An ecosystem is all the organisms living in a particuar area and all the abiotic factors


Energy is transferred through ecosystems

  • The main route by which energy enters an ecosystem is photosynthesis.
  • During photosynthesis plants convert sunlight energy into a form that can be used by other organisms - plants are producers (because they produce organic molecules using sunlight energy).
  • Energy is transferred through the living organisms of an ecosytem when organisms eat other organisms. Producers are eaten by organisms called primary consumers, who are then eaten by secondary consumers, then tertiary consumers etc.
  • Each of the stages are called trophic levels.
  • Food chains and food webs show how energy is transferred through an ecosystem.
  • Food chains show simple lines of energy transfer whereas food webs show lots of food chains in an ecosystem and how they overlap.
  • Energy locked up in things that can't be eaten gets recycled back into the ecosystem by microorganisms called decomposers - they break down dead or undigested material.
1 of 14

Not all energy gets transferred to the next trophi

  1. Not all the energy that's available to the organisms in a tropic level is transferred to the next trophic level - around 90% of the total energy available is lost in various ways.
  2. Some of the available energy (60%) is never taken in by the organisms in the first place. E.g.
  • Plants can't use all the light energy that reaches their leaves as some is the wrong wavelength, some is reflected and some passes straight through the leaves.
  • Some sunlight can't be used because it hits parts of the plant that don't photosynthesise.
  • Some parts of food aren't eaten by organisms so the energy isn't taken in.
  • Some parts of food are indigestible so pass through organisms and come out as waste.

      3. The rest of the available energy (40%) is taken in (absorbed) this is called the gross productivity. But not           all of this is available to the next trophic level either.

  • 30% of the total energy available is lost to the environment when organisms use energy produced from respiration for movement or body heat. This is called respiratory loss.
  • 10% of the total energy available becomes biomass - this is called the net productivity.

      4. Net productivity is the amount of energy that's available to the next tropic level.

2 of 14

Productivity and energy transfer between trophic l

The net productivity of a trophic level can be calculated when you know the gross productivity and the respiratory loss of the trophic level.

  • net productivity = gross productivity - respiratory loss

You can also calculate how efficent energy transfer is between trophic levels.

  • % efficiency of energy transfer between trophic levels = (net productivity of a level/net productivity of previous level) x100

Primary productivity can also be calculated. When you're talking about producers, net productivity is called net primary productivity (NPP) and gross productivity is called gross primary productivity (GPP).

  • NPP = GPP - plant respiration
3 of 14


Habitat - the place where an organisms lives.

Population - all the organisms of one species in a habitat.

Population size - the number of individuals of one species in a particular area.

Community - populations of different species in a habitat make up a community.

Abiotic factors - the non-living features of an ecosystem.

Biotic factors - the living features of the ecosystem.

Abundance - the number of individuals of one species in a particular area (the same as population size).

Distribution - where a species is within a particular area.

4 of 14

Population size varies because of abiotic factors.

The population size of any species varies because of abiotic factors. When abiotic conditions are ideal for a species, organisms can grow fast and reproduce successully.

  • For example, when the temperature of a mammal's surroundings is the ideal temperature fro metabolic reactions to take place, they don't have to use up as much energy maintaining their body temperature. This means more energy can be used for growth and reproduction, so their population size will increase.

When abiotic conditions aren't ideal for a species, organisms can't grow as fast or reproduce as successfully.

  • For example, when the temperature of a mammal's surroundings is significantly lower or higher than their optimum body temperature, they have to use a lot of energy to maintain the right body temperature. This means less energy will be available for growth and reproduction, so their population size will decrease.
5 of 14

Populations size varies because of Biotic factors.

Interspecific competition.

  1. Interspecific competition is whe  organisms of different species compete with each other for the same resources.
  2. Interspecific competition between two species can mean that the resources available to both populations are reduced. This means both populations will be limited by a lower amount of food. They'll have less energy for growth and reproduction, so the populations size will be lower for both species.

Intraspecific competition.

  1. Intraspecific competition is when organisms of the same species compete with each other for the same resources.
  2. The population of a species increases when resources are plentiful. As the population increase, there'll be more organisms competeing for the same amount of space and food.
  3. Eventually, resources such as food and space become limiting - there isn't enough for all the organisms. The population then begins to decline.
  4. A smaller population then means that there's less competition for space and food, which is better for growth and reproduction - so the population starts to grow again.
  5. The maximum stable population size of a species that an ecosystem can support is called the carrying capacity.


  1. Predation is where an organism kills and eats another organisms. The population sizes of predators and prey are interlinked - as the population of one changes, it causes the other population to change.
  2. As the prey population increases, there's more food for predators, so the predator population grows.
  3. As the predator population grows, more prey is eaten so the prey population then begins to fall.
  4. This means there's less food for the predators, so their population decreases, and so on.
6 of 14

Distribution varies

Organisms can only exist where the abiotic factors they can survuve in exist.

  1. Some plants only grow on south-facing slopes in the northern hemisphere because that's where solar input is greatest.
  2. Some plants don't grow near to the shoreline because the soil is too saline.
  3. Large trees can't grow in polar regions because the temperature is too low.

Biotic Factors:

Interspecific competiton can affect the distribution of species. If two species are competiting but one is better adapted to its surroundings than the other, the less well adapted species is likely to be out-competed - it won't be able to exist alongside the better adapted species.

7 of 14

Every species occupies a different niche.

A niche is the role of a species within its habitat. It includes:

  • its biotic interactions
  • its abiotic interactions

Every species has its own unique niche - a niche can only be occupied by one species.

It may look like two species are filling the same niche but there'll be slight differences.

The abundance of different species can be explained by the niche concept - two species occupying similar niches will compete, so fewer individuals of both species will be able to survive in the area.

The distribution of different specie can also be explained by the niche concept - organisms can only exist in habitat where all the conditions that make up their role exist.

8 of 14

Investigating populations of organisms

Investigating populations of organisms involves looking at the abundance and distribution of species in a particular area.

A random sample needs to be taken from the area being investigated.

  1. Choose an area to sample - a small area within the area being sampled.
  2. Samples should be random to aviod bias (e.g. random number generator)
  3. Use an appropriate technique to take a sample pf the population.
  4. Repeat the process, taking as many samples as possible. This gives a more reliable estimate for the whole area. (If your results aren't reliable then your conculsions won't be valid.)
  5. The number of individuals for the whole area can then be estimated by taking an average of the data collected in each sample and multiplying it by the size of the whole area. The percentage cover for the whole area can be estimated by taking the average of all the samples.
9 of 14

Frame quadrats, point quadrats and transects

  • A frame quadrat is a square frame divided into a grid of 100 smaller squares by strings attached across the frame.
  • They're placed on the ground at random points within the area being investigated. This can be done by selecting random coordinated.
  • The number of individuals of each species is recorded in each quadrat.
  • The percentage cover of a plant species can also be measured by counting how much of the quadrat is covered by the plant species - you count a square if it's more than half-covered. Percentage cover is a quick way to investigate populations because you don't have to count all the individual plants.
  • Frame quadrats are useful for quickly investigating area with species that fit within a small quadrat - most frame quadrats are 1m by 1m.
  • Areas with larger plants and trees need very large quadrats. Large quadrats aren't always in a frame - they can be marked out with a tape measure.
  • A point quadrat is a horizontal bar on two legs with a series of holes at set intervals along its length.
  • Point quadrats are placed on the ground at random points within the area being investigated.
  • Pins are dropped through the holes in the frame and every plant that each pin touches is recorded.
  • The number of individuals of each species is recorded in each quadrat.
  • The percentage cover of a species can also be measured by calculating the number of times a pin has touched a species as a percentage of the total number of pins dropped.
  • Point quadrats are especially useful in area where there's lots of dense vegetation close to the ground.
  • Line transects - a tape measure is placed along the transectand the species that touch the tape measure are recorded.
  • Belt transects - data is collected along the transect using frame quadrats placed next to each other.
  • Interrupted transects - instead of investigating the whole transect of either a line or a belt, you can take measurements at intervals.
10 of 14

Abiotic factors can also be measured.

Climate - the weather of the region.

  • temperature is measured using a thermometer.
  • rainfall is measured using a rain gauge - a funnel attached to a measuring cyclinder. The rain falls into the funnel and runs down into the measuring cylinder. The volume of water collected over a period of time can be measured.
  • Humidity is measured using an electronic hygrometer.

Oxygen availablity - this only needs o be measured in aquatic habitats. The amount of oxygen dissolved in the water is measured using an electronic device called a oxygen sensor.

Solar input is measured using an electronic device called a light sensor.

Edaphic factors (soil conditions):

  • pH is measured using indicator liquid - a sample of the soil is mixed with the liquid that changes colour depending on the pH. Electronic pH monitors can also be used.
  • moisture content - the mass of a sample of soil is measure before and after being dried out in an oven at 80-100oC. The difference in mass as a percentage of the original mass of the soil is then calculated. This shows the water content of the soil sample.

Topography - the shape and features of the Earth's surface.

  • relief (how the height of the land changes across a surface) can be measured by taking height readings using a GPS device at different points across the surface. Maps with contours can also be used.
  • slope angle can be measured using a clinometer. A simple clinometer is just a piece  of string with a weight on the end attached to the centre of a protractor. The flat edge  of the protractor is pointed up the hill and the slope angle is read off the protractor.
  • aspect (the direction a slope is facing) is measured using a compass
11 of 14


Succession is the process by which an ecosystem changes over time. The biotic conditions change as the abiotic conditions change. There are two types of succession.

  1. Primary succession - this happens on land that's been newly formed or exposed. There's no soil or organic material to start with.
  2. Secondary succession - this happens on land that's been cleared of all the plants, but where the soil remains.
12 of 14

Succession occurs in stages called seral stages

  1. Primary succession starts when species colonise a new land surface. Seeds and spores are blown in by the wind and begin to grow. The first species to colonise the area are called pioneer species - this is the first seral stage.
  • The abiotic conditions are hostile. Only pioneer species grow because they're specialised to cope with the harsh conditions.
  • The pioneer species change the abiotic conditions - they die and microorgansisms decompose the humus. This forms a basic soil.
  • This makes conditions less hostile, basic soil retains water, meaning new organisms can move in and grow. These then die, adding more humus, making the soil deeper. Larger plants can now grow in deeper soil, which retains even more soil.
  1. Secondary succession happens in the same way, but because there's already a soil layer succession starts at a later seral stage - the pioneer species in secondary succession are the larger plants.
  2. At each stage, different plants and animals that are better adapted for the improved conditions move in, out-compete the plants and aminals already there, and become the dominant species in an ecosystem.
  3. As succession goes on, the ecosystem becomes more complex. New species move in alongside existing species meaning the species diversity increases.
  4. The final seral stage is called the climax community - the ecosystem is supporting the largest smf most complex community of plants and animals it can. it won't change much more - it's in a steady state.
13 of 14

Climax communities and prevention.

Which species make up the climax community depends on what the climate's like in an ecosystem. The climax community for a particlar climate is called it climatic climax.

  • In a temperate climate there's plenty of available water, mild temperatures and not much change between the seasons. The climatic climax will contain larger trees because they can grow in these conditions once deep soils have devloped. In a polar climate there's not much available water, temperatures are low and there are massive changes between the seasons. Large trees won't ever grow here, so the climatic climax only contains herbs or shrubs, but it's still the climax community.

Human activities can prevent succession, stopping the normal climax community from developing. When succession is stopped artifcially like this, the climax community is called a plagioclimax.

  • A regularly mown grassy field won't develop woody plants, even if the climate of the ecosystem could support them. The growing points of the woody plants are cut off by the lawnmower, so larger plants can't establish themselves.
14 of 14


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Ecology, ecosystems and environmental biology resources »