Trophic structure, the nutrient cycle, and why clearance is so damaging

Overview

Trophic structure of an ecosystem, trophic structure of a tropical forest from the bottom up and three nutrient stores in an ecosystem.

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 meant by the trophic structure of an ecosystem, and why can one animal occupy more than one trophic level in a tropical forest?

   Model answer

   Trophic structure is the arrangement of an ecosystem's organisms into feeding levels, each level defined by how many steps it sits from the original capture of energy by plants. The first level is the producers, the second the herbivores that eat them, the third the carnivores that eat the herbivores, and so on, with decomposers breaking down dead material from every level. In a tropical forest a single animal frequently feeds at several of these levels at once: a bird that takes fruit, insects and the occasional small lizard is acting as a primary consumer, a secondary consumer and a tertiary consumer in the same day. Trophic level is therefore a property of a feeding relationship rather than a fixed label belonging to a species, which is why the forest is described as a food web rather than a set of separate food chains.

2. 2

   Describe the trophic structure of a tropical forest from the bottom up.

   Model answer

   Producers are the green plants: the emergent, canopy and under-canopy trees together with lianas, epiphytes, shrubs and ground plants, which fix solar energy by photosynthesis and make up the overwhelming majority of the living biomass. Primary consumers are the herbivores, dominated by leaf-eating and fruit-eating insects, birds, bats and primates in the canopy rather than by large grazing mammals. Secondary consumers are small carnivores such as insectivorous birds, frogs, lizards and small mammals. Tertiary consumers are the top predators, including big cats, snakes, eagles and, in the rivers, large fish and reptiles, and they are few in number. At every level detritivores and decomposers, particularly termites, ants, fungi and bacteria, break down dead tissue and return the nutrients.

3. 3

   Describe what a pyramid of biomass for a tropical forest would look like, and explain its shape.

   Model answer

   A pyramid with an extremely broad base and very rapidly narrowing tiers above it. The producer tier is enormous because the biomass is dominated by long-lived woody trunks, branches and roots, of the order of several hundred tonnes per hectare. The herbivore tier is a small fraction of it, the carnivore tier smaller again, and the top predator tier only a sliver. The shape follows from the fact that only about ten per cent of the energy at each level is passed to the next, the rest being lost as heat in respiration and in undigested material, so each successive level can support far less living tissue than the one below.

4. 4

   Why are there so few large predators in a tropical forest despite its enormous productivity?

   Model answer

   Because energy is lost at every transfer, so only about a tenth of the energy in each trophic level reaches the next. Four levels above the producers leaves very little to support top carnivores, which is why they are large-bodied, few in number and hold very large territories. In addition much of the forest's productivity is locked in woody tissue that few herbivores can eat, and the herbivores themselves are widely dispersed in the canopy, so predators must range widely to find them. Food chains in the forest are therefore short and food webs, though highly complex, support only sparse populations at the top.

5. 5

   Why is the tropical forest food web described as complex, and why does that complexity make it fragile rather than robust?

   Model answer

   It is complex because there are so many species and so many specialised feeding relationships: individual insects tied to a single tree species, pollinators tied to a single flower, seed dispersers on which particular trees depend for regeneration. The fragility follows from the specialisation, not from the number of links. Where a consumer depends on one food source, the loss of that source removes the consumer, and where a tree depends on one pollinator or disperser, the loss of the animal ends the tree's regeneration. Removing one species can therefore cascade through the web in a way that would not happen in a simpler ecosystem where most organisms are generalists with alternative foods.

6. 6

   What role do decomposers play in the tropical forest, and why are they so effective there?

   Model answer

   They break down dead leaves, wood, faeces and carcasses, releasing the nutrients locked in that dead organic matter back into the soil in a soluble form that roots can take up. They are exceptionally effective in the tropical forest because the constant warmth and moisture are ideal for bacterial and fungal activity, so decomposition is very rapid: a fallen leaf may be gone in weeks rather than the years it would take in a cool climate. Termites and ants shred material and greatly accelerate the process. This is why the litter layer on a rainforest floor is surprisingly thin despite the huge quantity of material falling onto it.

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