• define the term ecosystem

Ecosystem – all living organisms and non-living components in a specific habitat, and the interrelationships between them.

Habitat – the place where an organism lives.

Population – all of the organisms of one species , who live in the same place at the same time, and can breed together.

Community – all the organisms of different species, who live in the same place at the same time, and can interact with each other.

Niche – the role that each species plays in an ecosystem.

Autotroph – an organism that uses light energy to synthesise its own complex organic molecules.

Heterotroph – an organism that consumes complex organic molecules to gain nutrients, obtaining energy.

  • state that ecosystems are dynamic systems

Ecosystems are dynamic systems because the population rises and falls due to the interactions of living organisms between each other and with the physical environment. Any small changes in one thing can affect the others. For example, if a predator’s population size goes up, the population size of the prey will go down.

  • define the term biotic factor and abiotic factor, using named examples

Biotic factors – the effects of living organisms, e.g. food supply, predation, disease, competition.

Abiotic factors– the effects of non-living components, e.g. temperature, pH of soil, soil type, light intensity, oxygen concentrations, carbon dioxide concentrations.

  • define the term producer, consumer, decomposer and trophic levels

  • describe how energy is transferred through ecosystems

Food chains show how energy is transferred from one organism to another. Different food chains join together to make a food web, which helps us understand how energy flows through the whole ecosystem. The arrows in a food chain show the direction of energy transfer.

  • discuss the efficiency of energy transfers between trophic levels

Energy is lost at each trophic level and is unavailable to the next trophic level. Energy is used for respiration which is lost through heatenergy. The energy is stored in dead organisms and waste material which can only be accessed by decomposers. Because of this, there is less living tissue (biomass) at higher levels of a food chain. There are always less consumers as the pyramid gets higher due to energy loss at each trophic level.

  • outline how energy transfers between trophic levels can be measured


  • explain how human activities can manipulate the flow of energy through ecosystems

  • describe one example of primary succession resulting in a climax community

Succession – a change in a habitat causing a change in the make-up of a community.

Succession on Sand Dunes:

  1. Pioneer plants like sea rocket(Cakilemaritima)and prickly sandwort (Salsola kali) colonise the sand just above the high water mark – tolerate salt water, lack of freshwater and unstable sand.
  2. Wind-blown sand builds up around the base of these plants, forming a ‘mini’ sand dune. As plants die and decay, nutrients accumulate in this mini sand dune. As the dune gets bigger, plants like sea sandwort(Honkenyapeploides) and sea couch grass(Agropyronjunceiforme) colonise it, which have long roots to stabilise it in the sand.
  3. With more stability and accumulation of more nutrients, plants like sea spurge(Euphorbaparalias) and marram grass(Ammophilaarenaria)start to grow. Marram grass traps sand, as the sand accumulates, the shoots grow taller to stay above the growing dune trapping more sand.
  4. As the sand dune and nutrients build up, other plants colonise the sand, such as hare’s foot clover(Trifoliumarvense)and bird’s foottrefoil(Lotuscorniculatus). These have bacteria in their root nodules to convert nitrogen into nitrates. With nitrates available, more species like sand fescue(Festucarubra) and viper’sbugloss(Echiumvulgare) colonise the dunes, stabilising the dunes further.

  • describe how the distribution and abundance of organisms can be measured, using line transects, belt transects, quadrats and point quadrats

A transect is a line taken across a habitat. You stretch a tape measure across the habitat and take samples along the line.  You can use a:

  • Line Transect – recording each organism which is touching the line at suitable, regular intervals.
  • Belt Transect – placing a quadrat against the line, recording its contents, then placing the next quadrat immediately touching the first one, repeating this along the transect.
  • Interrupted Belt Transect – placing quadrats at regular intervals along the transect.


Abundant 80-100%
Common 60-80%
Frequent 40-60%
Occasional 20-40%
Rare 0-20%

A quadrat is a square frame used to define the size of the sample area. It’s important to choose the right size of the quadrat (normally 50cm or 1m quadrats are used) depending on the size of the area. The quadrat is placed randomly and the abundance is measures. You could:

  • Count the number of individuals of each species.
  • Estimate the percentage cover of each species – this is the proportion of the area within the quadrat which it occupies.
  • Use an abundance scale, such as the ACFOR scale, by estimating which one of these best describes the abundance of each species within the quadrat.

A point quadrat may be used. This is a frame holding a number of long needles or pointers. You lower the frame into the quadrat and record any plant touching the needles. It can also be useful for measuring the height of plants.

Population size = mean no. of individuals of the species in each quadrat

fraction of the total habitat are covered by a quadrat

It’s important to decide where to place the quadrats, how many samples to take, and how big they should be:

  • Use a sample which is representative of the whole habitat: randomly position the quadrats across the habitats, using random numbers to plot coordinates or take samples at regular distances across the habitat.
  • To work out how many quadrats are needed: do a pilot study – take random samples from across the habitat and make a cumulative frequency table. Plot cumulative frequency against quadrat number. The point where the curve levels off tells them that the minimum number of quadrats to use. Ecologists often double
  • To work out how big your quadrats should be: count the number of species you find in larger quadrats. Plot quadrat area on the x-axis, against the number of species you find in each one on the y-axis. Read the optimalquadrat size at the point where he curve starts to level off.
  • describe the role of decomposers in the decomposition of organic material

Decomposers, such as bacteria and fungi, break down dead and waste organic material. Bacteria and fungi feed saprotrophically so are called saprophytes. They secrete enzymes onto dead and waste material. The enzymes digest the material into small molecules, which are then absorbed into the organisms body. Having been absorbed, the molecules are stored or respired to release energy.

If bacteria and fungi did not break down dead organisms then energy and valuable nutrients would remain trapped in the dead organisms. Microbes get a supply of energy to stay alive, and the trapped nutrients are recycled.

  • describe how microorganisms recycle nitrogen within ecosystems (Only Nitrosomonas, Nitrobacter and Rhizobium need to be identified by name)

Nitrogen gas is very unreactive, so is impossible for plants to use it directly. Nitrogen is needed to make proteins and nucleic acids. Plants need fixed nitrogen as ammonium ions (NH4+) or nitrate ions (NO3). Bacteria is involved in the recycling of nitrogen.

Nitrogen Fixation:

  • Nitrogen fixation can occur when lightning strikes or through the Haber process – only 10% of total nitrogen fixation.
  • Nitrogen-fixing bacteria live freely in the soil and fix nitrogen using it to make amino acids. Nitrogen-fixing bacteria, such as Rhizobium, also live inside the root nodules of legumes (bean plants).
  • They have a mutualistic relationship with the plant, fixing nitrogen and receiving carbon compounds such as glucose in return.
  • Proteins, such as leghaemoglobin, in the nodules absorb oxygen to keep the conditions anaerobic. Under these conditions, the bacteria use nitrogen reductase to reduce nitrogen gas to ammonium ions (NH4+) that can then be used by the host plant.


  • Nitrification happens when chemoautotrophic bacteria in the soil absorb ammonium ions.
  • Ammonium ions (NH4+) are released by bacteria involved in putrefaction of proteins found in dead or waste organic matter.
  • Chemoautotrophic bacteria obtains energy by oxidising ammonium ions (NH4+) to nitrites (NO2) (Nitrosomonas bacteria), or by oxidising nitrites (NO2) to nitrates (NO3) (Nitrobacter bacteria).
  • As oxidation requires oxygen, these reactions only happen in well-aerated soils.
  • Plants need nitrates to make amino acids, proteins, enzymes, DNA, RNA, etc.


  • Other bacteria convert nitrates back to nitrogen gas.
  • When the bacteria grows under anaerobic conditions, such as waterlogged soils, they use nitrates (NO3) as a source of oxygen for their respiration and produce nitrogen gas (N2) and nitrous oxide (N2O).