General Principles of Ecology


  • Ecology is the study of the interactions between organisms and between organisms and their environment
  • An ecosystem is a group of clearly distinguished organisms that interact with their environment as a unit
  • The biosphere is that part of the planet in which living organisms can be found
  • A habitat is the place where a plant or animal lives
  • A population is all the members of the same species living in the habitat
  • A community is all the different populations of species living in the habitat

Environmental Factors (Terrestrial)

  • Abiotic factors: non-living factors, e.g. aspect: north-facing slopes are cooler and darker than south facing slopes (in Northern hemisphere only)
  • Biotic factors: living factors, e.g. available food: more food will enable more organisms to survive
  • Climatic factors: effects of weather, e.g. rain: more rain means more water, which supports more life
  • Edaphic factors: effects of soil, e.g. soil pH: pH affects growth of particular plants as pH affects enzyme action.

Environmental Factors (Aquatic)

  • Light
  • Currents
  • Wave action
  • Salt content
  • Oxygen concentration

Energy Flow

  • Sun: ultimate source of energy
  • Feeding allows ‘energy flow’ from organism(s) to organism(s)
  • Producers: photosynthetic organisms that make their own food
  • Consumers: organisms that obtain their food from other organisms
  1. Primary consumers: feed on producers (herbivores; decomposers; detritus feeders; omnivores)
  2. Secondary consumers: feed on primary consumers (carnivores; scavengers; omnivores)
  3. Tertiary consumers: feed on secondary consumers (carnivores; scavengers; omnivores)
  • Energy is passed from one trophic level to another.
  • Food chains tend to be limited to four or five organisms due to a large amount of energy being lost as it is transferred up the food chain.
  • Only a small amount is transferred from one organism to the next due to a large amount being lost as heat and energy being used up in respiration.

Food Chain

grazing food chain is a sequence of organisms in which each one is eaten by the next member in the chain

Examples of food chains:

  • Leaf → Caterpillar → Thrush → Falcon
  • Grass → Rabbit → Fox
  • Algae →  Limpet → Starfish→ Gull
  • Plankton → Barnacle → Whelk → Crab

Food web:

A food web consists of two or more interlinked food chains.

Pyramids of numbers:

pyramid of numbers represents the number of organisms at each stage in a food chain

Ecological pyramids are ways of comparing different communities of the ecosystem in order of different trophic levels. There are three general types:

  • Upright pyramids of numbers: the numbers of organisms at each trophic level decreases. (Organism’s body size usually (but not always) increases as you go up the pyramid)
  • Partially upright pyramids of numbers: the number of organisms at the beginning of the chain (producers) is very small when compared to the trophic levels that follow.
  • Inverted pyramids of numbers: the numbers of organisms at each trophic level increase

Limitations of the use of pyramids of numbers:

  • Pyramid of numbers does not take into account the actual numbers of organisms involved
  • Pyramid of numbers cannot be drawn to scale
  • Some are not technically pyramids – they are inverted


niche is the functional role an organism plays in a particular habitat, e.g fox’s role is to keep numbers of rabbits and hares down

Nutrient recycling

Nutrient recycling is the way in which elements (such as carbon and nitrogen) are exchanged between living and non-living components of an ecosystem

The Carbon Cycle:

The Nitrogen Cycle:

Human Impact on Ecosystem:


Pollution is any harmful addition to the environment

  1. Domestic pollution:
  • Example: carbon dioxide
  • Harmful effect: can cause global warming
  • Control measure: change to renewable forms of energy; e.g. solar panels
  1. Agricultural pollution:
  • Example: fertilisers
  • Harmful effect: can cause eutrophication (algal blooms) of rivers/lakes.
  • Control measure: Only spread fertiliser during dry weather
  1. Industrial pollution: e.g. sulphur dioxide
  • Example: sulphur dioxide
  • Harmful effect: can contribute to acidic rain
  • Control measure: use sulphur-free fuels OR change to renewable forms of energy; e.g. use electric vehicles


Conservation is the management of the environment


  • Storing slurry in leak-proof pits
  • Only spread slurry on land in summer
  • Do not spread fertilisers or spray insecticides in wet conditions
  • Growing of organic crops and livestock


  • Analysing water samples
  • Fishing quotas
  • Monitoring of fish stocks
  • Mesh sizes of nets are large to allow younger fish to escape


  • Tops of trees and branches are not wasted – they are converted to sawdust and then to MDF
  • Felled trees are replaced with young trees for the future
  • Effective and rapid action plans for forest fires

Waste Management

Problems associated with waste disposal:

  • Disease-causing micro-organisms (pathogens) are present and can be harmful
  • Toxic/harmful chemicals are present and can be harmful
  • Nutrient elements: Phosphorus and nitrogen can cause eutrophication of rivers and lakes
  • Landfill sites are unsightly and attract vermin/scavengers
  • Dumping waste at sea leads to pollution and harms sea life.
  • Incineration can release toxic chemicals which are harmful to the breathing system.

In general, wastes can be minimised by employing the three R’s:

  • Reduce: do not buy foods that use excess packaging
  • Reuse: household objects can be reused – for example ice cream tubs, glass bottles, etc.
  • Recycle: many materials used can be recycled, such as glass bottles, paper, plastics, metals, and organic waste

Waste Minimisation

  • Agriculture: Slurry (eutrophication) is stored in leak-proof pits and only spread on land when it is dry
  • Fisheries: Waste parts of fish are pulped, dried, and recycled as fertiliser and/or pig feed on farms
  • Forestry: Waste pieces of trees are processed into chip board, MDF board, etc.

Role of Micro-organisms in Waste Management and Pollution Control

  • Landfill sites: Bacteria break down the organic waste
  • Sewage: Small amounts of sewage are treated naturally by bacteria in water; large amounts of sewage have to be treated by sewage-treatment plants (also involves bacteria)

Sewage Treatment

Sewage can cause eutrophication of lakes and rivers if released directly into them – this causes fish kills. Therefore sewage must be treated. Sewage generally goes through three stages of treatment:

  1. Primary sewage treatment
  2. Secondary sewage treatment
  3. Tertiary sewage treatment

1. Primary Sewage Treatment

  • Physical method: Screening large objects by flowing sewage through metals grills
  • Sedimentation of sewage where it is stored in tanks and smaller particles such as grit settle out at the bottom producing sludge which is removed.

2. Secondary Sewage Treatment

  • Biological method: Bacteria and fungi are added to the water and sludge: organic matter in sludge and waste water is broken down with the production of methane gas, carbon dioxide and water
  • Chlorination: at the end of the biological process the waste water is usually treated with chlorine to destroy any remaining micro-organisms

3. Tertiary Sewage Treatment

  • Chemical method: Removal of minerals by addition of chemicals that cause the minerals to precipitate out of the waste water

Ecological Relationships

a) Competition
b) Predation
c) Parasitism
d) Symbiosis

a) Competition: organisms for a resource that is in short supply

  • Contest competition: physical struggle between two organisms with one organism obtaining all of the resource
  • Scramble competition: struggle between a number of organisms with all the organisms obtaining a small amount of the resource

Competition limits population growth.

Adaptation occurs to reduce competition: e.g. caterpillar eats cabbage; butterfly eats nectar

b) Predation: catching, killing, and eating of another organism, e.g. snake is predator and a mouse is its prey.

Factors that affect predator-prey relationships:

Predator-prey relationship over time
  • Availability of food
  • Concealment/camouflage
  • Movement (speed)

c) Parasitism one organism (the parasite lives in or on another organism (called the host) and the host is harmed;e.g. Athlete’s foot; Ringworm

d) Symbiosis occurs when two organisms of different species live in close association and at least one of them benefits, e.g. bacteria in human digestive system produce vitamin K and get food and a safe environment in return

Population Dynamics

Contemporary issues in population dynamics (human population):

  • Disease
  • Famine
  • War
  • Contraception