Ecosystem, revision notes

Overview

An ecosystem is formed when a community of different populations living together in an area interacts with the non-living, abiotic parts of their environment, such as light, temperature, water and soil. This chapter covers how communities are organised, the feeding relationships that link organisms together, the flow of energy and the cycling of nutrients through an ecosystem, and the factors that control the size of a population.

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.

Population, community and ecosystem (CS 24.1)

A population is all the individuals of one species living in the same area at the same time, able to interbreed with one another. A community is all the different populations living together in that area, and an ecosystem is that community together with the non-living, abiotic factors of its environment, such as light intensity, temperature, water availability and soil type, with which it constantly interacts.

Exam questions often test whether these three terms are used correctly at the right scale: a single species of fish in a pond is a population, all the species of fish, plants and other organisms in that pond together form a community, and the pond itself, including its water chemistry and temperature, is the ecosystem.

Abiotic factors do not simply sit alongside a community; they shape which species can survive there and how large each population can grow, while the community in turn can alter abiotic factors, for example when plant cover changes local humidity or when decomposers change soil nutrient content.

Feeding relationships: food chains and food webs (CS 24.1)

Producers make their own food by photosynthesis and form the base of every food chain; primary consumers eat producers directly, and secondary or tertiary consumers eat other consumers. Decomposers, mainly bacteria and fungi, break down dead organisms and waste at every level, returning nutrients to the environment rather than occupying a fixed position in the chain.

A food chain shows a single pathway of energy, drawn with arrows pointing from the organism being eaten to the organism eating it, for example grass to grasshopper to frog to snake. Because most organisms eat more than one type of food and are eaten by more than one predator, real communities are better represented by a food web, a network of overlapping food chains.

A common task gives a set of organisms and their feeding relationships in words and asks the student to construct a food chain or food web from them, or to classify each organism as a producer, a named type of consumer, or a decomposer. Arrow direction is one of the most frequently lost marks in this section.

Energy flow through an ecosystem (CS 24.1)

Energy from the Sun enters an ecosystem through photosynthesis and then flows in one direction only, from producers through the levels of consumers, and is never recycled. At each transfer between trophic levels, only a fraction of the energy is passed on; the rest is lost as heat during respiration, used for the organism's own life processes such as movement and growth, or remains locked in parts of the organism that are not eaten.

This progressive loss explains why a food chain rarely extends beyond four or five trophic levels: there is simply too little usable energy left to support another level of consumers. It also explains why the total mass of organisms generally decreases at each successive trophic level.

An exam question on this topic usually asks for an explanation, not just a description, of why energy decreases along a food chain, so an answer should name the specific losses, heat from respiration, use in life processes, and uneaten parts, rather than simply stating that 'energy is lost'.

Nutrient cycles: carbon and nitrogen (CS 24.1)

Unlike energy, which passes through an ecosystem only once, nutrients such as carbon and nitrogen are recycled repeatedly between living organisms and the non-living environment, so the same atoms are reused again and again rather than being constantly resupplied.

In the carbon cycle, producers remove carbon dioxide from the air during photosynthesis, consumers obtain that carbon by eating other organisms, and respiration by all organisms, together with the decomposition of dead matter, returns carbon dioxide to the atmosphere. In the nitrogen cycle, nitrogen-fixing bacteria convert atmospheric nitrogen gas into a form plants can absorb, decomposers release nitrogen compounds from dead organisms and waste, and other bacteria convert these compounds through further steps that eventually return nitrogen gas to the atmosphere.

Questions on nutrient cycles usually give a labelled diagram and ask a student to name the process at each arrow, so being able to say precisely which organisms or process, photosynthesis, respiration, decomposition, nitrogen fixation, is responsible for each step matters more than reciting the cycle from memory alone.

Ecological pyramids compared (CS 24.1)

An ecological pyramid represents each trophic level of a food chain as a horizontal bar stacked with producers at the bottom, and it is usually drawn narrowing towards the top because the amount of living material and available energy both decrease at each successive level.

Three types of pyramid are examined, and they do not always agree with one another in shape. A pyramid of numbers counts individual organisms and can occasionally show an unusual shape, for instance when a single large tree supports many small herbivores.

A pyramid of biomass measures the dry mass of organisms at each level and is more reliably pyramid-shaped, though it can still be inverted in some aquatic ecosystems. A pyramid of energy measures the energy available at each level over a fixed time and is always a true pyramid, since energy is always lost between levels and can never increase further up the chain.

Habitat, niche and interspecific relationships (CS 24.1)

A habitat is simply the physical place where an organism lives, such as a riverbank or the forest canopy, and can usually be named in a word or short phrase. A niche is much broader: it is the organism's functional role in the ecosystem, covering what it eats, what eats it, when it is active, and how it interacts with other species, so two organisms can share a habitat while occupying quite different niches.

Organisms within a community interact in three defined ways. Competition occurs when two organisms require the same limited resource, whether between members of the same species (intraspecific) or between different species (interspecific).

Predation occurs when a predator captures and consumes prey. Symbiosis describes a close, long-term relationship between two species, classified as mutualism when both species benefit, commensalism when one benefits and the other is unaffected, or parasitism when one benefits at the other's expense.

The most common exam trap in this section is memorising a fixed list of named pairs, such as bee and flower for mutualism, rather than being able to identify which of the three interaction types is being described in an unfamiliar example. Reading the description carefully to identify who benefits, who is harmed and who is unaffected is the actual skill being tested.

Population growth and carrying capacity (CS 24.2)

A population's size at any time depends on the balance between births and immigration, which add individuals, and deaths and emigration, which remove them. When a population is introduced into a new environment with abundant resources, it typically grows slowly at first, then very rapidly, and finally levels off in an S-shaped curve once it reaches the carrying capacity, the maximum population size the environment can sustainably support.

Factors that limit population size fall into two categories. Density-dependent factors, such as food supply, disease and predation, have a greater limiting effect as the population becomes larger and more crowded.

Density-independent factors, such as a flood, fire or extreme cold, reduce a population by a broadly similar proportion regardless of how large or small it already is.

A population can temporarily overshoot its carrying capacity when conditions are briefly favourable, but limited resources then cause the death rate to rise and the birth rate to fall, bringing the population back down towards a sustainable size; this overshoot-and-fall pattern often appears as a peak followed by a dip on a growth graph, and recognising it is a frequently tested reading skill.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Community and ecosystem?
2. Can you define and explain Feeding relationships?
3. Can you define and explain Energy flow?
4. Can you define and explain Nutrient cycles?
5. Can you define and explain Population ecology?
6. Can you define and explain Ecological pyramids?
7. Can you define and explain Habitat and niche?
8. Can you define and explain Interspecific relationships?
9. Can you define and explain Population growth and carrying capacity?
10. Can you define and explain Factors affecting population size?

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Source:SRC-DSKP-EN