Trophic structure and ecological pyramids

Overview

Trophic level, food chain and pyramid of numbers.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an autotroph?

   Model answer

   An autotroph is an organism that manufactures its own food from simple inorganic substances rather than by consuming other organisms. Green plants and algae are the important examples: they use chlorophyll to capture solar energy and combine carbon dioxide and water into carbohydrate by photosynthesis, releasing oxygen. Autotrophs are also called producers, and they occupy the first trophic level. They are the point at which energy enters the living part of the ecosystem, so everything else in the system ultimately depends on them.

2. 2

   What is a heterotroph?

   Model answer

   A heterotroph is an organism that cannot manufacture its own food and must obtain its energy and nutrients by consuming other organisms or their remains. All animals are heterotrophs, and so are fungi and most bacteria. They are also called consumers, and they occupy the second and all higher trophic levels: herbivores eat plants, carnivores eat other animals, and omnivores eat both.

3. 3

   What is a trophic level?

   Model answer

   A trophic level is the position an organism occupies in a feeding sequence, defined by the number of steps it is removed from the producers. The first trophic level is the producers, the second is the primary consumers or herbivores, the third is the secondary consumers, which are carnivores eating herbivores, and the fourth is the tertiary consumers or top carnivores. An organism that eats at more than one level, such as an omnivore, occupies more than one trophic level at once.

4. 4

   What do decomposers do, and why is the ecosystem dependent on them?

   Model answer

   Decomposers break down dead organic matter - fallen leaves, dead wood, corpses and animal waste - into simpler compounds, releasing the nutrients locked inside it back into the soil in a soluble form that plant roots can absorb. They include bacteria and fungi, which digest material chemically, and detritivores such as earthworms, woodlice, millipedes and beetles, which physically fragment litter and so increase the surface area available for microbial attack. The ecosystem depends on them because without decomposition the nutrients would stay locked in accumulating litter, the soil store would not be replenished, and plant growth would eventually cease for lack of nutrients even where light and water were plentiful. They also work on material from every trophic level, so they operate across the whole system rather than at one point in the chain.

5. 5

   What is a food chain?

   Model answer

   A food chain is a simple linear sequence showing the direction in which energy and nutrients are transferred as one organism is eaten by another, for example oak leaf to caterpillar to blue tit to sparrowhawk. The arrows point in the direction the energy travels, that is from the organism being eaten towards the one eating it. Each link represents one trophic level.

6. 6

   Why is a food web a more realistic model than a food chain?

   Model answer

   Because very few organisms feed on only one thing or are eaten by only one predator. A food web shows many interlocking food chains, with each species connected to several others, which reflects what actually happens in a woodland or grassland. It also shows something a chain cannot: that a web with many alternative pathways is more resilient, because if one species declines its predators can switch to other prey and the system absorbs the change, whereas in a simple chain with few alternatives the loss of one link can cause the collapse of those above it. This is why simple ecosystems such as the tundra are so much more vulnerable than complex ones.

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