Trophic Levels of Organisms (OCR GCSE Biology A (Gateway))

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

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

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

• 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

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

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! 

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