B3.1 Systems in balance – how do different species depend on each other?


A group of organisms that can breed together and produce fertile offspring is called a SPECIES. All things rely on their environment and other species to survive. The individuals in a species are adapted to living in their environment – ADAPTATIONS of living organisms to their environment increases the species’ chance of survival by making it more likely that individuals will survive to reproduce. If an organism reaches sexual maturity, it is more likely to pass on its genes, including the genes coding for its adaptations, to the next generation.

For example – POLAR BEAR they are well adapted for survival in the Arctic. They have:

  • A white appearance, as camouflage from prey on the snow and ice
  • Thick layers of fat and fur, for insulation against the cold
  • A small surface area to volume ration, to minimise heat loss
  • A greasy coat which sheds water after swimming


When the same RESOURCES (E.g. shelter, food, water, light availability, etc) are needed by different organisms in the same habitat then there is COMPETITION. The organisms that are most successful at competing survive and pass on those genes that code for adaptations – SURVIVAL OF THE FITTESET.

FOOD WEBS show how all the food chains in a given habitat are interrelated. It shows how the loss of one organism has an effect of other organisms in the food web. All the organisms in a food web are dependent on other parts of the web – this is called INTERDEPENDENCE. This means changes affecting one species in a food web could have an impact on other species that are part of the same food web.


Change in a n environment may cause the species to become extinct, for example if:

  • The environmental conditions change beyond its ability to adapt
  • A new species that is a competitor, predator or disease organism of that species is introduces
  • Another species (animal, plants or microorganisms) in its food web becomes extinct

Nearly all organisms are ultimately dependant on energy from the SUN. Energy from the sun enters the food chain when green plants absorb light in order to PHOTOSYNTHESISE. By the process of photosynthesis, organic compounds like glucose are made from carbon dioxide and water using this energy. Plants only absorb a small percentage of the sun’s energy for this process; this energy is stored in the chemicals which make up the plants’ cells.

Energy is transferred between organisms in an ecosystem:

  • When organisms are eaten
  • When dead organisms and waste materials are fed on by decay organisms. There are two types of decay organism:
    • DECOMPOSERS such as bacteria and fungi break down the dead materials and use the energy stored inside.
    • DETRITIVORES include animals such as earthworms and woodlice. These consume the DETRITUS (dead plants or animals and their waste), breaking it down into smaller particles that other detritivores and decomposers can use

In a food chain only around 10% of the energy is passed on to the next level. A large proportion of the energy is:

  • Loss to the environment as heat
  • Excreted as waste products
  • Trapped in indigestible materials such as bones, cellulose and fur

As less energy is transferred at each level of the food chain, the number of organisms at each level gets smaller.

The percentage of energy efficiency can be calculated using the formula

CARBON is a vital element for living things. It is used in all organic molecules, including sugars, proteins and amino acids. Carbon is recycled through the environment do that it is available for life processes. This can be seen in the CARBON CYCLE:

Plants and animals need NITROGEN to make proteins but they cannot get nitrogen directly from the air because, as a gas nitrogen is fairly unreactive. Plants are able to take up nitrogen compounds such as nitrates and ammonium salts from the soil.

Nitrogen, like carbon, has to be recycled to ensure that it is available for life processes. This can be seen in the NITROGEN CYCLE:


Nitrogen is recycled through the environment in the process of:

  • Nitrogen fixation (making nitrogen in the air) to form nitrogen compounds including nitrates
  • Conversion of nitrogen compounds to protein plants and animals
  • Transfer of nitrogen compounds through food chains
  • Excretion, death and decay of plants and animals resulting in release of nitrates into the soil
  • Uptake of nitrates by plants
  • Denitrification – this is where, in the absence of oxygen, denitrifying bacteria converts’ nitrates in dead plants and animal remains back into nitrogen gas. This completes the cycle, releasing nitrogen back into the atmosphere.



Biologists can measure changes in the environment by using indicators. These may be NON-LIVING or LIVING.


  • Nitrate levels can be measured using test kits with chemicals that change colour. The chemicals can then be matched against a chart indicating the amount of nitrate present in the sample
  • Temperature can be measured using a thermometer, or a data-logger, which is more accurate and reliable
  • Carbon dioxide levels can be measured using data-loggers


  • In the oceans, phytoplankton (plant) are useful for detecting the effect of temperature changes and for detecting changes in the food web
  • Lichens grow very slowly and are vulnerable to atmospheric pollutants and acid rain. A decline in their number can indicate pollution
  • River organisms, such as the larvae of mayfly (mayfly nymphs) can be used to indicate the quality of water. Mayfly nymphs can only live in clean river water with enough oxygen water. If a river has mayfly nymphs, then pollution levels will be low