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Living World - Background to ecosystems

An ecosystem is a natural system that comprises a community of plants and animals that interact with each other and their physical environment. There are often complex relationships that exist in ecosystems, between the non-living elements (soils, rocks, water, sunlight etc.) and the living elements (plants, animals, bacteria etc.).

Key terms

  • Consumer – Creature that eats herbivores and/or plant matter.
  • Decomposer – An organism such as a bacterium or fungus, that breaks down dead tissue, which is then recycled to the environment.
  • Food chain – The connections between different organisms (plants and animals) that rely upon one another as their source of food.
  • Food web – A complex hierarchy of plants and animals relying on each other for food.
  • Nutrient cycling – A set of processes whereby organisms extract minerals necessary for growth from soil or water, before passing them on through the food chain − and ultimately back to the soil and water.
  • Producer – An organism or plant that is able to absorb energy from the sun through photosynthesis.

We can look at ecosystems at different scales.  A back garden could be classified as a local ecosystem, as could a pond.  Larger ecosystems could be ranges of sand dunes, your local woodland or forest, or a lake.  At a global scale we look at ecosystems as biomes, and these include tropical rainforests, deserts and tundra environments. Biomes have common characteristics, such as buttress roots in rainforests, but not necessarily exactly the same species of plants and animals within them.

Small scale ecosystems – Freshwater ponds
Consider a local ecosystem.  In this case we will consider a FRESHWATER POND.

Freshwater Pond

You may have a freshwater pond in your back garden or in your school or the local park.   They contain a VARIETY of habitats for plants and animals;
• Animals and plants living in deeper water at the bottom of the pond will have less light and Oxygen to cope with and ADAPT to.
• Living things at the edges of a pond (the margins) have more light and Oxygen, but also have to cope with more wind etc.

We can CLASSIFY living organisms in an ecosystem as either PRODUCERS or CONSUMERS. PRODUCERS generally use energy from the environment, such as from the sun, and convert it into sugars (or glucose).  Plants do this in abundance, using PHOTOSYNTHESIS to use sunlight to synthesize nutrients from carbon dioxide and water. CONSUMERS then get their energy by eating the producers for their sugars. In our pond example, reeds living at the edge of the pond and water lilies would be our producers, and pond snails are a consumer as they eat the pond plants.


Ecosystems therefore have LINKS within them as energy flows from one food source to another.  There is a clear TROPHIC PYRAMID composed of many primary producers, a smaller number of primary consumers, and even smaller number of secondary consumers and a tiny number of tertiary consumers.  From this view point the primary producers are very important as the support the whole pyramid.
We can look at the flows in a basic LINEAR way along food chains – food moves up the line from producer to tertiary consumer.

Pond food chain

However, this is too simplistic.  Animals might eat many other plants and animals, not have just one source of food.  It is better to consider the flows as a FOOD WEB; that considers all of the connections between the plants and animals within an ecosystem like a pond.

Pond food web

All ecosystems are also highly reliant upon DECOMPOSERS and SCAVENGERS. These are organisms that break down dead material in an ecosystem and return the nutrients from them into the soil.  These nutrients can then be used by other plants for further GROWTH. Decomposers are often bacteria and fungi, but could also be types of insect like dung beetles, worms that we find in compost bins, or the rat tailed maggot found in ponds.
Nutrients then, are the food that plants need for growth, and include Potash, Nitrogen and Potassium. Nutrients can be added from rainwater, weathered out of rocks or recycled by decomposers.
Ecosystems are incredibly VULNERABLE to change. Invasive species can out-compete natives, as we have seen with Grey squirrels competing with native British red squirrels.   Diseases can be introduced and spread which can cause untold damage, as we are seeing with Ash die back in UK ash trees.  Within our pond, an increase in the number of water lilies could starve the bottom of the pond of both light and oxygen; this would impact on bottom feeders which would impact upon fish. Similarly, if someone added predatory fish they would eat smaller fish such as Sticklebacks and even small frogs, which would increase the numbers of slugs and flies normally eaten by the frogs.

Freshwater Frog

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