Arrows in a food chain show the direction of fl ow of both the energy and nutrients that keep organisms alive. Energy fl ow through an ecosystem can be quantifi ed and analysed. These studies reveal that, at each step in the food chain, energy is lost from the chain in various ways. Some is not consumed, some leaves the food chain as waste or when an animal dies, and some is used by living organisms as they respire (Figure 5.3). In all three cases, the lost energy cannot be passed to the next trophic level.
The end of the chain
When an organism dies, its remains provide nutrients for other groups of organisms called detritivores and saprotrophs. Detritivores are organisms that ingest dead organic matter, whereas saprotrophs are organisms that secrete digestive enzymes onto the organic matter and then absorb their nutrients in a digested form. Saprotrophs are therefore responsible for the decomposition of organic matter and are often referred to as decomposers. Saprotrophic bacteria and fungi are the most important decomposers for most ecosystems and are crucial to the recycling of nutrients such as nitrogen compounds.
Trophic levels
Every ecosystem has a structure that divides organisms into trophic levels on the basis of their food sources. Trophic means ‘feeding’ and every organism in a food chain is on a particular feeding level.
Green plants are producers and are at the lowest trophic level. Above them come all the consumer levels. The fi rst consumers, or primary consumers, are always herbivores. Any organism above the herbivores will be a carnivore and these can be listed as secondary consumer, tertiary consumer and so on. A food chain can therefore be summarised as:
producer → primary consumer → secondary consumer → tertiary consumer
Food webs
Few consumers feed on only one source of food. For example, this food chain describes one set of feeding relationships:
grass → beetle → tree creeper → sparrowhawk
But beetles eat a wide range of plants, tree creepers eat other types of insect and sparrowhawks eat other birds. So this food chain could be interlinked with many others. A food web like the one shown in Figure 5.2 shows a much more realistic picture of the feeding relations of the organisms in a habitat. Notice how organisms change trophic levels depending on what they are eating at any particular time. In Figure 5.2, for example, the fox is a primary consumer when it is eating a crab apple but a secondary consumer when it is eating a woodmouse.
1 In the food web in Figure 5.2, what is the trophic level of:
a the tree creeper when eating a caterpillar?
b the fox when eating a great tit?
sparrowhawk
tree creeper
energy from all organisms
decomposers
caterpillar beetle wood mouse rabbit
crab apple tree grass
bluebell oak tree
great tit
fox
tertiary consumer
primary consumers
producers
secondary consumers
heatenergy
light energy
respiration
waste elimination and death digestion, assimilation and growth photosynthesis
respiration detritivores
primary consumers
Processes photosynthetic
plants (producers)
tertiary consumers
secondary consumers
Figure 5.3 Energy losses at each trophic level of a food chain.
Figure 5.2 A food web in oak woodland.
Consider an area of African savannah where grass, antelopes and cheetahs form a simple food chain.
•
Energy loss 1 – not consumed. The grass stores energy from photosynthesis but the antelopes only eat some parts of the grass, so they do not consume all the energy it has stored.•
Energy loss 2 – not assimilated. The grass that is eaten passes through the digestive system of the antelope but not all of it is digested and absorbed, so some passes out in the faeces.•
Energy loss 3 – cell respiration. The antelope uses energy to move and to keep its body temperature constant. As a result, some energy is lost to the environment as heat.The assimilated energy remaining after respiration goes into building the antelope’s body and this energy becomes available to the cheetah when it eats the antelope.
Ecologists represent the transfer of energy between trophic levels in diagrams called energy pyramids. The width of each of the layers in the pyramid is proportional to the amount of energy it represents. So the antelope → cheetah energy transfer would appear as in Figure 5.4. This section of an energy pyramid shows that only about 10% of energy from the antelope passes to the cheetah and about 90% has been lost.
Energy losses occur at every step in a food chain, as the energy pyramid in Figure 5.5 illustrates. Every link in the chain results in losses, so that eventually there will be insuffi cient energy to support any further trophic levels. Most food chains commonly contain between three and fi ve organisms, and seldom more than six. The energy that enters an ecosystem as light is converted to stored chemical energy and fi nally lost as heat.
Energy fl ow and nutrient recycling
All the organic matter from an organism, including everything from living or dead material to waste, is eventually consumed by other organisms.
All these organisms respire and release energy as heat. All the energy that enters ecosystems as light energy, and is trapped by photosynthesis, will eventually be converted to heat and become unavailable to be used again by living things.
Nutrients, on the other hand, are continually recycled. A nitrogen atom may be absorbed as nitrate by a plant root and used to make an
energy in cheetah energy in antelope
sunlight
energy transfer
producers primary consumers secondary consumers tertiary consumers Figure 5.4 A simple energy pyramid for a
single energy transfer.
Figure 5.5 A generalised energy pyramid.
5 ECOLOGY AND EVOLUTION 105 amino acid. The amino acid may pass into an animal when the plant
material is eaten, and then pass out of the animal’s body during excretion.
Soil bacteria may convert urea in the excreted material back into nitrate and the cycle begins again.
Decomposers in the soil, the saprotrophic bacteria and fungi, are essential for the recycling of nutrients. You can fi nd out more about nutrient cycles on pages 106 and 443.
2 Defi ne the following terms.
a ‘species’ d ‘community’
b ‘habitat’ e ‘ecosystem’
c ‘population’ f ‘ecology’
3 Distinguish between an autotroph and a heterotroph.
4 Distinguish between consumers, detritivores and saprotrophs.
5 Give an example of a food chain containing four named organisms.
6 What do the arrows represent in a food chain?
7 Describe what is meant by a food web.
8 What is the initial energy source for most food chains?
9 Defi ne ‘trophic level’.
10 List the three ways energy is lost when moving from one trophic level to the next.
11 The leaves of a tree store 20 000 J m−2 y −1 of energy. What is the approximate amount of energy stored by the caterpillars that feed on the leaves?
12 Name the two types of decomposer.
13 State the difference between energy movement and nutrient movement in an ecosystem.
5.2 The greenhouse effect
Assessment statements