Food chains and webs

‘Interdependence’ means the way in which living organisms depend on each other in order to remain alive, grow and reproduce. For example, bees depend for their food on pollen and nectar from flowers. Flower depend on bees for pollination. Bees and flowers are, therefore interdependent.

Food chains

Every organism needs food to survive and they depend on each other for their food. In any ecosystem, there is a hierarchy of feeding relationships that influences how nutrients and energy pass through it. Many animals, such as rabbits, feed on plants. Such animals are called herbivores. Animals that eat other animals are called carnivores. A predator is a carnivore that kills and eats other animals. A fox is a predator that preys on rabbits. Scavengers are carnivores that eat the dead remains of animals killed by predators. These are not hard and fast definitions. Predators will sometimes scavenge for their food and scavengers may occasionally kill living animals. Animals obtain their energy by ingestion.

Basically, all animals depend on plants for their food. Foxes may eat rabbits, but rabbits feed on grass. A hawk eats a lizard, the lizard has just eaten a grasshopper but the grasshopper was feeding on a grass blade. This relationship is called a food chain.

The organisms at the beginning of a food chain are usually very numerous while the animals at the end of the chain are often large and few in number. The food pyramids in Figure below show this relationship.

There will be millions of microscopic, single-celled algae in a pond. These will be eaten by the larger but less numerous water fleas and other crustacea, which in turn will become the food of small fish such as minnow and stickleback. The hundreds of small fish may be able to provide enough food for only four or five large carnivores, like pike or perch.

▲ Plankton. The microscopic organisms that live in the surface waters of the sea or fresh water are called, collectively plankton. The single-celled algae are the phytoplankton. They are surrounded by water, salts and dissolved carbon dioxide. Their chloroplasts absorb sunlight and use its energy for making food by photosynthesis. Phytoplankton is eaten by small animals in the zooplankton, mainly crustacea. Small fish will eat crustacea.

Every organism fits somewhere in a food chain, and although the organisms that make up the food chain will vary from place to place, almost every food chain starts with a green plant. It may be any part of the plant – the leaves, roots, stems, fruits, flowers or nectar. Green plants start food chains because they are able to capture light energy from the Sun and synthesise sugars, amino acids, lipids and vitamins, using simple inorganic substances such as water, carbon dioxide and minerals. Plants are called autotrophs (which means ‘self feeding’) or producers because they ‘produce’ organic compounds by photosynthesis. Every other organism in a food chain gets its organic compounds from its food and so is called a heterotroph or consumer.

Figure shows three examples of food chain from different ecosystem. Notice that arrows in the food chain always point in the direction in which the energy and nutrients flow.

A food chain from the Kalahari desert

A food chain from a freshwater lake

A food chain from the Antarctic ocean

Pyramids of numbers

The width of the bands in Figure above (pyramids of number) is meant to represent the relative number of organisms at each trophic level. So the diagrams are sometimes called pyramids of numbers.

However, you can probably think of situations where a pyramid of numbers would not show the same effect. For example a single sycamore tree may provide food for thousands of greenfly. One oak tree may feed hundreds of caterpillars. In these cases the pyramid of numbers in upside-down.

Pyramids of biomass

As stated earlier, displaying food chains using pyramids of number can produce inverted pyramids. This is because the top consumers may be represented by large numbers of very small organisms, for example, fleas feeding on an owl. The way around this problem is to consider not the single tree, but the mass of the leaves that it produces in the growing season, and the mass of the insects that can live on them. Biomass is the term used when the mass of living organisms is being considered, and pyramids of biomass can be constructed as in Figure above. A pyramid of biomass is nearly always the correct pyramid shape.

You can calculate the efficiency of Biomass transfer

• The numbers show the amount of biomass available to the next level. So 43 kg is the amount of biomass available to the greenflies and 4.2 kg is the amount available to the ladybirds.
• You can work out how much biomass has been lost at each level by taking away the biomass that is available at that level from the biomass that was available at the previous level. For example:

Biomass lost at 1st trophic level = 43 kg – 4.2 kg = 38.8 kg

• You can also calculate the efficiency of biomass transfer between trophic levels:

So at the 1st tropic level, efficiency of biomass transfer = 4.2 kg ÷ 43 kg x 100 = 9.8% efficient

Pyramids of energy

The elements that make up living organisms are recycled i.e. they are used over and over again. This is not the case with energy, which flows from producers to consumers and is eventually lost to the atmosphere as heat.

Gross production and net production

A pyramid of energy shows energy flow in an ecosystem. The lowest bar of the pyramid represents gross primary production, the total amount of energy that flows through the producers. It is measured in kilojoules of energy per square metre per year (kJm^-2 y^-1).

Net primary production is the amount of energy available to herbivores from producers after subtracting the energy used by the plants for respiration.

This can be represented as:

net production = gross production – energy lost in respiration

Similar calculations can be carried out for each trophic level and the data used to construct pyramids of energy like the one show in the Figure below

Problems with trophic levels

grass ⟶ rabbit ⟶ fox

In a simple food chain, such as the one shown above, grass is the primary producer, the rabbit is the primary consumer and the fox is the secondary consumer, so each organism is said to occupy a separate trophic level. In practice, simple food chains rarely exist – foxes do not feed exclusively on rabbits – so more complicated food webs are constructed.

In the example of a food web shown below, several of the organisms do not occupy a single trophic level because they have a varied diet. The fox could be said to be primary consumer because it eats fruit. It could also be classed as a secondary or tertiary consumer because it eats both rabbits (primary consumers) and great tits (secondary consumers). In addition, food chains and webs usually contain organisms that feed on dead material. These are the detritivores and saprotrophs, together known as decomposers, which do not fit into a particular trophic level.

To overcome the difficulty of categorising organism like these, animals are often classified according to their main food source.

As one moves up a food chain or food web, energy is lost at each trophic level through respiration and waste. Table below confirms the efficiency of transfer from one level to the next is only about 10%. This is why ecosystems rarely contain more than four or five trophic levels. There is simply not enough energy to support another level . This is demonstrated in a pyramid of biomass, which shows decreasing amounts of biomass at each level.

Food webs

The food webs for land, sea and fresh water, or for ponds, rivers and streams, will all be different. Food webs will also change with the seasons when the food supply changes.

If some event interferes with a food web, all the organisms in it are affected in some way. For example, if the rabbits in Figure below were to die out, the foxes, owls and stoats would eat more beetles and rats. Something like this happened in 1954 when the disease myxomatosis wiped out nearly all the rabbits in England. Foxes are more voles, beetles and blackberries, and attacks on lambs and chickens increased. Even the vegetation was affected because the tree seedlings that the rabbits used to nibble on were able to grow. As a result, woody scrubland started to develop on what had been grassy downs.

The effects of over-harvesting

Overfishing

Small populations of humans, taking fish from lakes or oceans and using fairly basic methods of capture, had little effect on fish numbers. At present, however, commercial fishing has intensified to the point where some fish stocks are threatened or can no longer sustain fishing. In the past 100 years, fishing fleets have increased and the catching methods have become more sophisticated.

Eventually the stocks are so depleted that it is no longer economical to exploit them. The costs of the boats, the fuel and the wages of the crew exceed the value of the catch. Men are laid off, boats lie rusting in the harbour and the economy of the fishing community and those who depend on it is destroyed. Overfishing has severely reduced stocks of many fish species: herring in the North Sea, halibut in the Pacific and anchovies off the Peruvian coast, for example. In 1965, 1.3 million tonnes of herring were caught in the North Sea. By 1977 the catch had diminished to 44000 tonnes, i.e about 3% of the 1965 catch.

Similarly, whaling has reduced the population of many whale species to levels that give cause for concern. Whales were the first marine organisms to face extinction through overfishing. This happened in the early 1800s when they were killed for their blubber (a thick fat layer around the body of the mammal) for use as lamp oil. The blue whale’s numbers have been reduced from about 2000000 to 6000 as a result of intensive hunting.

Overfishing can reduce the populations of fish species and can also do great damage to the environment where they live. For example, the use of heavy nets dragged along the sea floor to catch the fish can wreck coral reefs, destroying the habitats of many other animal species. Even if the reef is not damaged, fishing for the top predators such as grouper fish has a direct effect on the food chain: fish lower down the chain increase in numbers, and overgraze on the reef. This process is happening on the Great Barrier Reef in Australia. Grouper fish are very slow growing and take a long time to become sexually mature, so the chances of them recovering from overfishing are low and they are becoming endangered.

Introducing foreign species to a habitat

One of the earliest examples of this process was the accidental introduction of rats to the Galapagos Islands by pirates or whalers in the 17th or 18th centuries. The rats had no natural predators and food was plentiful: they fed on the eggs of birds, reptiles and tortoises, along with young animals. The Galapagos Islands provide a habitat for many rare species, which became endangered as a result of the presence of the rats. A programme of rat extermination is now being carried out on the islands to protect their unique biodiversity.

The prickly pear cactus, Opuntia, was introduced to Australia in 1839 for use as a living fence to control the movement of cattle, but its growth got out of control the movement of cattle, because of the lack of herbivores that eat it. Millions of acres of land became unusable. A moth, Cactoblastis cactorum, whose young feed on the cactus, was successfully introduced from Argentina and helped to control the spread of the cactus. Other places with similar problems, for example the island of Nevis in the West Indies, followed Australia’s example, but with less successful results. The moth had no natural predators and ate other native cactus species as well as the prickly pear, bringing them to the brink of extinction. The moth is now spreading to parts of the United States of America and poses a threat to other cactus species.

Food chains and webs can also be disrupted by the use of pesticides and other poisons, sometimes released accidentally during human activities.

Energy transfer

When a herbivorous animal eats a plant (the caterpillar feeding on a leaf), the chemical energy stored in that plant leaf is transferred to the herbivore. Similarly, when a carnivore (the blue tit) eats the herbivore, the carnivore gains the energy stored in the herbivore. If the carnivore is eaten by another carnivore (the kestrel), the energy is transferred again.

Use of sunlight

This figure of 1% will vary with the type of vegetation being considered and with climatic factors, such as availability of water and the soil temperature. Sugar-cane grown in ideal conditions can convert 3% of the Sun’s energy into photosynthetic products; sugar-beet at the height of its growth has nearly a 9% efficiency. Tropical forests and swamps are far more productive than grassland but it is difficult, and in some cases undesirable, to harvest and utilise their products.

In order to allow crop plants to approach their maximum efficiency they must be provided with sufficient water and mineral salts. This can be achieved by irrigation and the application of fertiliser.

Energy transfer between organisms

Having considered the energy conversion from sunlight to plant products, the next step is to study the efficiency of transmission of energy from plant products to primary consumers. On land, primary consumers eat only a small proportion of the available vegetation. In a deciduous forest only about 2% is eaten; in grazing land, 40% of the grass may be eaten by cows. In open water, however, where the producers are microscopic plants (phytoplankton) and are swallowed whole by the primary consumers in the zooplankton, 90% or more may be eaten. In the land communities, the parts of the vegetation not eaten by the primary consumers will eventually die and be used as a source of energy by the decomposers.

Arrows in a food chain show the direction of flow of both the energy and nutrients that keep organisms alive. Energy flow through an ecosystem can be quantified 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.

This figure will vary with the diet and the age of the animal. In a fully grown animal all the digested food will be used for energy and replacement and none will contribute to growth. Economically it is desirable to harvest the primary consumers before their rate of growth starts to fall off.

In Figure above the width of the horizontal bands is proportional to the masses (dry weight) of the organisms in a shallow pond.

It is very unusual for food chains to have more than five trophic levels because on average, about 90% of the energy is lost at each level. Consequently, very little of the energy entering the chain through the producer is available to the top consumer. The food chain below shows how the energy reduces through the chain. It is based on grass obtaining 100 units of energy.

Energy transfer in agriculture

In human communities, the use of plant products to feed animals that provide meat, eggs and dairy products is wasteful, because only 10% of the plant material is converted to animal products. It is more economical to eat bread made from the wheat than to feed the wheat to hens and then eat the eggs and chicken meat. This is because eating the wheat bread avoids using any part of its energy to keep the chickens alive and active. Energy losses can be reduced by keeping hens indoors in small cages, where they lose little heat to the atmosphere and cannot use much energy in movement. The same principles can be applied to ‘intensive’ methods of rearing calves. However, many people feel that these methods are less than humane, and the saving of energy is far less than if the plant products were eaten directly by humans, as is the case in vegetarians.

Consideration of the energy flow of a modern agricultural system reveals other sources of inefficiency. To produce 1 tonne of nitrogenous fertiliser takes energy equivalent to burning 5 tonnes of coal. Calculations show that if the energy needed to produce the fertiliser is added to the energy used to produce a tractor and to power it, the energy derived from the food so produced is less than that expended in producing it.

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.

Recycling

They produce extracellular enzymes that digest the decaying matter and then they absorb the soluble products back into their cells. In so doing, they remove the dead remains of plants and animals, which would otherwise collect on the Earth’s surface. They also break these remains down into substances that can be used by other organisms. Some bacteria, for example, break down the protein of dead plants and animals and release nitrates, which are taken up by plant roots and are built into new amino acids and proteins. This use and reuse of materials in the living world is called recycling.

Using Quadrats

Organism live in different places because the environment varies

• As you know, a habitat is the place where an organisms lives, e.g. a playing field.
• The distribution of an organism is where the organism is found e.g. in a part of the playing field.
• Where an organism is found is affected by environmental factors. An organism might be more common in one area than another due to differences in environmental factors between the two areas. For example, in the playing field, you might find that daisies are more common in the open than under trees, because there’s more light available in the open.
• There are a couple of ways to study the distribution of an organism. You can:
  • measure how common an organism is in two sample areas (e.g. using quadrats) and compare them.
  • study how the distribution changes across an area e.g. by placing quadrats along a transect.

Both of these methods give quantitative data (numbers) about the distribution.

Using quadrats to study the distribution of small organisms

• Place a 1m² quadrat on the ground at a random point within the first sample area. E.g. divide the area into a grid and use a random number generator to pick coordinates.
• Count all the organisms within the quadrat.
• Repeat steps 1 and 2 as many times as you can.
• Work out the mean number of organisms per quadrat within the first sample area.

• Repeat steps 1 to 4 in the second sample area.
• Finally compare the two means. E.g. you might find 2 daisies per m² in the shade, and 22 daisies per m² (lots more) in the open field.

You can also work out the population size of an organism in one area

Using Transects

Use transacts to study the distribution of organisms along a line

You can use lines called transects to help find out how organisms (like plants) are distributed across an area – e.g. if an organism becomes more or less common as you move from a hedge towards the middle of a field. Here’s what to do:

You can estimate the percentage cover of a quadrat

If it’s difficult to count all the individual organisms in the quadrat (e.g. if they’re grass) you can calculate the percentage cover. This means estimating the percentage area of the quadrat covered by a particular type of organism, e.g. by counting the number of little squares covered by the organisms.

Example: Some students were measuring the distribution of organisms from one corner of a school playing field to another, using quadrats placed at regular intervals along a transect. Below is a picture of one of the quadrats. Calculate the percentage cover of each organism, A and B.

(42/100) x 100

= 0.42 x 100 = 42%

Do the same for organism B

Type B = 47 squares

(47/100) x 100

= 0.47 x 100 = 47%