(4) Food webs - interdependence - examples explained

Interdependence means organisms depend on other organisms for resources such as food or shelter in order to survive and reproduce. Therefore, any change in the population of one species, can have knock on effect on another species in the same community. See also notes on ecosystems.

Because of all the different species that co-exist in an environment - the ecosystem, it means there are lots of possible interconnected food chains, though all must start with a producer - usually a photosynthesising plant.

A food web links many, if not all, of the different food chains together.

All the species in a food web are interdependent.

In a community each species depends on others for many things e.g. food (plants eaten by animals, prey hunted by predator, parasites on hosts), mutualism - pollination of flowers by insects, plant seed dispersal by animals, shelter - nests made of twigs and leaves and beaver dams.

This unfortunately means that if one organism population changes, then others populations are affected too - I've called these 'disruptions' - 'what affects what', but not all have negative results - there are often 'gainers' as well as 'losers'. Summed up as 'knock on' effects.

 

Examples of food webs and interdependence

The trophic levels are indicated i.e. producer, primary/secondary/tertiary consumers AND examples of what may happen if there is a sudden increase or decrease in the population of a particular organism.

Diagram of food web example 1

Food web 1. Part of a typical complex food web in a lake or river.

Trophic level 1: The photosynthesising producers are the algae and pond weed.

Trophic level 2: The primary consumers are the tadpoles and aquatic insects.

Trophic level 3: The secondary consumers can be the perch, stickleback and frogs

Trophic level 4: Tertiary consumers at the top of the food chain are the pike, otter and heron (top predators).

(some organisms can be in different trophic levels depending on the food chain sequence)

'Disruptions'

(i) Suppose the lake becomes overgrown with weed reducing the light entering the water. The weeds under the water decay and use up all the oxygen (eutrophication effect). The reduction of oxygen affects all the consumers and their numbers are considerably reduced, often to zero in the water.

(ii) Suppose the number of aquatic insects is reduced due to adverse weather conditions. There is less food for the stickleback and perch fishes and frogs so their numbers decline. In turn there is less food for the pike, otter and heron further up the food chain, so their numbers decline too. The heron can easily fly to new waters in search of food, but fishes might not be able to move out of the particular lake or river system. At the same time there will be more food for the tadpoles (less competition for the algae and pondweed), so more frogs survive, but the perch population may increase too because there are more tadpoles to eat.

Things can get very complicated and often can only be quantitatively described and understood by a computer programme whose algorithms describe the rise and fall of the different populations of the species in the habitat.

 

Diagram of food web example 2

Food web 2. Part of a typical complex food web in a park or garden.

Trophic level 1: The photosynthesising producers are the fruit bushes (or any other plants).

Trophic level 2: The primary consumers are the caterpillars, insects like the greenfly, snails, worms and blue tits.

Trophic level 3: The secondary consumers can be the blue tit, ladybird, thrush and sparrow hawk.

Trophic level 4: Tertiary consumer at the top of the food chain is the sparrow hawk.

(some organisms like blue tits or the sparrow hawk can be in two different trophic levels depending on the food chain sequence e.g. blue tits might feed directly on both greenfly and ladybirds)

'Disruptions'

(i) Suppose the population of particular organism like the ladybird is decreased by some bacterial or viral infection. This will affect several food chains and hence the populations of other organisms. With less ladybirds to eat the greenfly, the greenfly population can increase.

(ii) Although the greenfly increase in population, there are less ladybirds for the blue tits, so the blue tit population might decline, but their numbers might remain the same because the greenfly population increases - an alternative food for the blue tits.

Diagram of  food web example 3

Food web 3. Part of a typical complex food web in grassy woodland

Trophic level 1: The photosynthesising producers are the varieties of plants and their fruits and nuts etc.

Trophic level 2: The primary consumers are the aphids, beetles, woodlice, worms, slugs, squirrels and rabbits.

Trophic level 3: The secondary consumers can be the insect eating birds, mouse, badger, weasel and owl.

Trophic level 4: Tertiary consumers at the top of the food chain are the owl and fox (bar the fleas!).

(some organisms like the owl can be in two different trophic levels depending on the food chain sequence)

'Disruptions'

(i) If there was a decline in rabbit population, with less food for the foxes, the fox population would begin to decline, but with less foxes around, the rabbit population begins to rise, but then there is more food for the foxes, so its population can rise.

This leads to complex cycles in the rise and fall of the populations of the species involved.

A classic example is the relationship between rabbit (the prey) and fox (predator) populations in a particular community. Predator-prey cycles are always out of phase with each other because it takes some time for one population to respond to changes in the other population.

Note that if a predator eats all their chosen prey, it will die out for lack of food or move to another habitat.

In stable communities the numbers of predators and preys rises and falls but in a balanced way.

 

Diagram of food web example 4

Food web 4. Part of a typical complex marine food web in the sea.

Trophic level 1: The photosynthesising producers are the plankton, algae and seaweed.

Trophic level 2: The primary consumers are fishes like herring, mussels, crabs and limpets.

Trophic level 3: The secondary consumers are the dolphin, seal, crabs, seagull and starfish.

(some organisms like the crab can be in two different trophic levels depending on the food chain sequence)

'Disruptions'

(i) Every so often you get a massive increase in algae (an 'algal bloom') which provides lots of extra food for the primary consumers and so on. Every species benefits up the food chains.

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Biology Revision Notes: Food Webs and Interdependence of Organisms

What is a Food Web?

A food web is a complex network of interconnected food chains showing how different organisms in an ecosystem obtain their energy.

Unlike a simple food chain, which follows a linear path (e.g., grass → rabbit → fox), a food web illustrates multiple feeding relationships among various organisms.

Interdependence in a Food Web

Organisms within a food web depend on each other for survival.

This is known as interdependence, meaning that a change in the population of one species can affect many others. Key relationships include:

• Producers: Plants and algae that create their own food via photosynthesis. They provide energy for primary consumers.

• Primary Consumers: Herbivores that eat producers, such as rabbits, caterpillars, and deer.

• Secondary Consumers: Carnivores that feed on primary consumers, like foxes or frogs.

• Tertiary Consumers: Top predators that eat secondary consumers, such as eagles or sharks.

• Decomposers: Organisms like bacteria and fungi that break down dead matter, recycling nutrients back into the soil.

Consequences of Changes in a Food Web

Since organisms rely on each other, disruptions can have widespread effects. For example:

• A decline in producers (due to pollution or deforestation) can reduce food for primary consumers.

• Overhunting of predators can lead to an increase in prey populations, possibly causing overgrazing or depletion of resources.

• The removal of decomposers affects nutrient cycling, leading to poor soil quality and weakened plant growth.

Key Takeaways for Your Exam

• Food webs show multiple feeding connections in an ecosystem.

• Organisms are interdependent, meaning changes in one population affect others.

• Producers, consumers, and decomposers all play crucial roles in maintaining ecosystem balance.

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Summary of learning objectives and key words or phrases

Be able to explain and interpret what a food web is.

Be able to explain what is meant by interdependence between organisms?

Understand the importance of interdependence of plants and animals in a food web.

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