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.

Study Question 2

Practical Activity 1

Study Question 3

Biotic Inter-Relationships


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

  1. 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.

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

  1. 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.

  • Predation

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.

  • Parasitism
  •  This is the relationship where an organism [parasite] obtains nutrients from another live organism [host] without killing it. The parasite obtains food and shelter from the host causing some harmful effects. Parasites may weaken the host and also transmit diseases which may kill their host thus reducing their number an d distribution. There are two types of parasites;
  • Ecto-parasites
  • Endo-parasites

Study Question  5

  • Symbiosis

This is an association between two of different species in which both benefit. For example the association of colon bacteria with humans and other animals, especially plant-eating animals, the ox-pecker bird and the ox etc.

The Rhizobium bacteria help the leguminous plants to fix nitrogen while the bacteria obtain shelter and carbohydrates from the plants.


  • Saprophytism

This is where organisms obtain nutrients from dead organisms causing decomposition hence releasing nutrients into the ecosystem. Saprophytes include the bacteria and fungi.

The Nitrogen Cycle

This refers to the cycling of nitrogen and its compounds in the natural environment.

  • Although nitrogen is abundant in the atmosphere as nitrogen gas, it cannot be utilised by plants. It has to be converted into a form that can be absorbed by plants through a process called nitrogen fixation.
  • Nitrogen fixation is done in two ways;
  • Biological fixation. This can occur in two forms
    • Nitrogen fixation by symbiotic bacteria such as Rhizobium spp. They are found in the root nodules of legumes. They convert nitrogen gas into ammonia which is then converted into nitrates for plant utilisation.
    • Nitrogen fixation by free living bacteria e.g. Clostridium, Azotobacter, and some algae such as Anabaena, chlorella and Nostoc.
  • Non-Biological nitrogen fixation. This is done by lightning. During thunderstorms, lightning energy combines atmospheric nitrogen gas with oxygen to form nitrous and nitric acid. These are then converted into nitrates.
  • Plants absorb nitrates and convert them into plant proteins. Animals feed on these plants and obtain the proteins. They are then digested into amino acids and become assimilated into animal proteins.
  • When living organisms die, saprophytic bacteria and fungi break down the proteins in their bodies into ammonia. Nitrifying bacteria convert this ammonia into nitrates thorough a process called nitrification. Nitrosomonas and Nitrococcus convert ammonia into nitrites and Nitrobacterconvert nitrites into Nitrates.
  • Some soil micro organisms such as Pseudomonas denitrificans & Thiobacillus denitrificansutilise the oxygen in the nitrates reducing it to nitrites, ammonia and eventually into nitrogen gas. This is called de-nitrification.
  • This reduces the amount of nitrogen available to plants but it frees the nitrogen so that it becomes available for the cycle to continue.


Practical activity 2

Study question 6

Energy Flow in an Ecosystem

The sun is the natural source of energy. This energy is transferred to the following feeding levels;

  • Producers
  • Primary consumers
  • Secondary consumers
  • Tertiary consumers
  • Quaternary consumers
  • These feeding levels are called Trophic levels


They break down organic materials into simple substances which are made available for re-use by other organisms. Decomposers are mainly fungi and bacteria.

Food Chains

This is the representation of energy flow from a producer to other organisms linearly. Green plants are eaten by herbivores which are eaten by carnivores.

Producers’          Primary consumers        Secondary consumers        Tertiary consumers Quaternary consumers

Some energy is lost as it is moved from one trophic level to the next. This is through respiration, defecation, excretion and in form of heat.

Fig. 2.7


When the decomposers are included in a food chain, they are placed at the end.

Study Question 7

Food Webs

These are several interconnected food chains. Simple food chains rarely exist since in any ecosystem, many populations interact.


Study Question 8

Ecological Pyramids

These give a simplified representation of feeding relationships and energy flow in an ecosystem. They are of three types.

  • Pyramid of numbers
  • Pyramid of biomass
  • Pyramid of energy

Pyramid of Numbers

There is a progress decrease in the number of organisms as one move from the producers all the way to the quaternary consumers. Producers have the greatest number followed in a decreasing order by primary, secondary, tertiary and quaternary consumers.

Construction of Pyramid of Numbers

  1. Use data provided or collected.
  2. From the data, identify and draw the most suitable food chain.
  3. Indicate the numbers at each trophic level in the food chain.
  4. Choose a suitable scale for the data.
  5. Using the chosen scale draw a horizontal rectangular bar to represent the number of the producers as the base of the pyramid.
  6. Progressively draw horizontal bars of the other trophic levels in their ascending order.
    1. Ensure that the width of the bars is uniform.

Study Question 9

Interpretation of Pyramid of Numbers

  • Generally the body size of organisms increases at each trophic level from the base to the apex of the pyramid as their number decreases.
  • At each trophic level much energy is lost through respiration, excretion, sweating, defecation etc. therefore less energy is transmitted to the succeeding trophic level. Fewer organisms can therefore be supported.
  • Inverted pyramid of numbers also exist. For example where one mango tree supports several monkeys each being fed on by several fleas.

Pyramid of Biomass

Biomass of an organism is its constant dry weight. In an ecosystem, the producers have the highest biomass followed in decreasing order by primary, secondary, tertiary and quaternary consumers.

Study Question 10

Practical activity 3

Study Question 11


Populations change in size, structure and organisation.

Characteristics of a population.

  • Density. This is the number of individuals per unit area. E.g. 50 gazelles per Km2.
  • Dispersion. This is the distribution or spread of organisms in a habitat.
  • Population growth. This refers to the rate of increase in numbers.

Population Estimation Methods

Usually a representative sample is used to estimate the population of organism in a big habitat. A sample is a small number of individuals taken from the habitat that is a representative of the whole population. The following methods are used when sampling.

  • Quadrat method.
  • Line transect.
  • Belt transect.
  • Capture-recapture method.

Adaptations of plants to various Habitats

An adaptation is a change to suit environment: the development of physical, physiological or behavioural characteristics that allow organisms to survive and reproduce in their habitats. There are four main groups of plants namely;

  • Xerophytes.
  • Mesophytes.
  • Hydrophytes.
  • Halophytes.


These are plants adapted to survive in the dry habitats. These habitats have the following characteristics.

  1. Unpredictable and poorly distributed rainfall between 250-350mm per year.
  2. Very high day temperatures and very low night temperatures hence high diurnal temperature range.
  3. They are very windy.
  4. Low humidity.

Adaptations of Xerophytes

  1. Shedding of leaves during the dry season to reduce the surface exposed to transpiration.
  2. Reduced leaves in size such as in pine or modified into spines as in cactus. This reduces the surface area over which transpiration occurs.
  3. Leaves have a thick waxy cuticle to reduce the rate of transpiration.
  4. Some store water in large parenchyma cells contained in succulent stems and leaves.
  5. Some have reversed stomatal rhythm.
  6. Sunken stomata
  7. Folded leaves reduced the surface area.
  8. Reduced number of stomata
  9. Some have deep roots to absorb water from deep in the soil. Others have superficial roots growing horizontally close to the surface to absorb water after a light


These are plants growing in well watered areas. Such habitats have the following general characteristics.

  • Adequate rainfall; 950-1800mm that is well distributed throughout the year.
  • Relatively high humidity
  • Thick clouds
  • Moderate to high temperatures
  • Shallow water table
  • Less windy

Adaptations of Mesophytes

They show various adaptations depending on where they grow. Some of these adaptations are for reduction of water loss, others for increased water, loss and some are also adapted to light conditions.

Forest Ecosystem

  1. Vegetation grows fast to compete for light.
  2. Trees grow very tall to compete for light.
  3. Some develop buttress roots or prop roots for extra support such as the Ficus natalensis.
  4. Climbers such as lianas support themselves on stems of tall trees to reach light.
  5. Epiphytes support themselves on the branches of tall trees.
  6. Others are adapted to carry out photosynthesis under low light intensity by having many chloroplasts that are sensitive to low light intensity.
  7. They show leaf mosaic pattern to minimise overlapping enhancing trapping of light for photosynthesis.
  8. Thosein areas with a lot of water have broad leaves, thin cuticle and many stomata on both surfaces to encourage high rate of transpiration.
  9. Those in dry areas have waxy and shiny cuticle to reflect light. Others are deep rooted to obtain water from deep in the soil.


These are plants growing in fresh water either partially or wholly. Such habitats have the following general characteristics.

  • Low concentration of dissolved gases such as oxygen
  • Presence of waves and currents
  • Inadequate light in water

Adaptations of hydrophytes

  1. Broad leaves with maximum number of stomata on upper leaf surface providing a large surface are for transpiration.
  2. They have a large air filled tissue called aerenchyma tissue. The air reduces the density hence creating buoyancy to the plants and also aids in gaseous exchange.
  3. Submerged ones have dissected leaves to offer large surface area for light absorption required during photosynthesis.
  4. They have chloroplasts sensitive to low light intensity.
  5. They have poorly developed leaves and lack the root hairs to reduce water absorption
  6. Flowers are raised above the water to allow for pollination.



These are plants which are able to tolerate very salty conditions in soil and marine water. Such habitats have the following general characteristics.

  • High concentration of mineral salts
  • Low concentration of dissolved gases
  • Low light intensity in marine water
  • Presence of waves and currents in marine water

Adaptations of Halophytes

  1. They root cells which concentrate a lot of salts to enable them to absorb water by osmosis.
  2. Some have salt glands that secrete excess salts.
  3. Many have water storage tissues.
  4. Some like the mangroves have breathing roots called pneumatophores. These rise above the water surface to obtain oxygen from the atmosphere.
  5. Mangroves growing on mud flats have buttress roots for support.
  6. Submerged halophytes are adapted to photosynthesise under low light intensity.

Their fruits are adapted for dispersal by having aerenchymatous tissue for air storage to make them buoyant.