 5 KB

However, closely related species can live together without competition. For example, when Paramecium caudatum and Paramecium bursoria are grown in the same culture, there is no competition because each species occupies a different part of the culture. Similarly, browsers and grazers can occupy same habitat without competition because they feed at different levels of the same plants. For example, the zebras eat the softer shoots, followed by the wild beasts, and the gazelles which eat the fibrous left over of the same grass. 

Study Question 4 

i.) Intra-specific competition. This is the competition between members of the same species for the same resources. 

When there is competition the best adapted organisms survive while the less adapted ones may die or be forced to migrate. 


This is the relationship where one organism kills another for food and feed on it either as a whole or a part of it. The predator is the one which kills while the prey is the one being killed for food. 

Predators have various adaptations to enable them to be efficient in capturing the prey. These include; 

  • Sharp eyesight as in eagles, kites and hawk 
  • Fast flight, 
  • Modified beaks 
  • Strong jaws with carnassial’s teeth as in leopards and lions. 
  • Large claws on strong forelimbs. 
  • Colour camouflage such as the spotted pattern of the leopard blends well with the background colour of the bushes and trees. 
  • Moving against the wind while stalking the prey. Preys also have structural and behavioural adaptations. These include: 
  • Swift movement e.g. the antelope and gazelle 
  • Camouflage e.g. in gazelles and stripes of the zebra. 
  • Large eyes on the sides of the head to giving them a wide field of view 
  • Confrontational display in porcupine 

NB/. When the number of the prey increases that of the predators also increases. An increase in the number of predators leads to a decrease in the population of the prey. This decrease in prey population leads to a fall in predator population which in turn gives space for the increase in the population of the prey. This is the basis of biological control. See the graph below. 


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