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Organisms and their Environment

Energy Flow

  • The sun is the principal source of energy input to biological systems
  • The earth receives two main types of energy from the sun: light (solar) and heat
  • Photosynthetic plants and some bacteria can trap light energy and convert it into chemical energy

Food Chains and Food Webs

  • A food chain shows the transfer of energy from one organism to the next, beginning with a producer
  • Energy is transferred between organisms in a food chain by ingestion

Transference of Energy between Trophic Levels

  • Energy is lost at each trophic level (90% is lost, and only 10% is passed)
  • Energy is lost through the process of respiration (as heat)
  • Energy is lost in excretion
  • Some of the material in the organism being eaten is not used by the consumer, for example, a locust does not eat the roots of maize, and some of the parts eaten are not digestible


  • A food chain usually starts with a photosynthetic plant, which gains its energy from the sun
  • The arrows used to link each organism to the next represent the direction of energy flow. They always point towards the ‘eater’, and away from the plant
  • The feeding level is known as the trophic level
  • Plants are producers (they make/produce food for other organisms)
  • Animals that eat plants are primary consumers (a consumer is an ‘eater’). They are also called carnivores
  • Trophic level is the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass

Why the Transfer of Energy from One Trophic Level to Another is Inefficient

  • Energy transfer is inefficient because energy is lost while moving from one trophic level to another. This is because:
    • not the entire organism is consumed or digested. Parts, such as a root, woody stems, bones, etc., are not eaten, and some materials such as cellulose cannot be digested
    • energy is used up by organisms in each trophic level for movement and transport inside their bodies
    • energy is used in respiration and is released from the body of the organism as heat
    • energy becomes lost in excretion

Why Food Chains usually have Fewer than Five Trophic Levels

  • On an average, about 90% of energy is lost and 10% is passed at each level in a food chain, so the further along the chain you go, the less energy there is
  • There is plenty of energy available for producers, there is less for the primary consumers, and the least in secondary consumers
  • Thus, towards the ends of a food chain, the organisms get fewer in number
  • Only a small percentage reaches the fourth trophic level
  • There isn’t enough energy in the fifth trophiic level to support another level


  • A food web is a network of interconnected food chains
  • A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
  • A consumer is an organism that gets its energy by feeding on other organisms. Consumers may be classed as primary, secondary, and tertiary according to their position in a food chain
  • A herbivore is an animal that gets its energy by eating plants
  • A carnivore is an animal that gets its energy by eating other animals
  • A decomposer is an organism that gets its energy from dead or waste organic material

Why there is Greater Efficiency in Supplying Plants as Human Food, and that there is a Relative Inefficiency in Feeding Crop Plants to Livestock that will be used as Food

  • When we eat meat, eggs, or cheese, or drink milk, we are feeding further along the food chain.
  • There is less energy available for us from the original source
  • It will be more efficient to eat grass in the field rather than letting cattle eat it

Food Pyramids of Numbers and Biomass

  • A food pyramid shows the relative sizes of different components at the various trophic levels of a food chain
  • There are three types of ecological pyramids we use: numbers, biomass, and energy
  • In a food pyramid, each trophic level in a food chain is represented by a horizontal bar, with the width of the bar representing the number of organisms, the amount of biomass or the amount of energy available at that level
  • The base of the pyramid represents the producer, the second level is the primary consumer, and so on
  1. Pyramids of Numbers
    • A pyramid of numbers shows the relative number of organisms at each stage of a food chain
    • Example: Clover → snail → thrush → hawk
  2. Pyramids of Biomass
    • Sometimes a pyramid of numbers is not the nest way to represent a food chain.
    • In this case a pyramid of biomass (the dry mass of an organism) is a better diagram to use
    • It shows the total mass of organisms ar each stage of a food chain
    • In general, all producers have a higher biomass than the primary consumers, so a pyramid will always be produced
    • The total energy (and biomass) present at a lower tier of the pyramid, must be greater than the higher tiers in order to support the energy requirements of the subsequent organisms

The Advantages of using a Pyramid of Biomass rather than a Pyramid of Numbers to Represent a Food Chain

  • A pyramid of biomass is better than a pyramid of numbers because when you compare masses, it gives us a better idea of the amount of energy that is available at each trophic level
  • In a pyramid of biomass, it shows the amount of energy that is lost through each step
  • This is the amount each trophic level decreases in comparison to the previous

Nutrient Cycles


  • Most of the chemicals that make up living tissue contain carbon
  • When organism die, the carbon is recycled so that it can be used by future generations
  • Four main processes are involved: photosynthesis, respiration, decomposition, and combustion
  • Carbon enters the atmosphere as CO2 from respiration and combustion
  • CO2 is absorbed by the producers to make carbohydrates in photosynthesis
  • Animals feed on the plant passing the carbon compounds along the food chain
  • Most of the carbon they consume is exhaled as CO2 formed during respiration
  • The animals and plants eventually die
  • The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as CO2
  • In some conditions, decomposition is blocked
  • The plant and animal material may then be available as fossil fuel in the future for combustion

The Effects of the Combustion of Fossil Fuels and the Cutting Down of Forests on the Carbon-dioxide Concentrations in the Atmosphere

  • Photosynthesis takes carbon-dioxide out of the atmosphere and replaces it with oxygen
  • Respiration and combustion use up oxygen from the atmosphere and replace it with carbon-dioxide
  • When fossils are burnt, the carbon in them combines with oxygen from the air, and forms carbon-dioxide. This process is called combustion
  • Combustion of fossil fuels is thought to be having an effect on the balance of carbon-dioxide
  • The extra carbon-dioxide may be causing the percentage of carbon-dioxide in the air to increase
  • The loss of the trees may reduce the amount of photosynthesis taking place
  • As a result, the concentration of carbon-dioxide increases and oxygen decreases in the atmosphere
  • The rise in the levels of carbon-dioxide levels in the atmosphere could be dangerous as it may cause global warming


  • Energy from the sun heats the surface of the earth
  • Plants release water vapor into the air through transpiration
  • Water evaporates from oceans, rivers, lakes, and soil
  • The warm, moist air rises because it is less dense
  • Water vapor condenses into water droplets as it cools down, forming clouds
  • As water droplets get bigger and heavier, they begin to fall as rain, snow, and sleet (precipitation), draining into streams, rivers, lakes, and the seas
  • Plant roots take up water by osmosis
  • In addition, animals lose water to the environment through exhaling and sweating, and in urine and feces


  • Nitrogen is essential for the formation of amino acids to make proteins
  • The nitrogen cycle describes the ways in which nitrogen is recycled
  • The element nitrogen is a very unreactive gas
  • Plants are not able to change it into nitrogen compounds, but it is needed to form proteins
  • Nitrogen compounds become available for plants in the soil in a number of ways, including:
    • Nitrogen-fixing bacteria (some plants – legumes such as peas, beans, and clover – have roots with nodules that contain these bacteria, so the plant receives a direct source of nitrates)
    • Breakdown of dead plants and animals by decomposers (bacteria, fungi, and invertebrates)
    • The addition of artificial fertilizers, compost (decaying plant materials) and manure (decaying animal waste – urine and feces), lightning – its energy causes nitrogen to react with oxygen
  • Plants absorb nitrates into their roots by active uptake. The nitrates are combined with glucose (from photosynthesis) to form protein
  • Proteins are passed through the food chain as animals eat the plants
  • When animals digest proteins, the amino acids released can be reorganized to form different proteins
  • Some soil bacteria – denitrifying bacteria – breakdown nitrogen compounds and release nitrogen back into the atmosphere. This is a destructive process, commonly occurring in waterlogged soil
  • Farmers try to keep soil well drained to prevent this happening – a shortage of nitrates in the soil stunts the growth of crop plants
  • Nitrates and other ammonium compounds are very soluble, so they are easy leached out of the soil and can cause pollution
  • Farmers can increase the fertility (nitrogen compound concentration) of their soil by:
    • adding artificial fertilizers
    • adding manure or compost
    • growing leguminous plants, then digging the roots (with their nodules) into the soil

Population Size

  • Population is a group of organisms of one species, living in the same area, at the same time
  • Community is all of the populations of different species in an ecosystem
  • Ecosystem is a unit containing the community of organisms and their environment, interacting together, e.g. a decomposing log, or a lake

Factors Affecting the Rate of Population Growth

Food Supply

  • Abundant food will enable organisms to breed more successfully to produce more offspring.
  • Shortage of food can result in death or force emigration, reducing the population


  • If there is heavy predation of a population, the breeding rate may not be sufficient to produce enough organisms to replace those eaten, so the population will drop in numbers
  • There tends to be a time lag in population size change for predators and their prey; as predator numbers increase, prey number drops, and as predator numbers drop, prey numbers rise again (unless there are other limiting factors)


  • This is a particular problem in large populations, because disease can spread easily from one individual to another
  • Epidemics can reduce population sizes very rapidly

Population Growth in n Environment with Limited Resources

  • When a limiting factor influences population growth, a sigmoid (S-shaped) curve is created
  • You need to be able to place the terms, lag, log, stationary, and death phase on a graph of population growth
  • Lag Phase: The new population takes time to settle and mature before breeding begins. When this happens, a doubling of small numbers does not have a big impact on the total populations’ size, so the line of the graph rises only slowly with time
  • Log (Exponential) Phase: There are no limiting factors. Rapid bredding in an increasing population causes a significant increase in numbers, A steady doubling in numbers per unit of time produces a straight line
  • Stationary Phase: Limiting factors, such as shortage of food, cause the rate of reproduction to slow down and there are more deaths in the population. When the birth rate and death rate are equal, the line of the graph becomes horizontal
  • Death Phase: As food runs out, more organism die than are born, so the number in the population drops

Population Growth in the Absence of Limiting Factors

  • If there is no limiting factors, there will be no stationary or death phase – the log phase will continue upwards, instead of the line leveling off
  • This has happened with human populationgrowth
  • Human population size has increased exponentially because of improvements on food supply, and the development of medicine to control diseases
  • Infant mortality has decreased, while life expectancy has increased
  • Such a rapid increase in population size has social implications:
    • increased demand for basic resources (food, water, space, medical care, fossil fuels)
    • increased pressures on the environment (more land needed for housing, growing crops, road buildings, more wood for fuel, and housing), and more pollution
    • larger population of young people → greater demands on education. Moore old people → greater demands on healthcare
    • Abundant food supplies can lead to more people becoming obese → greater demands on healthcare (heart disease, diabetes, blindness). In the long term it reduces average life expectancy, as poor health becomes a limiting factor