Energy Flow and Food Webs

Energy Flow

The Sun is the principal source of energy input to biological systems. Producers take in light energy from the Sun and store it as chemical energy in their organic nutrients. This chemical energy is passed to other organisms when they feed, and it is eventually transferred to the environment.

Worked example (describing energy flow): State the principal source of energy for living organisms, and the form of this energy. Step 1. The principal source of energy input to biological systems is the Sun. Step 2. It arrives as light energy. Answer: the Sun; light energy.

Worked example (energy in organisms): State the form in which energy is stored in the organic nutrients of a plant. Step 1. Producers use light energy to make organic nutrients. Step 2. The energy is stored in these nutrients as chemical energy. Answer: chemical energy.

Food Chains

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Each arrow points to the organism that eats the one before it.

Consumers may be classed as primary, secondary, tertiary and quaternary according to their position in a food chain.

Worked example (constructing a food chain): A hawk eats snakes. Snakes eat frogs. Frogs eat grasshoppers. Grasshoppers eat grass. Construct the food chain. Step 1. Start with the producer: grass. Step 2. Add each feeder, with arrows pointing to the organism that eats the one before. Answer: grass → grasshopper → frog → snake → hawk.

Worked example (interpreting a food chain): In the chain grass → grasshopper → frog → snake → hawk, name the herbivore and the tertiary consumer. Step 1. The herbivore eats plants: the grasshopper eats grass, so it is the herbivore (and the primary consumer). Step 2. The tertiary consumer is the third consumer in the chain: the snake. Answer: herbivore: grasshopper; tertiary consumer: snake.

Food Webs

A food web is a network of interconnected food chains. Many food chains share the same organisms.

Worked example (finding a food chain): Write one food chain from Fig 19.13 that has four organisms. Step 1. Start with a producer: grass. Step 2. Follow the arrows: grass is eaten by the mouse, the mouse is eaten by the snake, and the snake is eaten by the hawk. Answer: grass → mouse → snake → hawk.

Worked example (interpreting a food web): Use Fig 19.13 to name two organisms that eat the mouse. Step 1. Find the mouse. Look for arrows that start at the mouse. Step 2. The arrows from the mouse point to the snake and to the fox. Answer: snake and fox.

Worked example (classifying organisms in a web): Use Fig 19.13 to name two carnivores and one producer. Step 1. Carnivores eat other animals: for example the frog, snake, fox and hawk. Step 2. Producers make their own organic nutrients: grass and shrub. Answer: e.g. snake and fox; grass.

Trophic Levels

A trophic level is the position of an organism in a food chain, food web or ecological pyramid. The trophic levels are producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers.

A primary consumer feeds on a producer, a secondary consumer feeds on a primary consumer, and so on. Decomposers get their energy from dead or waste organic material from organisms at every trophic level.

Worked example (naming trophic levels): In Fig 19.13, the snake eats the mouse, and the mouse eats grass. State the trophic level of the mouse and of the snake. Step 1. Grass is the producer. Step 2. The mouse eats the producer, so it is a primary consumer. The snake eats the mouse, so it is a secondary consumer. Answer: mouse: primary; snake: secondary.

Trophic levels in Fig 19.13

Human Impact on Food Chains

Food chains and food webs can be used to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat.

Worked example (overharvesting): Large fish eat small fish, and small fish eat algae. Humans catch too many large fish. Describe the effect on the small fish and on the algae. Step 1. Fewer large fish eat the small fish, so the small fish increase. Step 2. More small fish eat more algae, so the algae decrease. Answer: small fish increase; algae decrease.

Worked example (a foreign species): Rats are introduced to an island where birds nest on the ground. The rats eat the birds' eggs. Describe the effect on the birds. Step 1. The rats are a foreign species that eat the eggs. Step 2. Fewer eggs hatch, so the number of birds decreases. Answer: bird numbers decrease.

Pyramids of Numbers and Biomass

An ecological pyramid has the producers as the bottom bar, with a bar for each trophic level above. A pyramid of numbers shows how many organisms there are at each level. A pyramid of biomass shows the mass of living material at each level.

In a pyramid of numbers, one large producer, such as an oak tree, can feed many small consumers, so the shape is not a pyramid. A pyramid of biomass takes account of the mass of the organisms, so it is usually a pyramid shape. This is the advantage of a pyramid of biomass over a pyramid of numbers.

Worked example (interpreting a pyramid): Use Fig 19.15: state why the pyramid for the oak tree is not a pyramid shape. Step 1. There is only one oak tree at the producer level. Step 2. It supports many caterpillars, so the next bar is wider. Answer: one large producer feeds many consumers.

Pyramid of Energy

ExtendedPyramids of energy

A pyramid of energy shows how much energy is transferred through each trophic level in a given area over a given time. Each bar is smaller than the one below it.

ExtendedAn advantage of a pyramid of energy

A pyramid of energy is more reliable than pyramids of numbers or biomass. A pyramid of biomass shows the mass at one time only. Organisms differ in size and in how fast they grow, but a pyramid of energy shows the energy over a period of time, so the levels can be compared fairly.

Worked example (advantage of an energy pyramid): State one advantage of a pyramid of energy over a pyramid of numbers. Step 1. A pyramid of numbers ignores the size of the organisms, so it can be inverted. Step 2. A pyramid of energy is always a pyramid shape, because at each level some energy is not passed on, so each level has less than the one below. Answer: always a pyramid shape.

Energy Transfer Between Levels

ExtendedWhy transfer is often not efficient

Only a small fraction of the energy at one trophic level reaches the next. Much of the energy is not passed on to the next level because:

Much of the energy is transferred to the environment as heat from respiration. Some is in waste such as faeces and urine, and some is in parts that are not eaten, such as bones or roots. Waste and parts not eaten pass to decomposers.

ExtendedFewer than five trophic levels

At each step much of the energy is transferred to the environment or is not passed on, so less is available for the next trophic level. After a few levels there is too little energy left to support another level, so food chains usually have fewer than five trophic levels.

ExtendedCrops and livestock

It is more energy efficient for humans to eat crop plants than to eat livestock fed on crop plants. Eating the crop is a shorter food chain.

Worked example (energy efficiency): Explain why more people can be fed from a field of wheat eaten directly than from the same wheat fed to cattle. Step 1. Feeding wheat to cattle adds a trophic level, and the cattle release much of the energy as heat from respiration, and some is in waste and uneaten parts. Step 2. Eating the wheat gives humans a larger share of the energy, so more people are fed. Answer: fewer energy losses in a shorter chain.