An ecosystem is a community of organisms together with the non living factors around them. Energy enters from light and thins out at each feeding step.
The idea
Biotic factors are the living influences: predation, competition, disease, and the food an organism eats. Abiotic factors are non living: temperature, light, water, pH and salinity. A population is one species in a place. A community is all the populations. Add the abiotic surroundings and you have an ecosystem. A food chain shows one path of feeding. A food web shows how those paths join. Arrows follow energy, from the organism eaten to the organism that eats it.
A pyramid of energy shows the energy stored at each level in a given time. It is always narrow at the top, because at every transfer most energy is used in life processes or lost as heat, and only a small part becomes new biomass. A pyramid of numbers can be inverted, for example one tree feeding many insects, so numbers are a poor guide. Energy is the reliable pyramid. Producers are the base. Remove them and the levels above starve.
What earns the marks
Define the term the question names. Ecosystem is not a synonym for habitat. A habitat is the place. An ecosystem is the community plus the non living factors. Say why a pyramid of numbers can mislead, if that is the question, and do not claim a pyramid of energy can be inverted. It cannot, if it is truly energy per unit time. Avoid a made up percentage. "Only a small part is passed on" is the point.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
4 marks
(a) Distinguish between a habitat and an ecosystem. (2 marks)
(b) Give one abiotic factor and one biotic factor that could affect a population of tilapia in a pond. (2 marks)
Model answer
(a) A habitat is the place where an organism lives. An ecosystem is the community of organisms together with the non living factors in that place.
(b) Abiotic: the amount of dissolved oxygen, or the temperature. Biotic: predation, or competition for food.
Mark scheme
Habitat is the place or the type of surroundings1 mark
Ecosystem includes the community and the non living factors1 mark
A genuine abiotic factor for water, such as oxygen, temperature, pH or light1 mark
A genuine biotic factor, such as predators, parasites, competitors or food organisms1 mark
2Question 2
4 marks
Explain why a pyramid of energy is not inverted, whereas a pyramid of numbers sometimes is. (4 marks)
Model answer
Energy is lost at every transfer, so each level stores less energy than the one below it. Numbers need not fall. One tree is a single organism and it can feed hundreds of insects, so the number pyramid is upside down even though the energy pyramid is not.
Mark scheme
Energy is lost at each transfer, as heat or in metabolism1 mark
So each higher level has less energy than the level below1 mark
Numbers can rise if many small organisms feed on one large one1 mark
A correct example, such as one tree and many insects1 mark
3Question 3
1 mark
State what the arrows in a food web represent. (1 mark)
Model answer
The direction in which energy flows, from food to consumer.
Mark scheme
Direction of energy transfer, or from prey to predator1 mark
Practise Biology with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
What is the difference between a habitat and an ecosystem?+
A habitat is where an organism lives. An ecosystem is the living community in a place together with the non living factors, such as temperature, light and water. A pond habitat for tilapia is not the whole ecosystem until you include the other organisms and those factors.
Why does a pyramid of energy narrow toward the top?+
Each organism uses energy to live, and much of it leaves as heat. Only a small part is built into the body and can be eaten by the next level. The next level therefore has less energy available than the one below. All WASSCE Biology past questions