Food Chains

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Slide 1

In this video, we’re going to talk about food chains. Food chains show the ways in which organisms in a habitat feed: who produces food, and who eats who. For example, krill might eat phytoplankton, and a whale shark might eat the krill. They also demonstrate the flow of energy in ecosystems.

Slide 2

Living things need energy in order to keep on living. They need energy to grow, run, and jump. Every time an animal moves, it uses energy. Animals even use energy when they are sleeping. Where do animals get that energy? They get it from the food they eat. Plants produce their own food, but they need the energy from the Sun to do it. Food chains show how living things get their energy. They demonstrate the feeding relationships between animals and plants in an ecosystem: who eats whom, and the way that nutrients and energy are passed from one organism to another.

Slide 3

More formally, a food chain is a sequence of organisms in a biotic community through which food passes. Don’t panic if that didn’t make much sense! A biotic community is a collection of interacting organisms that live in the same habitat. The food chain illustrates the feeding relationships between these organisms: members of one level of the food chain become food for the members of the next level of the food chain. Food chains provide a list of what eats what in a habitat. They may include producers, different sorts of consumers and decomposers.. Producers are usually plants which use energy from the Sun to create the food for herbivores. The herbivores are one level of consumer (as they consume the plants). Herbivores, in turn, are often eaten by carnivores, which form another level of consumer and so on. Decomposers are organisms which feed on rotting material and return its energy to the food chain.
 
Slide 4

Energy and nutrients flow from one level of the food chain to the next as organisms eat organisms from lower levels of the food chain. Think of the arrows in the food chain as meaning “is eaten by”. So, in the food chain on the left, the grass is eaten by the cow and the cow is eaten by the person. If you eat a hamburger for lunch, you might be part of this food chain. The producer is the grass (it gets its energy from the sunlight). The cow becomes a primary consumer as it eats the grass, and the girl is a secondary consumer as she eats the cow which has eaten the grass. Hopefully no one eats the girl, so she is an apex predator in this food chain. The food chain starts with Grass, which is eaten by the Cow, which is eaten by the Human.

Slide 5

Here are another couple of food chains. In the food chain on the left, the producer is the flower. (point to each using the mouse) It is eaten by a caterpillar, which, in turn, is eaten by the frog. A snake eats the frog and an owl eats the snake. Each arrow in the food chain represents the transfer of energy (in the form of food) from the preceding level to the next.

(point to each using the mouse) The sun is not part of the food chain on the right. The Algae (the producer) makes its food using the energy from the Sun. It is eaten by the prawn, which is then eaten by the fish, and the Human eats the fish. No one eats the human. Energy passes from the algae to the prawn to the fish and, finally to the human.


Slide 6

The lengths of food chains are important properties of biological communities. We can calculate the length of a food chain by counting the number of links joining the producer to the top consumer (or apex predator). The two food chains on the left each have a length of 5. There are 5 feeding arrows in each. The length of a food chain is limited by the fact that energy is not used 100% efficiently as we pass from one level of the food chain to the next. Food chain length measures the capacity of the environment to transmit energy from one level of the food chain (trophic level) to the next. More complex ecosystems can potentially contain longer food chains, so the length of the food chains in a habitat reflects the complexity of the ecosystem.

Slide 7

Most food chains contain four or five steps. So, most food chains have a length of 4 or 5. Scientists call each step in the food chain a trophic (or feeding) level. Trophic levels tell us how many feeding steps separate the organism from the base (or producer) level of the food chain.

Slide 8

The first level of a food chain is made up of the producers. These are usually plants in dry environments. Next, we have the primary consumers who eat the plants. As they eat only plants, these animals are herbivores. The third level is made up of the secondary consumers. These are the carnivores and omnivores who eat the herbivores. At the fourth feeding level, we have the tertiary consumers. They are carnivores and omnivores which eat the secondary consumers. Finally, in a food chain with 5 levels, we have the apex predators at the top on the fifth level. These are carnivores and omnivores which eat the tertiary consumers. No one eats the apex predators.
 
Slide 9

Let’s have a look at a couple of examples. On the left we have a terrestrial food chain. At the first trophic level, we have the producers, which are plants. The second trophic level consists of grasshoppers, who eat the plants. Grasshoppers are primary consumers. They, in turn, are eaten by mice. Mice are secondary consumers and are placed at the third feeding (or trophic) level of the food chain. Eagles eat the mice. They are tertiary consumers and are placed at the fourth feeding (or trophic) level of the food chain. At the top is a tiger, who eats the eagle. The tiger is an apex predator. Nobody would dare to eat the tiger. Mr tiger is placed at the 5th feeding (or trophic) level of the food chain.

In the aquatic food chain on the right, the seaweed is the producer, and sits at the first feeding (or trophic) level. The snail is the primary consumer as it eats the seaweed. It sits at the second feeding (or trophic) level. Lobsters eat the snails. They are secondary consumers, sitting at the 3rd feeding (or trophic) level. Tuna eat the lobsters. They are tertiary consumers, sitting at the 4th feeding (or trophic) level. Finally the shark eats the tuna. Nobody messes with the shark. It is an apex predator, sitting at the 5th feeding (or trophic) level.

Slide 10

Food chains always start with producers. You might wonder why we don’t start them with sunlight, because the sunlight gives the producers the energy to create food, but the point is that food chains only contain biotic elements of the ecosystem. The sun is abiotic. Producers take the Sun’s energy and transfer it into the food chain. Part of the Sun’s energy is then transferred to the organisms at the other levels of the food chain as they eat the organisms from the preceding level.
 
Slide 11

Most food chains have length four or (at most) five. Let’s have a think about why we don’t get food chains with hundreds of levels. What stops this from happening?
The key is that food chains transfer energy from one level to the next, but they don’t do it perfectly. Energy is lost at each level. Eventually, there isn’t enough energy to transfer from the top level to the next, and the food chain stops.

Slide 12

Only about 10% of the energy received by each level is transferred to the next level of the food chain. Some of the energy is used to allow the organisms at the next level to move and grow, but only about 10% of the energy is stored as biomass in the organisms. This is the energy that is available for transfer to the next level of the food chain. Fewer copies of each organism exist at each successively higher level of the food chain. After a certain number of feeding levels, there is too little energy available to support the existence of the next level of the food chain. We reach the apex predator level – the point where consumers have no predators. Most food chains have only 4 or 5 levels because of this energy limitations.
 
Slide 13

Two types of food chains occur in nature, depending on the type of their primary producers. Grazing food chains have plants as their producers. The plants get their energy from sunlight. Detritus food chains have decomposers as their producers. The decomposers get their energy from decaying organic matter.

Slide 14

Most food chains are grazing food chains. These food chains start out with living green plants. The plants are then eaten by grazing herbivores such as cows, and these herbivores are then eaten by carnivores and/or omnivores. Grazing food chains directly depend on energy from solar radiation. The producers capture this solar energy and convert it into food. This energy is then transferred to herbivores as they eat the plants.
 
Slide 15

Grazing food chains can be either aquatic or terrestrial. This slide shows two examples of grazing food chains. The aquatic food chain on the left starts with phytoplankton (although it shows a picture of seaweed). Phytoplankton absorb energy from the sun to produce their own food. Zooplankton are the primary consumers, who eat the phytoplankton. Finally the fish eat the zooplankton. Depending on the ecosystem, someone else might eat the fish, but we aren’t given that information in this food chain.

On the right, we have a terrestrial grazing food chain. Our producer is grass. It converts energy from the sunlight into food in a process called photosynthesis. Our primary consumer is the rabbit, which eats the grass. The secondary consumer is a fox, which eats the rabbit. Someone else might eat the fox, but we don’t know that for sure as we have no further information. As far as we can tell, the fox is the apex predator in this ecosystem.
 
Slide 16

The second type of food chain is the detritus food chain. This chain gets its energy from dead and decaying organic matter. Microorganisms feed on this detritus, and then other organisms eat the microorganisms and detritus. The chain also includes the predators of other organisms.

Detritus food chains are not directly dependent on energy from the Sun. Of course, that the dead organic matter came from may have directly depended on solar energy.
However, the energy driving a detritus food chain comes from organic matter that was produced in another ecosystem.
 
Slide 17

This slide shows a detritus food chain in a mangrove. It is really a food web as it contains a number of different branches. (Use the mouse to point as you go – better if you can find a laser pointer controller – have a look) Dead organic matter from the mangrove forest and sea grass drops down into the water, where it becomes detritus. In one branch of the food web, it is eaten by worms, which are eaten by small fish. Other branches show the detritus eaten by crabs, shrimp and bivalves. These, in turn are eaten by small fish. The small fish are eaten by larger fish or egrets.

Slide 18

Let’s quickly review the main characteristics of food chains. For food chains to be sustainable, they have to start with a producer. Usually the producer takes energy from sunlight and turns it into food for inclusion in the food chain, but sometimes the producer takes its energy from dead organic matter. Energy in the food chain flows in only one direction: from the Sun (or dead organic matter) to the Producer and then to the Consumers of various levels. Other nutrients continue to circulate in the food chain, after being released from dead organisms by decomposers. Decomposers are needed to sustain food chains.
 
Slide 19

Food chains are straight unless they are linked to other food chains as we saw in the food web of the mangrove forest. Most are short, with only 3 to 5 trophic levels as the transfer of energy between levels of a food chain is inefficient. As we increase the trophic level, the size of the population of each organism decreases, with the smallest population size for the apex predators or top carnivores. Finally, organisms can work at more than one trophic level. For example, a snake might be both a secondary and a tertiary consumer, feeding on both herbivorous rats and carnivorous frogs.


Slide 20

Let’s finish off by talking about why scientists study food chains. First, food chains help us to understand the feeding relationships between organisms in an ecosystem, and their interactions. They give us an appreciation of the flow of energy throughout an ecosystem and the ways in which the matter of an ecosystem circulates. Finally, they help us to understand the movement of toxic substances in an ecosystem. For example, if an industrial plant releases heavy metals into a waterway, those heavy metals will be absorbed by fish, which may then be eaten by humans who show the symptoms of heavy metal poisoning. Food chains help us to understand the biomagnification of toxins: biomagnification the increasing concentration of substances like toxic chemicals in the cells of organisms that are tolerant to them. This concentration increases as we reach higher levels of the food chain. The levels of these pollutants can increase in the organisms over time, as larger animals eat smaller, contaminated animals. Consequently, an understanding of food chains has important implications for our health as well as our understanding of ecosystems.