Trophic Levels of Organisms (OCR Gateway GCSE Biology)

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Food Chains & Food Webs

Trophic levels

  • Trophic levels are used to describe the feeding relationships between organisms
  • The Sun is the source of energy for nearly all life on Earth
  • Energy flows from the Sun to the first trophic level (producers) in the form of light
    • Producers do exactly that - they make food
  • Producers then convert light energy into chemical energy and it flows in this form from one consumer to the next
    • For example, plants (one type of producer) convert a small percentage of the light energy that falls on them into glucose, some of which is used immediately in respiration and some of which is stored as biomass
    • When a primary consumer (e.g. a herbivore such as a rabbit) feeds on a plant, the chemical energy stored in the plant's biomass is passed on to the primary consumer
  • Eventually, all energy is transferred to the environment – energy is passed on from one level to the next with some being used and lost at each stage
    • Energy is lost to the environment when heat energy is transferred from organisms to their surroundings

Trophic Levels Table

Energy table, IGCSE & GCSE Biology revision notes

Food chains

  • A simple way to illustrate the feeding interactions between the organisms in a community is with a food chain
  • A food chain shows the transfer of energy from one organism to the next
    • The arrow points in the direction that energy flows
    • NOT in the direction of feeding
  • The source of all energy in a food chain is light energy from the sun
  • The arrows in a food chain show the transfer of energy from one trophic level of the food chain to the next
  • Animals (known as consumers) can be at different trophic levels within the same food web
    • Because they may eat both producers and consumers
    • Including primary, secondary and/or tertiary consumers

Food chain, IGCSE & GCSE Biology revision notes

An example of a food chain (the sun is not included in food chains as it is not a living organism)

Food Chain & Webs Definitions Table

Types of Variation table, IGCSE & GCSE Biology revision notes

  • Omnivores feed on a mix of other animals and plant-based food
    • Examples include humans, starlings, seagulls, woodpeckers, raccoons
    • Many animals labelled as carnivores will in fact eat some plant-based material e.g. polar bears

Food chain showing trophic levels, downloadable IGCSE & GCSE Biology revision notes

Trophic levels for a simple food chain

Food webs

  • A food web is a network of interconnected food chains
  • Food webs are more realistic ways of showing connections between organisms within an ecosystem (compared to individual food chains)
    • Because animals rarely exist on just one type of food source

Food web, IGCSE & GCSE Biology revision notes

A food web shows the interdependence of organisms

  • Food webs give us a lot of information about the transfer of energy in an ecosystem
  • They also show interdependence - how the change in one population can affect others within the food web
  • Animals (known as consumers) can be at different trophic levels within the same food web
    • Because they could be omnivores (animals that feed on both plants and animals)
    • Or they could be predators that feed on both primary, secondary and/or tertiary consumers! 

Food web showing trophic levels_2, downloadable AS & A Level Biology revision notes

Trophic levels for a simple food web – note that some organisms can belong to more than one trophic level (such as the squirrel, fox and eagle in this food web)

Changes to populations at trophic levels

  • If a producer is removed from a food web, this deprives the whole food web of its energy source
    • An example could be a volcanic ash cloud that blocks all light entering a habitat
      • No plants would be able to photosynthesise, so no consumers would be able to feed
      • This is bad for all species in the ecosystem
  • Less well understood is the effect of removing a top (apex) predator from a food web, which can be equally serious to an ecosystem
    • In the food web above, let's assume that illegal egg collectors have reduced the eagle population to zero
    • Squirrels would fare better, with no predation by eagles to worry about
    • Foxes would also do well, with no competition from eagles for the squirrels as prey
    • However, the increased number of squirrels would put pressure on the grasshopper population and plant population
    • The extra feeding on plants may mean that fewer young plants can grow to maturity, which could lead to uncolonised ground
    • This could destabilise the soil and cause rain damage via run-off when it rains, leaving the ground barren
  • An ecosystem can be destroyed by removing its top predators, as well as its producers

Examiner Tip

It's tempting to think that because a predator might chase its prey when hunting, then the arrow in a food chain should go from
predator → prey (or OWL → MOUSE in the example above).
In fact, the prey provides energy to the predator so the arrow direction should be
prey → predator (MOUSE → OWL)

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Phil

Author: Phil

Expertise: Biology

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.