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
- An example could be a volcanic ash cloud that blocks all light entering a habitat
- 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)
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)
