Communities (AQA GCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
Levels of Organisation in an Ecosystem
There are 4 levels of organisation within an ecosystem:
Individual organism: a single member of a species
Population: a number of individuals of the same species living in the same area at the same time
Community: multiple populations (of different species) living and interacting in the same area
Ecosystem: the interaction between a community (the living, biotic part) and the non-living (abiotic) parts of the environment
Levels of organisation in an ecosystem
Competition & Adaption
You should understand the following 2 terms:
1. Competition:
If a group of organisms all need the same resource in order to survive and reproduce (have offspring) but there is a limited amount of the resource available, they are said to compete for the resource
Competition can be between members of the same species (intraspecific competition) or between members of different species (interspecific competition)
Intraspecific competition between two grey squirrels (same species) for a limited resource
Interspecific competition between a grey squirrel and a red squirrel (different species) for a limited resource
2. Adaptation:
If an organism has certain features, behaviours, or other characteristics that help it to survive and reproduce in its habitat, it is said to be adapted to its habitat
Ecosystems
An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment
Ecosystems can vary greatly in size and scale. Some ecosystems are small – a single tree can be seen as an ecosystem. Some ecosystems are very large – the whole of Antarctica can be seen as one ecosystem.
Some other examples of ecosystems include:
A garden pond
A woodland
A coral reef
A desert
Interactions within an Ecosystem
In order to survive and reproduce (have offspring), organisms need certain resources from their surroundings (from the ecosystem they are living in)
This means that members of a species will often interact with members of its own species or other species
Some examples of these interactions include:
Predators (carnivores) eating prey
Herbivores eating plants
Plant species being pollinated by bees
Competition in Plants & Animals
Plants in a community or habitat may compete with each other for certain limited resources:
Resources competed for by plants table
Animals in a community or habitat may compete with each other for certain limited resources:
Resources competed for by animals table
Interdependence
Within a community, each species depends on other species for food, shelter, pollination, seed dispersal etc
If one species is removed it can affect the whole
This is called interdependence
A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant
A food web shows the interdependence of organisms
For example, in the food web above, if the population of earthworms decreased:
The population of grass plants would increase as there are now fewer species feeding off them
The populations of frogs and mice would decrease significantly as earthworms are their only food source
The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)
Examiner Tips and Tricks
Questions about interdependence in food webs are common and simple to gain marks on if you answer them fully and correctly. Do not say an animal or plant would ‘die out’ as this is unlikely to happen – stick to using the words decrease or increase. If in doubt, always give your reason for the increase or decrease in population.
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