CIE Categories Archives: 19. Organisms and their Environment

19.4) Population size

19.4) Population size


Population: is a group of organisms of one species, living and interacting 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, eg. a decomposing log or a lake.


Factors affecting the rate of population growth for a population of an organism:

  • Food supply
  • Predation
  • Disease – when a disease spreads globally it is called a pandemic.


Human population:

  • About 20 years ago, the human population was increasing at the rate of 2% a year, this means the world population was doubling every 35 years.
  • This doubles for demand for food, water, space and other resources.
  • Infant mortality: the death rate for children less than 1 year old.
  • Life expectancy: the average age to which a newborn baby can be expected to live.
  • Fertility rate: the average number of children a woman would have.
  • Agricultural development and economic expansion led to improvements in nutrition, housing and sanitation, and to clean water supplies.
  • These improvements reduced the incidence of infectious diseases in the general population, and better-fed children could resist these infections when they did meet them.
  • The social changes probably affect the population growth more than did the discovery of new drugs or improved medical techniques.
  • Longer and better education: marriage is postponed and a better-educated couple will have learned about methods of family limitation.
  • Application of family planning method: either natural methods of birth control or use of contraceptives is much more common.
  • Because of these techniques – particularly immunisation -diphtheria, tuberculosis and polio are now rare, and by 1977 smallpox had been wiped out by the World Health Organization (WHO)’s vaccination campaign.


Sigmoid population growth curves:

  1. Lag phase. The population is small. Although the numbers double at each generation, this does not result in a large increase.


  1. Exponential(log) phase. Continued doubling of the population at each generation produces a logarithmic growth rate (eg. 64-128-256-512-1024). When a population of 4 doubles, it is not likely to strain the resources of the habitat, but when a population of 1021 doubles there is likely to be considerable competition for food and space and the growth rate starts to slow down.


  1. Stationary phase. The resources will no longer support an increasing population. At this stage, limiting factors come into play. The food supply may limit further expansion of the population, diseases may start to spread through the dense population and overcrowding may lead to a fall in reproduction rate. Now the mortality(death) rate equals the reproduction rate, so the population number stays the same.


  1. Death phase. The mortality rate is now greater than the reproduction rate, so the population numbers begin to drop Fewer offspring will live long enough to reproduce. The decline in population numbers can happen because the food supply is insufficient, waste products contaminate the habitat or disease spreads through the population.


Abiotic and biotic limiting factors:

  • Plant populations will be affected by abiotic ( non-biological) factors such as rainfall, temperature and light intensity.
  • Biotic (biological) factors affecting plants include their leaves being eaten by browsing and grazing animals or by caterpillars and other insects, and the spread of fungus disease.
  • Animal populations will to be limited by these factors.
  • The size of an animal population will also be affected by immigration and emigration.
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19.3) Nutrient cycles

19.3) Nutrient cycles


The carbon cycle:

  • Carbon is an element that occurs in all the compounds which make up living organisms.
  • Plants get their carbon from carbon dioxide in the atmosphere and animals get their carbon from plants.
  • The carbon cycle, therefore, is mainly concerned with what happens to carbon dioxide.

Removing CO2 from the atmosphere:

  • Plants use carbon dioxide from the atmosphere for photosynthesis.
  • The carbon becomes part of complex molecules in the plants, such as proteins, fats and carbohydrates.


Passing carbon from one organism to the next:

  • When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal.
  • Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants.
  • The carbon then becomes part of these organisms.


Returning CO2 to the atmosphere:

  • Carbon dioxide is released to the atmosphere through respiration by animals, plants and microorganisms.
  • It is also released by the combustion of wood and fossil fuels (such as coal, oil and natural gas).
  • The use of fossil fuels is gradually increasing the carbon dioxide levels in the atmosphere.
  • Decomposition or decay also releases carbon dioxide.
  • This process happens faster in warm, moist conditions with plenty of oxygen because it involves microorganisms.
  • Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.


The effects of the combustion of fossil fuels:

  • Since the Industrial Revolution, we have been burning the fossil fuels such as coal and petroleum and releasing extra CO2 into the atmosphere. As a result, the concentration of CO2 has increased from 0.029% to 0.035% since 1860.
  • Although it is not possible to prove beyond all reasonable doubt that production of CO2 and other ‘greenhouse gases’ is causing a rise in the Earth’s temperature, ie. global warming, the majority of scientists and climatologists agree that it is happening now and will get worse unless we take drastic action to reduce the output of these gases.
  • Another factor contributing to the increase in atmospheric gaseous CO2 is deforestation.
  • Trees are responsible for removing gaseous CO2 and trapping the carbon in organic molecules (carbohydrates, proteins and fats).
  • When they are cut down the amount of photosynthesis globally is reduced.
  • Often deforestation is achieved by a process called ‘slash and burn’, where the felled trees are burned to provide land for agriculture and this releases even more atmospheric CO2.


The water cycle:

  • The water cycle is also known as the hydrological cycle. It describes how water moves on, above, or just below the surface of our planet.
  • Water molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes. Water can change state.


  • Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and lakes. The warm air rises, carrying water vapour with it.


  • Transpiration from plants releases water vapour into the air.


  • The moist air cools down as it rises. Water vapour condenses back into liquid water, and this condensation process produces clouds.


  • As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet. This is called precipitation (it is not the same as precipitation in Chemistry, refer to A Lin’s notes).


The nitrogen cycle:

  • Nitrogen compounds found in cells include proteins.
  • Nitrogen from the air is converted into soluble ions that plant roots can absorb. It forms part of nitrogen compounds in the plants, and is then passed from one organism to the next.
  • It is returned to the atmosphere as nitrogen gas. This is the nitrogen cycle.
  • When a plant or animal dies, it tissues decompose, partly as a result of the action of saprotrophic bacteria
  • One of the important products of the decay of animal and plant protein is ammonia (NH3, a compound of nitrogen), which is washed into the soil. It dissolves readily in water to form ammonium ions (NH4-).
  • The excretory products of animals contain nitrogenous waste products such as ammonia, urea and uric acid.
  • Urea is formed in the liver of humans as a result of deamination.
  • The organic matter in animal droppings is also decomposed by soil bacteria.


Processes that add nitrates to soil:

Nitrifying bacteria:

  • These are bacteria living in the soil, which use the ammonia from excretory products and decaying organisms as a source of energy.
  • In the process of getting energy from ammonia called nitrification, the bacteria produce nitrates.
  • The ‘nitrite’ bacteria oxidise ammonium compounds to nitrites (NH4- ⇒ NO2-).
  • ‘Nitrate’ bacteria oxidise nitrites to nitrates (NO2- ⇒ NO3-).
  • Although plant roots can take up ammonia in the form of its compounds, they take up nitrates more readily, so the nitrifying bacteria increase the fertility of the soil by making nitrates available to the plants.


Nitrogen-fixing bacteria:

  • This is a special group of nitrifying bacteria that can absorb nitrogen as a gas from the air spaces in the soil, and build it into compounds of ammonia.
  • Nitrogen gas cannot itself be used by plants. When it has been made into a compound of ammonia, however it can easily be changed to nitrates by other nitrifying bacteria.
  • The process of building the gas, nitrogen, into compounds of ammonia is called nitrogen fixation.
  • Some of the nitrogen-fixing bacteria live freely in the soil. Others live in the roots of leguminous plants (peas, beans, clover), where they causes swellings called root nodules.
  • These leguminous plants are able to thrive in soils where nitrates are scarce, because the nitrogen-fixing bacteria in their nodules make compounds of nitrogen available for them.
  • Leguminous plants are also included in crop rotations to increase the nitrate content of the soil.



  • The high temperature of lightning discharge causes some of the nitrogen and oxygen in the air to combine and form oxides of nitrogen.
  • These dissolve in the rain and are washed into the soil as weak acids, where they form nitrates.
  • Although several million tonnes of nitrate may reach the Earth’s surface in this way each year, this forms only a small fraction of the total nitrogen being recycled.


Processes that removes nitrates from the soil:

Uptake by plants:

  • Plant roots absorb nitrates from the soil and combines them with carbohydrates to make amino acids, which are built up into proteins.
  • These proteins are then available to animals, which feed on the plants and digest the proteins in them.



  • Nitrates are very soluble and as rainwater passes through the soil it dissolves the nitrates and carries them away in the run-off or to deeper layers of the soil.


Denitrifying bacteria:

These are bacteria that obtain their energy by breaking down nitrates to nitrogen gas, which then escapes from the soil into the atmosphere.

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19.2) Food chains and food webs

19.2) Food chains and food webs


Food chain: shows the transfer of energy from one organism to the next, beginning with a producer.

Food web: is a network of interconnected food chains.

Producer: is an organism that makes it own organic nutrients, usually using energy from sunlight, through photosynthesis.

Consumer: is an organism that gets its energy from feeding on other organisms.

Herbivore: is an animal that gets its energy by eating plants.

Carnivore: is an animal that gets it energy by eating other animals.

Decomposer: is an organism that gets its energy from dead or waste organic material.


Interdependence means the way in which living organisms depend on each other in order to remain alive, grow and reproduce.

Predator is a carnivore that kills and eats other animals.

Scavengers are carnivores that eat the dead remains of animals killed by predators.

Energy is transferred between organisms in a food chain by ingestion.

  • A food chain shows what eats what in a particular habitat. It shows the flow of energy and materials from one organism to the next, beginning with a producer.
  • The arrows between each organism in the chain always point in the direction of energy flow from the food to the feeder.

A food web is a network of interconnected food chains. It shows the energy flow through part of an ecosystem.


Producers are plants that produce food.

Primary consumer are animals that eat the plants.

Secondary consumer are animals that prey on the plant-eaters.

Tertiary consumers are animals that feed on secondary consumers.

Quaternary consumer is an animal that is at the top of the food chain.

Pyramid of biomass:

  • Biomass is the total dry mass of one animal or plant species in a food chain or food web.
  • A pyramid of biomass shows the biomass at each trophic level, rather than the population.
  • Nearly always the correct pyramid shape.
  • more accurate indication of how much energy is passed on at each trophic level.



The effect of over-harvesting:

Over-harvesting causes the reduction in numbers of a species to the point where it is endangered or made extinct. As a result biodiversity is affected.



Introducing foreign species to a habitat:

  • An example of this process was the accidental introduction of rats to the Galapagos Islands.
  • The rats had no natural predators and food was plentiful: they fed on the eggs of bird, reptiles and tortoises along with young animals.
  • The galapagos Islands provide a habitat for many rare species, which became endangered as a result of the presence of the rats.


Energy transfer:

Trophic level: is the position of an organism in a food chain, food web or pyramid of numbers or biomass.

  • Energy decreases as it moves up trophic levels because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms from the next level.
  • 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.
  • The transfer of energy from primary to secondary consumers of probably more efficient, since a greater proportion of the animal food is digested and absorbed that is the case with plant material.
  • It is very unusual for food chains to have more than five trophic levels because, on average, about 90% of the energy is lost at each level.

  • Short food chains are more efficient than long ones in providing energy to the top consumer. On the right side are two food chains and energy values for each level in them. Both food chains have a human being as the top consumer.
  • Ten times more energy is available to the human in the second food chain than in the first.
  • Some farmers try to maximize meat production by reducing movement of their animals (keeping them in pens or cages with a food supply) and keeping them warm in winter. This means less stored energy is wasted by the animals.


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19.1) Energy flow

19.1) Energy flow


Nearly all living things depend on the Sun to provide energy. This is harnessed by photosynthesising plants and the energy is then passed through food chains.


  • With the exception of atomic energy and tidal power, all the energy released on Earth is derived from sunlight.
  • The energy released by animals comes, ultimately, from plants that they or their prey eat and the plants depend on sunlight for making their food.
  • Photosynthesis is a process in which light energy is trapped by plants and converted into chemical energy (stored in molecules such as carbohydrates, fats and proteins).
  • Since all animals depend, in the end, on plants for their food, they therefore depend indirectly on sunlight.
  • Eventually, through one process or another, all the chemical energy in organisms is transferred to the environment.
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