0610 · 19.2Food chains and food webs

Food chains and trophic levels

0:00Imagine that people remove most of the rabbits from a grassland. You might expect the grass to increase, but what happens to the foxes, and could other animals be affected too? Feeding relationships connect organisms, so changing one population can affect several others. In this video, we will follow those connections, explain why energy becomes less available along a food chain, and discover what ecological pyramids actually show. You need just two ideas to begin.

0:25Photosynthesis allows plants to use light energy to make organic nutrients, including carbohydrates. Those nutrients contain chemical energy, which organisms can release through respiration to support their activities. When one organism eats another, some of the chemical energy in the food passes to the organism eating it. A food chain shows the transfer of energy from one organism to the next, beginning with a producer. In our example, grass is eaten by a rabbit, which is eaten by a fox.

0:50The arrows point in the direction of energy flow, towards the eater. So the arrow from grass to rabbit means that energy in the grass is transferred to the rabbit, not that the grass eats the rabbit. A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. Grass is a producer, and so are trees and photosynthetic algae. Producers do not create energy, and they do not obtain ready-made food from the soil.

1:15They use energy to make organic nutrients from simpler substances. A consumer is an organism that gets its energy by feeding on other organisms. A herbivore is an animal that gets its energy by eating plants, so our rabbit is a herbivore. A carnivore is an animal that gets its energy by eating other animals, so our fox is a carnivore. These words describe how animals obtain food, while their position in a particular food chain gives us another way to classify them.

1:42A trophic level is the position of an organism in a food chain, food web or ecological pyramid. Producers occupy the first trophic level. An organism that eats a producer is a primary consumer, and one that eats a primary consumer is a secondary consumer. Eating a secondary consumer makes an organism a tertiary consumer, and eating a tertiary consumer makes it a quaternary consumer. Follow this longer example carefully.

2:06Grass is eaten by a grasshopper, the grasshopper by a frog, the frog by a snake, and the snake by a hawk. The grasshopper is the primary consumer, the frog is the secondary consumer, and the snake is the tertiary consumer. The hawk is the quaternary consumer, occupying the fifth trophic level because the producer also counts as a level. Let us construct a chain rather than simply read one.

2:29Suppose you are told that caterpillars eat oak leaves and small birds eat caterpillars. Begin with the oak tree because it is the producer, then place the caterpillar next, followed by the bird. Draw an arrow from the oak tree to the caterpillar and another from the caterpillar to the bird, checking each time that energy moves towards the eater. Not all feeding begins with living material being eaten. A decomposer is an organism that gets its energy from dead or waste organic material.

2:54Many bacteria and fungi are decomposers, obtaining nutrients from materials such as dead leaves, animal remains and waste. Dead material can come from every trophic level, so decomposers are connected to the whole feeding system rather than being one final consumer above all the others.

Food webs and population changes

3:09Real feeding relationships are usually more complicated than a single chain. A food web is a network of interconnected food chains. In this web, grass supplies both grasshoppers and rabbits, while hawks feed on both rabbits and frogs. The shared organisms connect separate feeding routes, showing that one species may have several food sources or several predators. To interpret a food web, start with an organism and examine its arrows.

3:33Arrows pointing towards it show which organisms supply its food energy, while arrows pointing away show which organisms feed on it. To identify a complete food chain, trace one continuous route from a producer through the consumers. Do not jump between organisms unless an arrow actually connects them in that direction. Notice that the hawk does not have one fixed consumer category in this web. Along the route from grass to rabbit to hawk, it is a secondary consumer.

3:58Along the route from grass to grasshopper to frog to hawk, it is a tertiary consumer. Consumer categories depend on feeding position, so the same organism can occupy different trophic levels in different chains. Now imagine that humans overharvest the rabbits, removing them faster than their population can replace them. With fewer rabbits feeding on grass, the grass may increase. Hawks lose one source of food, so their population may decrease if alternative prey cannot provide enough food.

4:25Hawks might also eat more frogs, showing why a food web can reveal effects that a single chain would miss. Use cautious reasoning when predicting these changes. A decrease in one prey species does not automatically mean that every predator dies, because the web may show alternative food sources. Likewise, more grass does not guarantee that every herbivore increases, since other conditions may limit them. Your explanation should follow the feeding relationships shown and describe likely effects rather than claiming certainty without evidence.

4:54Introducing a foreign species means bringing a species into a habitat where it did not previously occur. Suppose an introduced predator eats the frogs in our web. Frog numbers may decrease, allowing grasshoppers to increase because fewer are eaten, and increased grasshopper feeding may reduce the grass. Hawks could also lose frog prey, while an introduced species that shares their prey could compete with them for food.

Energy losses in food chains

5:16Food webs show where energy can move, but they do not show that every transfer is complete. A rabbit does not transfer all the energy it obtains from grass into new rabbit tissue. It uses much of that energy to stay alive, and some food energy never becomes part of its body. Only the energy remaining in material that a fox actually eats can enter the fox. First, some parts of an organism are not eaten, such as roots left in the ground or bones left by a predator.

5:40Second, some eaten material is not digested and absorbed, so its energy leaves in faeces through egestion. Some energy-containing substances also leave in excretory products. These routes reduce the energy available to the next consumer in the chain. Third, organisms release energy through respiration, and energy is transferred to the surroundings as heat. Respiration also supplies energy for activities such as movement, which ultimately results in further heat transfer. That energy is therefore not retained as chemical energy in new biomass for the next consumer.

6:12Energy has not been destroyed, but it has become unavailable for transfer along that food chain. This explains why food chains usually have fewer than five trophic levels. At every transfer, less energy remains available to support the next level. Eventually, too little is available to support another population of consumers. Five-level chains can exist, like our grasshopper example, but the existence of quaternary consumers does not mean that every habitat can support them.

6:37The same reasoning explains why it is more energy efficient for humans to eat crop plants than livestock fed on those crops. If humans eat the crops directly, there is one transfer from the crop plants to humans. Feeding the crops to livestock adds another transfer before the food reaches us. The livestock lose energy through respiration, movement and waste, leaving less energy in the animal tissue that humans eat.

6:58This is a comparison of energy transfer, not a claim that every food has the same nutritional value. The key point is that an extra consumer level introduces additional energy losses. Starting with the same quantity of crop energy, more can reach humans through the shorter feeding route.

Pyramids of numbers and biomass

7:13Now we can use ecological pyramids to represent different features of these feeding relationships. A pyramid of numbers represents the number of organisms at each trophic level. Put producers at the bottom and their consumers above them, with each bar’s width proportional to the number of organisms. In this illustrative example, there are one thousand grass plants, one hundred rabbits and ten foxes. The bars become narrower upwards, but that shape is not guaranteed because a pyramid of numbers counts organisms without considering their size.

7:42Consider one oak tree supporting twenty caterpillars, which support two small birds. The producer bar is narrow because there is only one tree, while the primary consumer bar is much wider. This does not mean that the caterpillars contain more living material than the tree. It means only that there are more individual caterpillars, which exposes an important limitation of counting organisms. Biomass is the mass of living material in organisms, and ecological comparisons commonly use dry mass.

8:08Removing water makes the comparison less dependent on differences in water content. A pyramid of biomass represents the total biomass at each trophic level in a given area at a particular time. Instead of treating a tree and a caterpillar as equivalent individuals, it compares how much living material they contain. A pyramid of biomass therefore accounts for differences in organism size and usually gives a better indication of the material available as food.

8:32In the oak example, the tree’s biomass can be much greater than the combined biomass of the caterpillars. The biomass pyramid can have a broad producer base even though the numbers pyramid has a narrow one. More individuals does not necessarily mean more biomass. Let us work through drawing a biomass pyramid from illustrative data. In the same sampled area, grass has a dry biomass of eight hundred grams per square metre, grasshoppers eighty, and frogs eight.

8:58First identify the feeding order, so grass goes at the bottom, grasshoppers in the middle, and frogs at the top. We use total biomass for each trophic level, not the mass of just one organism. Next choose a scale that keeps the bar widths proportional. For example, the grass bar could be twenty centimetres wide, making the grasshopper bar two centimetres wide and the frog bar two millimetres wide. Centre the bars above one another, keep their heights equal, and label the organisms and units.

9:24The width shows biomass, while the vertical order shows the trophic levels. However, biomass is a snapshot rather than a measure of energy transferred over time. In some aquatic food chains, tiny producers grow and are eaten rapidly, so their biomass at one moment may be smaller than the biomass of their consumers. A biomass pyramid can therefore be inverted at its base. The producers still supply the food, but their standing biomass does not show how much they produce over a longer period.

Pyramids of energy

9:51A pyramid of energy represents the energy transferred at each trophic level per unit area per unit time. Those last words matter because energy transfer is measured over a period, not just at one moment. Units might be kilojoules per square metre per year. As with the other pyramids, place producers at the bottom and make the bar widths proportional to the values. In this illustrative energy pyramid, the producer level transfers twenty thousand kilojoules per square metre per year, the primary consumers two thousand, and the secondary consumers two hundred.

10:21To interpret it, compare each level with the one below. The primary consumers receive only part of the energy represented at the producer level, and less remains at the secondary consumer level. These example values are not a rule that every ecosystem follows. A pyramid of energy is always upright because energy is lost from the feeding pathway at every transfer. Unlike a pyramid of numbers, it is not distorted by counting one large organism as equivalent to one small organism.

10:47Unlike a biomass pyramid, it measures transfer over time rather than standing material at one moment. It therefore gives the clearest comparison of energy flow between trophic levels. Keep the three measurements separate when interpreting a diagram. Numbers means how many organisms, biomass means how much living material, and energy means how much energy is transferred over a stated time. A wide bar means a large value of whichever quantity the diagram measures.

11:12You cannot assume that a wide numbers bar also represents the greatest biomass or the greatest energy transfer.

Exam answers and recap

11:19Let us bring this together with an exam-style question. Using this food web, describe the likely effects of overharvesting rabbits on the grass and hawks, and explain why less energy reaches hawks than is available in producers. Begin by tracing the arrows connecting rabbits to their food and predators. Then separate the population changes from the reasons for inefficient energy transfer, so each part of the question receives a direct answer.

11:40A suitable answer would be this. Grass may increase because fewer rabbits feed on it. Hawks may decrease because fewer rabbits are available as prey. Energy is transferred to the surroundings as heat from respiration. Energy is used for movement and ultimately transferred as heat, while further energy remains in uneaten material or leaves in egested and excreted material. If the question asks you to discuss the advantages of a pyramid of energy, make clear comparisons rather than just calling it better.

12:08It measures energy transfer over time rather than biomass at a single moment. It avoids the problem of counting differently sized organisms as equivalent individuals. It is always upright because less energy is available at each successive trophic level. Watch for three common mistakes that can undermine otherwise good reasoning. Arrows point towards the eater because they show energy transfer, not the direction an animal moves while hunting. Producers make organic nutrients, but they do not create energy.

12:35Finally, energy lost from a food chain has not vanished from existence. Much has been transferred to the surroundings as heat, and some remains in material outside that feeding route. Let us recap the whole picture. Food chains trace energy transfer from a producer through consumers, while food webs connect several feeding routes. Trophic levels describe feeding positions, from producers through primary, secondary, tertiary and quaternary consumers. Decomposers obtain energy from dead or waste organic material, and changes caused by overharvesting or introduced species can spread through the web.

13:06Pyramids of numbers count organisms, pyramids of biomass compare living material, and pyramids of energy compare energy transfer over time. Energy transfer between trophic levels is inefficient because not all material is eaten or absorbed, and organisms lose energy through waste and heat from respiration and activity. These losses limit food-chain length and explain why eating crops directly is more energy efficient than eating livestock fed on those crops. Once you follow both the arrows and the losses, the whole system becomes much easier to understand.