Charles Darwin is said to have told a story about a village with a large number of old ladies. This village produced higher yields of hay than the nearby villages. Darwin reasoned that the old ladies kept more cats than other people and that these cats caught more field mice which were important predators of wild bees. Since these bees were essential for the pollination of red clover (and clover improved the yield of hay), Darwin concluded that food chains were the answer to the superior hay yield. He was also highlighting the fact that inter-relationships between plants and animals can be quite complicated.
At any one time in a habitat, there will be a combination of animals associated with the plant community. A first example is a commercial crop, the strawberry, where the situation is relatively simple. The strawberry is the main source of energy for the other organisms, and is referred to, along with any weeds present, as the primary producer in that habitat. Any pest (e.g. aphid) or disease (e.g. mildew) feeding on the strawberries is termed a primary consumer , whilst a ladybird eating the aphid is called a secondary consumer. A habitat may include also tertiary and even quaternary consumers.
Any combination of species such as the above is referred to as a food chain and each stage within a food chain is called a trophic level. In the strawberry, this could be represented as:
strawberry ^ aphids ^ ladybird
In the soil, the following food chain might occur:
In the pond habitat, a food chain could be:
green algae ^ Daphnia crustacean ^ minnow fish.
Within any production horticulture crop, there will be comparable food chains to the ones described above. It is normally observed that in a monoculture such as strawberry, there will be a relatively short period of time (up to 5 years) for a complex food chain to develop (involving several species within each trophic level). However, in a long-term stable habitat, such as oak wood land or a mature garden growing perennials, there will be many plant species (primary producers), allowing many food chains to occur simultaneously. Furthermore, primary consumer species, e.g. caterpillars and pigeons, may be eating from several different plant types, whilst secondary consumers such as predatory beetles and tits will be devouring a range of primary consumers on several plant species. In this way, a more complex, interconnected community is developed, called a food web (see Figure 3.4).
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