AQA GCSE Biology

Revision Notes

7.1.1 Communities

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, IGCSE & GCSE Biology revision notes

Levels of organisation in an ecosystem

Competition & Adaption

You need to 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 (grey squirrels), IGCSE & GCSE Biology revision notesIntraspecific competition between two grey squirrels (same species) for a limited resource

 

 

Interspecific competition (grey and red squirrels), IGCSE & GCSE Biology revision notes

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

Resources competed for by plants table, IGCSE & GCSE Biology revision notes

  • Animals in a community or habitat may compete with each other for certain limited resources:

Resources competed for by animals table

Resources competed for by animals table, IGCSE & GCSE Biology revision notes

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

Food web, IGCSE & GCSE Biology revision notes

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)

Exam Tip

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.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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