Ecosystem, practice questions

C, A population is all the individuals of one species living in the same area at the same time.

B, Arrows show the direction of energy flow, from the organism eaten to the organism eating it.

C, Producers make their own food by photosynthesis and form the base of the food chain.

B, Energy is lost as heat during respiration and used for the organism's own life processes at each level.

C, Nutrients such as carbon and nitrogen are recycled repeatedly; energy flows through an ecosystem only once.

C, A pyramid of energy is always a true pyramid because energy is always lost between trophic levels.

B, A habitat is the physical place an organism lives; a niche is its functional role.

C, Commensalism is a relationship in which one species benefits and the other is neither helped nor harmed.

B, Carrying capacity is the maximum population size an environment can sustainably support.

C, A flood affects a population by a similar proportion regardless of its size, making it density-independent; food supply, disease and predation are density-dependent.

• (a) One valid food chain is grass → grasshopper → hawk (arrows pointing towards the organism that eats).
• (b) A food web shows that most organisms eat more than one type of food and are eaten by more than one predator, unlike a single food chain, which shows only one pathway.
• This gives a more realistic picture of an ecosystem's feeding relationships.
• (c) An organism in a food web has alternative food sources if one prey species becomes scarce.
• This makes the community less vulnerable if one species is lost, compared with a community linked only by single food chains.
• A food web therefore reflects the interdependence of many species rather than a single linear relationship.

• (a) One oak tree can support a very large number of caterpillars, so the pyramid of numbers has a narrow base and a wider second level, making it appear inverted.
• (b) The pyramid of energy measures the energy available at each level over a fixed time.
• Energy is always lost as heat and through uneaten material at each transfer, so the energy available to consumers must be less than that available to the producer.
• This means a pyramid of energy is always a true pyramid shape, unlike a pyramid of numbers.
• (c) The pyramid of energy gives the more reliable comparison because it reflects the actual amount of energy, not simply the number of organisms, moving through the food chain.
• A single large producer can distort a pyramid of numbers even though the underlying energy relationships still follow the normal pattern.

• (a) The curve levels off at the carrying capacity, the maximum population size the lake can sustainably support.
• (b) One density-dependent factor is food supply, which becomes more limiting as the population grows larger.
• One density-independent factor is a sudden drop in water temperature, which would affect the population by a similar proportion regardless of its size.
• (c) If the population temporarily exceeded the carrying capacity, limited resources such as food and oxygen would cause the death rate to rise and the birth rate to fall.
• The population would then fall back towards the carrying capacity.
• This overshoot-and-fall pattern would appear as a peak followed by a dip on the population growth graph.

A population is all the individuals of one species living in an area; a community is all the populations of different species living together in that area; and an ecosystem is the community together with the non-living, abiotic factors of its environment, such as light, temperature, water and soil, with which it interacts.

Producers make their own food by photosynthesis, primary consumers eat producers, and secondary or tertiary consumers eat other consumers; decomposers break down dead organisms and waste, returning nutrients to the environment. These feeding relationships link organisms together into food chains, and overlapping food chains together form a food web.

Energy from the Sun enters an ecosystem through photosynthesis and flows one way through the food chain from producers to consumers, decreasing at every trophic level because much of it is lost as heat during respiration, used for the organism's own life processes, or contained in parts that are not eaten.

Unlike energy, which flows through an ecosystem only once, nutrients such as carbon and nitrogen are recycled repeatedly between living organisms and the non-living environment through processes including photosynthesis, respiration, decomposition, and, for nitrogen, fixation by bacteria, so the same atoms are used again and again.

A population's size at any time depends on the balance between the processes that add individuals, birth and immigration, and the processes that remove them, death and emigration, together with limiting factors in the environment such as food supply, disease, predators and competitors.

A pyramid of numbers, biomass or energy represents each trophic level of a food chain as a bar, usually narrowing towards the top because both the amount of living material and the amount of available energy decrease at each successive level, though a pyramid of numbers can occasionally have an unusual shape.

A habitat is the physical place where an organism lives, such as a pond or a forest floor, while a niche is the organism's functional role within its ecosystem, including what it eats, what eats it, and how it interacts with other organisms. Two species can share a habitat but rarely occupy exactly the same niche for long.

Organisms interact in defined ways: competition occurs when two organisms require the same limited resource, predation occurs when a predator captures and consumes prey, and symbiosis describes a close, long-term relationship between two species that is classified as mutualism when both benefit, commensalism when one benefits and the other is unaffected, or parasitism when one benefits at the other's expense.

A population introduced into a new environment typically grows slowly at first, then rapidly as resources are plentiful, and finally levels off in an S-shaped curve once it reaches the carrying capacity, the maximum population size the environment can sustainably support with its available food, space and other resources.

Density-dependent factors, such as food availability, disease and predation, have a greater limiting effect as a population becomes larger and more crowded, while density-independent factors, such as a flood, fire or extreme cold, reduce a population by a similar proportion regardless of how large or small it already is.