4.1 – Communities and Ecosystems

4.1 – Communities and Ecosystems

4.1.1 – Define species, habitat, population, community, ecosystem and ecology

Species – A group of organisms that can interbreed and produce fertile offspring.

They are a group of individuals of common ancestry that closely resemble each other and that are normally capable of interbreeding to produce fertile offspring.

Habitat – The environment in which a species normally lives or the location of a living organism.

If this area is extremely small, we call it a microhabitat, such as the crevices in the bark of a tree in which some insects live. Conditions in a microhabitat are different to those of the surrounding habitat.

Population – A group of organisms of the same species who live in the same area at the same time.

The members of a population have a high chance of interbreeding, assuming the species concerned reproduces sexually. The boundaries of populations are often hard to define.

Community – A group of populations living and interacting with each other in the same area.

Ecosystem – A community and its abiotic environment.

It is a stable, settled unit of nature consisting of a community of organisms, interacting with each other and with their surrounding physical and chemical environment. These vary greatly in size.

Ecology – The study of relationships between living organisms and their environment

4.1.2 – Distinguish between autotroph and heterotroph

Autotroph – An organism that synthesizes its organic molecules from simple inorganic substances

Heterotroph – An organism that obtains organic molecules from other organisms

4.1.3 – Distinguish between consumers, detritivores and saprotrophs

Consumers – An organism that ingests other organic matter that is living or recently killed

Detritivore – An organism that ingests non-living organic matter, also known as a decomposer.

Saprotroph – An organism that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion

4.1.4 – Describe what is meant by a food chain, giving three examples, each with at least three linkages (four organisms)

A food chain is a representation of the relationships between organisms based on their diet. A → B indicates that A is ‘eaten’ by B (that is, the arrow indicates the direction energy flow). Each food chain should include a producer and consumers, but not decomposers. Named organisms at either species or genus level should be used. Common species names can be used instead of binomial names. However, general names such as ‘tree’ or ‘fish’ should not be used.



4.1.5 – Describe what is meant by a food web

A food web is a diagram that shows how food chains are linked together in to more complex feeding relationships. Advantages of a food web include:

Advantages of a food web include:

  • Shows the much more complex interactions between species within a community or ecosystem
  • There is more than one producer supporting a community, which this shows
  • It shows that a single producer can be a food source for a number of primary consumers
  • A consumer may have a number of different food sources on the same or different trophic levels
  • A consumer can be an omnivore, feeding as a primary consumer and at also at higher trophic levels

Food webs, as they are very detailed and complicated, often reflect the interest of its author. Species of interest are detailed by name, whereas less important or interesting species are grouped into a large family.

4.1.6 – Define trophic level

The trophic level of an organism defines the feeding relationship of that organism to other organisms in a food chain. In a food web, a consumer can occupy a number of different trophic levels, depending on which organism is the prey.


4.1.7 – Deduce the trophic level of organisms in a food chain and a food web

Looking at the food web, assess the trophic level by looking at how many previous organisms it feeds off. You should also be able to identify them with the levels of producer, primary consumer, secondary consumer, and so on. The terms herbivore and carnivore are not always applicable.

4.1.8 – Construct a food web containing up to 10 organisms, using appropriate information

Where possible, identify the trophic levels for each organism.


4.1.9 – State that light is the initial energy source for almost all communities

To maintain food chains, food webs, communities and all their interactions, energy is required. Sunlight is the source of this energy for most communities, both aquatic and terrestrial. The principle trap of sunlight energy is the protein molecule chlorophyll, found in the chloroplasts of producers cells, mainly green plants.

4.1.10 – Explain the energy flow in a food chain

Energy losses between trophic levels include material not consumed or material not assimilated, and heat loss through cell respiration. Essentially the loss of heat from respiration

Photosynthesis converts light into energy. Not all solar energy will come into contact with chlorophyll and will therefore not be trapped in the synthesis of organic compounds.

Death and the consumption of dead organisms by detritivores, or as food not assimilated because of incomplete digestion.

Energy loss can occur in undigested food, which is used by saprophytes (decomposers), or in the reactions of respiration. Ultimately, all energy will be lost as heat.


4.1.11 – State that energy transformations are never 100% efficient

The transfer of energy from one trophic level to the next in inefficient. Only 10-20% of the energy on one trophic level will be assimilated at the next higher level.

In extreme environments like the arctic, the initial trapping of energy by producers is low, making the food chains much shorter. Likewise, in tropical rainforests, where the trapping of energy is more efficient, the food chains are longer, and the food webs are more complex.

This explains why larger predators at the top of the food chain are so rare. The energy loss throughout the food chain means that the number of organisms decreases at each step. In the higher trophic levels, organisms become less and less common. They are also more prone to extinction as they rely on the organisms below them. Any decrease in numbers in these organisms cause a chain reaction, resulting in possible extinction.

4.1.12 – Explain reasons for the shape of pyramids of energy

A pyramid of energy shows the flow of energy from one trophic level to the next in a community. The units of pyramids of energy are, therefore, energy per unit area per unit time, such as kJ m-2 yr-1

In a typical pyramid of energy, the initial solar energy is not shown. The narrowing shape shows the gradual loss of energy as you move up the food chain to the higher trophic levels. The scale to which it is drawn (energy/area/unit time) is written at the base of the pyramid.


4.1.13 – Explain that energy enters and leaves ecosystems, but nutrients must be recycled

Energy Flows

At every trophic level, energy is lost as heat. The narrowing of the energy pyramid shows that all energy is eventually radiated into space as heat.

Matter Cycles

New matter is not created, nor is it lost the way energy is. Instead, producers (autotrophs) take organic molecules and convert them into organic compounds, helping them to be recycled and re-used. Consumers then take in this organic matter as they feed and use it for their own growth. Such cycles of matter include the carbon, nitrogen and oxygen cycles.


4.1.14 – State that saprotrophic bacteria and fungi (decomposers) recycle nutrients

Nutrients, unlike energy, are not lost, but are recycled and re-used. Decomposers (saprotrophic bacteria and fungi) recycle organic molecules (nutrients) found in dead organisms. This is a complex process, serving many functions. These include the formation of soil, recycling nutrients stored in organic molecules, and reduction of high energy carbon compounds. Mineral elements are absorbed by plants as ions from the soil solution. This cycling process is called biogeochemical cycles, in which all essential elements take part. The biological process of decomposition begins when saprotrophic bacteria and

The biological process of decomposition begins when saprotrophic bacteria and fungi secrete extra-cellular digestive enzymes onto the dead organism. The enzymes hydrolyse the biological molecules that the dead organism is composed of. The hydrolysed molecules are soluble, and are absorbed by the fungi or bacteria. Organic molecules are oxidised to release carbon dioxide back into the atmosphere, and nitrogen in the form of nitrate, nitrite and ammonium. This also produces energy for the saprophyte, but returns in the various forms of matter to the abiotic environment.