Ecosystem

Food chains, food webs, ecological pyramids and the one-way flow of energy through an ecosystem are frequently examined, usually through a diagram that a student must interpret, construct or explain, so practising these diagrams alongside the underlying reason for each pattern is essential preparation.

Beyond feeding relationships, organisms within a community interact in other ways that this chapter also covers: competition occurs when two organisms need the same limited resource, predation occurs when one organism captures and eats another, and symbiosis describes close, long-term relationships such as mutualism, commensalism and parasitism, each defined by whether one, both or neither species benefits. Recognising which of these three interaction types is being described in an unfamiliar example, rather than memorising a fixed list of named pairs, is what most exam questions on this part of the chapter actually test.

Population ecology completes the chapter by explaining how a population changes in size over time, typically following an S-shaped growth curve that levels off once the population reaches the carrying capacity of its environment, and by distinguishing the density-dependent factors, such as food supply and disease, from the density-independent factors, such as flood or drought, that limit how large a population can grow. Environmental sustainability, the theme this chapter belongs to, depends on keeping human activity within limits that populations and ecosystems can absorb without the carrying capacity itself being permanently reduced.

Content standards in this chapter

1. 24.1 Community and Ecosystem
2. 24.2 Population Ecology

Key concepts

Community and ecosystem

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.

Feeding relationships

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 flow

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.

Nutrient cycles

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.

Population ecology

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.

Ecological pyramids

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.

Habitat and niche

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.

Interspecific relationships

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.

Population growth and carrying capacity

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.

Factors affecting population size

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.

How this chapter is examined

SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.

Common exam angles

• Constructing a food chain or food web from a list of given organisms, using arrows in the correct direction.
• Explaining why a food chain rarely has more than four or five trophic levels.
• Interpreting a pyramid of number, biomass or energy, including an unusual shape.
• Distinguishing habitat from niche using a named organism as a worked example.
• Identifying the type of interaction shown in a described relationship between two species, such as mutualism or parasitism.
• Interpreting a population growth curve and identifying the carrying capacity from the graph.

Common mistakes

Study this chapter

Processes in this chapter

• Nitrogen cycle
• Carbon cycle
• Energy flow in ecosystems
• Water cycle
• Decomposition

Key terms

EcosystemFood chainFood webProducerDecomposer

Frequently asked questions

Source:SRC-DSKP-EN, SRC-FORMAT