Interdependence & Competition (OCR Gateway GCSE Biology: Combined Science)

Revision Note

Test Yourself




Biology Project Lead


  • One species may be dependent on another within a community
    • An obvious example is that a predator species is dependent on there being an adequate supply of its prey
  • An extension of dependence is the concept of interdependence
    • All organisms in an ecosystem depend upon one another
    • The success or failure of one species can affect the success or failure of the others
  • The types of interaction between species can vary
  • 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) feeding on prey
    • Herbivores eating plants
    • Plant species being pollinated by bees
    • Seed dispersal via animals eating the fruit of certain trees and passing the seed in its faeces
  •  If one species is removed it can affect the whole community
    • 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)


  • Parasites live off a host organism
  • By taking what they need from the host and giving nothing in return
  • The host is harmed, although the parasite would wish to keep the host alive as a dead host is no longer as useful to it
  • An example is the tapeworm
    • Tapeworms attach themselves to the insides of the intestines of animals such as cows, pigs, and humans
    • They feed on the host's partly digested food, depriving the host of nutrients
  • Mistletoe is a plant parasite, and gains its water and nutrients from a host tree by anchoring to the tree and tapping into its tissues
    • Some mistletoe varieties can photosynthesise to a small extent, but only until they have fully anchored to their host tree and are gaining all their food and nutrients from it
  • Parasitism is a win-lose scenario


  • Unlike parasitism, two species can interact to mutually benefit
    • Flowering plants produce elaborate, coloured flowers containing sugar-rich nectar
    • Bees are attracted visually and with the scent of the flowers and nectar
    • Bees get a nutritious meal, and in return...
    • The flowers are able to be pollinated to ensure their survival
  • Certain species of shark have a mutualistic relationship with a much smaller fish called a cleaner wrasse
    • The shark requires its teeth to be razor-sharp and free of debris from the last kill
    • The wrasse (which might otherwise be part of the shark's normal prey) are allowed into the shark's mouth to pick off the debris and clean the shark's teeth
    • The shark benefits by having its teeth cleaned and made ready for the next kill
    • The wrasse benefits as it feeds by removing debris 

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.


Competition in plants & animals

  • 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)
  • 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, downloadable IGCSE & GCSE Biology revision notes

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

Exam Tip

You may wonder how a predator and prey are interdependent - it feels as if only the predator benefits, right?
In the case of the feeding relationship between wolves and elk in parts of the USA and Canada, the prey (elk) may well lose its weakest and least mobile members to predation as the wolves pick off the easiest animals to kill. The elk benefit indirectly as this strengthens their gene pool by eliminating characteristics that may lead to weakness. The weaker animals that have been killed cannot go on to reproduce, so any genetic weaknesses that they possess die out with them. 

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?


Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.