5.1: Communities and Ecosystems

Question 1

Define species, habitat, population, community, ecosystem and ecology:

Answer 1

Species: a group of organisms that can interbreed and produce fertile offspring.

Habitat: the environment in which a species normally lives or the location of a living organism.

Population: a group of organisms of the same species who live in the same area at the same time.

Community: a group of populations living and interacting with each other in an area.

Ecosystem: a community and its abiotic environment.

Ecology: the study of relationships between living organisms and between organisms and their environment.

Question 2

Distinguish between autotroph and heterotroph:

Answer 2

Autotrophs are organisms that synthesise their organic molecules from simple inorganic substances whereas heterotrophs are organisms that obtain organic molecules from other organisms.

Question 3

Distinguish between consumers, detritivores and saprotrophs:

Answer 3

Consumer: an organism that ingests other organic matter that is living or recently killed.

Detritivore: an organism that ingests non-living organic matter.

Saprotroph: an organism that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion.

Question 4

Describe what is meant by a food chain, giving an example:

Answer 4

A food chain shows the direction of energy flow from one species to another. For example, an arrow from A to B means that A is being eaten by B and therefore indicates the direction of the energy flow.

Question 5

Describe what is meant by a food web:

Answer 5

A food web is a diagram that shows all the feeding relationships in a community with arrows which show the direction of the energy flow.

Question 6

Define trophic level

Answer 6

Trophic level: the trophic level of an organism is its position in the food chain. Producers, primary consumers, secondary consumers and tertiary consumers are examples of trophic levels.

Question 7

Deduce the trophic level of organisms in a food chain and a food web:

Answer 7

Plants or any other photosynthetic organisms are the producers. Primary consumers are the species that eat the producers. Secondary consumers are the species that eat the primary consumers and tertiary consumers in turn eat the secondary consumers.

Question 8

State that light is the initial energy source for almost all communities.

Answer 8

Light is the initial energy source for almost all communities.

Question 9

Explain the energy flow in a food chain:

Answer 9

Producers receive their energy from light energy (the sun) by means of photosynthesis. After this, the energy in organic matter flows from producers to primary consumers to secondary consumers to tertiary consumers. This is because producers will be eaten by primary consumers which in turn will be eaten by secondary consumers and so on. However, between these trophic levels, energy is always lost. All of the trophic levels lose energy as heat through cell respiration. Also, as the organic matter passes from one trophic level to the next, not all of it is digested and so we have loss of energy in organic matter through feces. This energy then passes on to the detritivores and saprotrophs. Another energy loss occurs through tissue loss and death which can happen at any trophic level. Once again, this energy would be passed on to detritivores and saprotrophs as they digest these. Detritivores and saprotrophs in turn lose energy as heat through cell respiration.

Question 10

State that energy transformations are never 100% efficient.

Answer 10

Energy transformations are never 100% efficient.

Question 11

Explain the reason for the shape of pyramids of energy:

Answer 11

• A pyramid of energy is a graphical representation of the amount of energy of each tropic level in a food chain
• They are expressed in units of energy per area per time (e.g. kJ m2 year -1)
• Each level of the pyramid of energy should be approximately one tenth the size of the level preceding it, as energy transformations are ~10% efficient

Question 12

Explain that energy enters and leaves ecosystems, but nutrients must be recycled:

Answer 12

1. Energy is not recycled. Constantly being supplied to the ecosystem through light energy.
2. Energy is lost from the ecosystem in the form of heat through cell respiration.
3. Nutrients must be recycled as there is only a limited supply of them.
4. They are absorbed by the environment, used by organisms and then returned to the environment.

Question 13

State that saprotrophic bacteria and fungi (decomposers) recycle nutrients.

Answer 13

Saprotrophic bacteria and fungi (decomposers) recycle nutrients.