Ecology and Ecosystems: Biology 0610 (Cambridge O Level / IGCSE)

A tropical pitcher plant has ordinary green leaves that photosynthesise like any other plant. Some of its leaves are also modified into pitcher-shaped traps. Insects are lured inside the pitchers, drown in a pool of digestive fluid, and are broken down. The plant absorbs minerals released from the digested insects, but photosynthesis remains its main source of energy.

Which term correctly describes the pitcher plant's role in a food web, and why?

Ecologists studying a single, very large mangrove tree growing at the edge of a tidal creek recorded the organisms living on and around it. Among the tree's roots they counted 850 mangrove tree crabs, which feed only on the tree's fallen leaves and bark. Over the wider creek, they also recorded 15 collared kingfishers, which hunt over several mangrove trees and prey mainly on the mangrove tree crabs.

(a) State the trophic level of the mangrove tree crab and of the collared kingfisher in this food chain. [2]

(b) The mangrove tree itself is counted as a single producer, yet it supports 850 mangrove tree crabs. Describe the shape of a pyramid of numbers drawn using these figures, and explain why it does not look like a typical, upright pyramid. [3]

(c) State one advantage of drawing a pyramid of biomass, rather than a pyramid of numbers, to represent this food chain. [2]

(d) The collared kingfishers were also seen occasionally catching small fish from tidal pools nearby. Use this observation to explain the difference between a food chain and a food web. [2]

In an area of savanna grassland, ecologists measured the energy stored in the tissues of the organisms in one food chain, in kilojoules per square metre per year (kJ/m²/year):

• Elephant grass (producer): 250 000
• Grasshoppers that eat the grass (primary consumers): 20 000
• Fork-tailed drongos that eat the grasshoppers (secondary consumers): 1600
• Pale chanting goshawks that eat the drongos (tertiary consumers): 80

(a) Describe the shape of a pyramid of energy drawn using these four values, and state whether a pyramid of energy can ever be the opposite shape (wider at a higher trophic level than at the level below it). [2]

(b) Calculate the percentage of the energy in the elephant grass that is transferred to the grasshoppers. Show your working. [2]

(c) Explain, in terms of energy losses, why less than all of the energy stored in the grasshoppers is transferred to the drongos that eat them. [3]

(d) Explain, using the energy values given, why this food chain is unlikely to support a fifth trophic level, a predator that eats the goshawks. [2]

A market gardener grows French beans, a type of legume, in one plot. Swellings called root nodules on the bean roots contain nitrogen-fixing bacteria. After harvesting the beans, the gardener digs the whole bean plant, roots, stems and leaves, back into the soil as green manure instead of removing it.

The following season, maize is grown in the same plot without adding any artificial fertiliser, and it grows well. At the end of the maize season, the old maize stalks are collected and burned on a bonfire, and the ash is spread back onto the soil.

(a) Name the process by which the bacteria in the bean's root nodules convert nitrogen gas from the air into a form the plant can use. [1]

(b) Explain how nitrogen locked up in the proteins of the buried bean plant eventually becomes available to the following season's maize plants as nitrate ions. Refer to the roles of decomposers and nitrifying bacteria. [4]

(c) Other soil bacteria convert some nitrate ions in the soil back into nitrogen gas, reducing the amount available to crops. Name this process. [1]

(d) Explain what happens to the carbon that was stored in the maize stalks when they are burned, naming the process involved. [2]

A student set up a sealed culture bottle containing a fixed amount of banana-based fruit-fly medium and introduced a small starting population of fruit flies. The bottle was kept at a constant, suitable temperature, and the number of adult fruit flies present was counted every two days:

During which day interval was the fruit fly population growing at its fastest rate?

Carbon dioxide is constantly added to and removed from the atmosphere by different living processes in an ecosystem.

Which process releases carbon dioxide into the atmosphere and is carried out by producers, consumers and decomposers alike?

A small, undisturbed woodland contains oak trees, deer that browse on low leaves, and fallen leaves and twigs that build up on the woodland floor. Fungi and bacteria in the leaf litter break down this dead plant material over time. Occasionally, a fallen branch is collected by a park ranger and burned on a campfire.

(a) Name the process by which the oak trees remove carbon dioxide from the atmosphere and use the carbon to build organic compounds in their tissues. [1]

(b) State two different living processes, occurring in this woodland, that return carbon dioxide to the atmosphere. [2]

(c) Explain what happens to the carbon stored in the dead leaves and twigs as fungi and bacteria break them down in the leaf litter. [3]

(d) Name the process occurring when the fallen branch is burned on the campfire, and state what happens to the carbon it contained. [2]

Ecologists studying a garden pond recorded the estimated numbers of organisms in one food chain:

• Duckweed (a tiny floating producer): 8000 plants
• Water fleas, which feed on the duckweed: 400
• Small fish, which feed on the water fleas: 25
• Herons, which feed on the small fish: 2

(a) State the trophic level of the water flea and of the small fish in this food chain. [2]

(b) Using the numbers given, describe the shape of a pyramid of numbers for this food chain, and explain why it has this shape. [3]

(c) A student claims that a pyramid of numbers is always a perfectly regular, upright triangle for any food chain. Using an example, explain why this claim is not always true. [2]

Twelve rabbits were introduced onto a small island that had abundant grass but no rabbits and no natural predators. Over the next few years, the rabbit population grew rapidly, then levelled off at a roughly stable size.

(a) Name two factors, other than predation, that could limit the size of the rabbit population as it approaches its stable size on the island. [2]

(b) Explain how competition for food, in particular, would slow the rate of population growth as rabbit numbers increase. [3]

(c) Several years later, a small number of foxes were introduced to the island. Predict and explain the likely short-term effect of the foxes on the rabbit population. [2]

(d) Define the term "carrying capacity", as illustrated by the stable rabbit population size reached before the foxes were introduced. [1]

In a food chain, typically only around 10% of the energy stored in the tissues of organisms at one trophic level is transferred to the tissues of organisms at the next trophic level.

Which of the following is not a reason for this loss of energy between two trophic levels that are already part of the same food chain?