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A-level Geography - Ecosystems: Change and Challenge: Food Chains and Food Webs

Food Chains, Trophic Levels and Ecological Pyramids

The following presentation gives a clear explanation about Food Chains, Trophic Levels and Ecological Pyramids:

Food Chain and Food Web


Food Chain - A food chain is the series of organisms showing feeding relationships. A food chain almost always begins with a green plant (producer) which is eaten by an animal (consumer). The arrow means 'is eaten by', and shows the flow of matter and energy along the food chain.



Food Web - a network of interrelated food chains in a given area. Because organisms in a habitat generally eat more than one thing, the energy produced in plants connects with several organisms living together. It shows how the loss of one organism has an effect of other organisms in the food web. This is called the interdependence of living things


The following diagram shows the difference between a food chain and a food web:


1What are the products of photosynthesis?


2What colour is chlorophyll?


3Where does photosynthesis take place?


4A green plant is usually a:


5What does a food chain always start with?


6Which is the secondary consumer in this food chain:

grass -> grasshopper -> frog -> hawk


7What does an omnivore eat?


8What is wrong with this pyramid of numbers?

from top to bottom: the sparrowhawk is represented by a thin bar, the thrush is represented by a slightly thicker bar, the clover has the widest bar in this pyramid, and the snail is at the bottom with a slightly narrower bar than the clover


9What can we conclude from this pyramid of numbers?

There is a small bar for the producer, a very large bar for the primary consumer, and a smaller bar for the secondary consumer


10How many predators are there in this food web?

Shows a food web. The grass is eaten by rabbit, insects and slugs. Rabbit are eaten by foxes. Insects and slugs are both eaten by voles and thrushes.


11What is likely to happen to the population of slugs if the population of thrushes decreases?

Shows a food web. The grass is eaten by rabbit, insects and slugs. Rabbit are eaten by foxes. Insects and slugs are both eaten by voles and thrushes.


12What is likely to happen to the population of thrushes if the population of voles increases?

Shows a food web. The grass is eaten by rabbit, insects and slugs. Rabbit are eaten by foxes. Insects and slugs are both eaten by voles and thrushes.


13What is likely to happen to the population of foxes if the population of rabbits decreases?

Shows a food web. The grass is eaten by rabbit, insects and slugs. Rabbit are eaten by foxes. Insects and slugs are both eaten by voles and thrushes.


In this food web, what is represented by the arrows?

2Which of these organisms is a producer?

3What's the longest food chain in this food web?

4For this particular food web a pyramid of biomass would be widest:

5Which of the following is NOT a renewable form of energy:

1. In the food relationship where the lion eats the wildebeest, and the wildebeest eats plants ...

  1. The lion is the prey and the wildebeest is the predator
  2. The lion is the predator and the wildebeest is the prey
  3. The lion is the primary consumer and the wildebeest is the secondary consumer
  4. The lion is the secondary consumer and the wildebeest is the primary consumer
A and C only A and D only
B and C only B and D only

2. If there is a shortage of wildebeest in the area, what are some things the lions might do?

  1. move to another area to hunt for food
  2. eat the bark and roots of trees
  3. hunt for other animals
C only C and B only
A and C only A and B only

3. Animals that feed on both plants and animals are called …

carnivores herbivores detritivores omnivores

4. There are more herbivores than carnivores in a food chain because

  1. much energy is lost in the transfer of energy from herbivores to carnivores
  2. a larger number of herbivores is needed to support a smaller number of carnivores
  3. this is the way for the food chain to stay in equilibrium
A only B and C only
A and B only All of the above

5. Over a period of time, the number of wildebeest drops. Which of the following statements could explain the drop in the wildebeest population?

  1. A drought occurred
  2. An increase in the lion population
  3. The death rate of the wildebeest was lower than the birth rate
  4. An increase in the number of predators
A and B only B and C only
A, B and C only A, B and D only

6. When a population of animal 'X' was introduced into the community, the number of lions increased while the number of wildebeest decreased. Animal 'X' could be a …

herbivore e.g., zebra
carnivore that is a predator of wildebeest and lions
carnivore that is a predator of wildebeest but a prey of the lions

Types of Food Web

A food web is made up of two or more interconnected food chains. It shows the food relationships among organisms in a community.


Food Web - Tropical Equatorial Forest

Tropical Rainforest Food Web
The food web of tropical rainforests is highly complicated and many different food chains are interlinked through this biome. However, it can be classified roughly into 4 levels. The first level includes plants (leaves, flowers & fruits), plankton, larvae, spiders and insects. Plankton eaters and insects along with plants are at the second level. The different types of plankton eaters include fish, frogs, possums, bandicoots, birds, echidnas, kangaroos and wallabies. Such types of creatures are abundant in nature and form the diet of those placed above in the hierarchy. Small-sized animal eaters are placed at the third level of food web. Quolls, snakes, platypus, dunnarts, owls, kookaburras, etc. are the small animal eaters at this level. Bigger carnivores like crocodiles, pythons, feral cats, dingoes and feral dogs are placed at the top (fourth) level. Thus, carnivores are at the top and plants at the base of this tropical rainforest food pyramid. For example, dingoes (Canis lupus dingo) find themselves at the top of the food pyramid in Australian rainforests. 

The different organisms mentioned in 4 levels of the food chain are dependent on each other for obtaining nutrition. Those organisms perched on the topmost tier feed on those at the bottom. Scavengers or decomposing microbes feed on top-level organisms after their death. Organic matter formed as a result of decomposition gets mixed in the soil and thus, food cycle is completed.

Energy Flow in Food Web
Flow of energy is a common factor which binds the members of food chain together. Energy originating from the sun is disintegrated through the process of decomposition. Plants use the process of photosynthesis to prepare food for themselves. A chemical called chlorophyll plays an important role in the process of photosynthesis. Solar energy is used in the conversion of carbon dioxide and water into sugars. Polymerization of sugars facilitates their storage and thus, long-chain carbohydrates are formed. The sulfates, nitrates and phosphates present in soil are used in the preparation of proteins. The proteins and carbohydrates prepared by plants are used as a source of energy by herbivores and eventually carnivores. Biomass goes on decreasing from the base of food pyramid to its pinnacle. The reason behind it is the loss of energy in the environment which results from entropy.

Food Web - Temperate Deciduous Woodland


The Primary Producers of this forest are the plants that use their chlorophyll to create food for their own growth as well for the animals. The Ground Layer of this forest is covered with green mosses and small plants. Mushrooms, grasses, and wild flowers grow among the mosses, covering the ground with a thick, moist, green carpet. Dead leaves, logs, needles, and twigs, provide food for the detritivores, who digest the dead materials and make them available for recycling.


The Primary Consumers are mostly small animals. The forest provides food, shelter in the moss, mild temperatures and humidity, which encourages the growth of many kinds of insects. It is a paradise for mosquitoes! As well as insects, there are small mammals: voles (tiny mammals like mice), chipmunks, squirrels, and seed eating birds. These eat seeds, grasses, and even mushrooms. Larger animals, such as deer and elk, also find food in this biome.


Secondary Consumers are also at home on the forest floor. Tiny shrews devour the insects, and frogs catch insects as they fly by. There are many insect-eating birds, like the woodpecker, are adapted to finding insects in the trees. Weasels eat small animals, and racoons eat animals, fishes, frogs, and fruit. Owls eat voles and chipmunks. Insects live as parasites on other animals. Large secondary consumers, such as wolves, bears, and cougars, are the only ones who can bring down deer or elk. There are other animals in this food web, such as foxes, beavers, black birds, and porcupines. Because the plants do so well, animals flourish too.

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