Feeding relationships

The ultimate source of energy for most living organisms is the sun. Green plants are able to trap energy from the sun and use it to convert carbon dioxide and water into food energy in the form of sugar in a process known as photosynthesis. Plants are said to make their own food and are known as producers. Animals all depend either directly or indirectly on green plants for their food and are therefore known as consumers.

• Herbivores which feed directly on plants are known as primary consumers

• Carnivores which feed on other animals are known as secondary consumers

A simple food chain shows how energy is transferred from the sun through living organisms. The carnivore at the end of the food chain is known as the top carnivore or tertiary consumer.

Most animals eat more than one kind of food and so in any ecosystem food chains connect to form a food web.

Where two animals are feeding on the same food source competition may occur. Competition occurs between animals and also plants when any resource is limited.

The different feeding levels at each stage of the food chain are known as trophic levels

• Green plants or producers form trophic level 1
• Primary consumers form trophic level 2
• Secondary consumers form trophic level 3
• The tertiary consumer forms trophic level 4

At each trophic level energy is used up and waste material produced, so there is less energy available for transfer at each stage and a food pyramid is formed

A single tree which is very large can sipport a large number of caterpillars which are relatively small. This would give a top-heavy pyramid if numbers were used. Using biomass gives a more accurate picture.

Predator / prey relationships and similarly the relationships between herbivores and plants are very important in regulating population sizes and maintaining a balance in the ecosystem. These relationships are frequently disturbed by humans. Removal of species (e.g.by the use of biocides and the over-exploitation of food stocks) and the introduction of new species can have a knock-on effect throughout the ecosystem.

Case study: the holly leaf-miner

The holly leaf-miner (Phytomyza ilicis) is a small fly which lays its eggs in leaves of the holly, where the larva feeds on the middle layers of the leaf, forming pale coloured blotches. The larvae are predated by birds such as blue tits and larvae and pupae are parasitized by a number of small wasps. One wasp (Pleurotropis amyntas) parasitises the larva of other parasitic wasps.

A simple food chain involving this species would be

However, by adding in links to more food chains we can begin to show that like most species the Holly leaf miner is almost certainly part of a complex food web.

The v-shaped flap on the leaf shows that this larva has been eaten by a bird	The pin-sized hole on the leaf shows that the larva has been parasitised by a wasp	Dissection of this mine has revealed the empty pupal case of a holly leaf-miner - the adult fly will have emerged successfully

In a collection of holly leaves it is easy to count the number of leaves infected by the fly. It is also not difficult to identify leaves:

• with healthy larvae
• with pupae
• where larvae have been parasitized
• where pupae have been parasitized
• where adult flies have successfully emerged
• predated by birds

Using this information it is possible to draw up a food chain with population sizes as shown below.

Remember this is based on numbers and evidence of cause of death. A much clearer picture would be available if it were possible to measure biomass at each stage of the food chain.

Using population estimates of the fly, the different parasites and the amount of predation by birds you can think about possible future population changes. For example if you find evidence for a large amount of bird predation, you can consider how might this affect the population of the holly leaf-miner, its parasites and the blue tit in the following year.