How do living and non-living parts of an ecosystem fit together, and how does energy flow through it?
Ecosystems, habitats and niches; biotic and abiotic factors; food chains, food webs and energy flow through trophic levels; and the interdependence of organisms within a community.
An SQA National 5 Environmental Science answer on ecosystems and interdependence, covering habitats and niches, biotic and abiotic factors, food chains and food webs, the flow of energy through trophic levels, and how organisms depend on one another in a community.
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What this dot point is asking
The SQA wants you to describe how an ecosystem is built from living (biotic) and non-living (abiotic) parts, to use the terms habitat, niche, population and community correctly, to read and build food chains and food webs, to explain how energy flows and is lost through trophic levels, and to explain why the organisms in a community depend on one another.
The building blocks: ecosystem, community, population
These three terms nest inside one another. A single oak wood is an ecosystem; all the living things in it (oaks, squirrels, fungi, beetles) make up the community; and all the grey squirrels in the wood make up one population. The non-living part of the ecosystem includes the soil, the air, the water and the climate.
Habitat and niche
A habitat is the place where an organism lives, for example a rock pool or a hedgerow. A niche is the role an organism plays in its community: what it eats, what eats it, when it is active and the conditions it needs. Two species can share a habitat but occupy different niches, which lets them avoid competing directly for the same resources.
Biotic and abiotic factors
The conditions in an ecosystem come from two kinds of factor:
- Biotic factors are the living influences on an organism: predation (being eaten), competition (for food, light, space or a mate), disease, and the availability of food.
- Abiotic factors are the non-living influences: light intensity, temperature, pH, moisture (water availability), oxygen levels and the concentration of minerals.
Both kinds of factor affect how many organisms can live in an area and where they are found. For example, low light under a dense tree canopy (an abiotic factor) limits which plants can grow there.
Producers, consumers and food chains
A food chain shows the order in which organisms feed and the direction energy flows. The arrow always points from the organism being eaten to the organism that eats it, because the arrow shows the flow of energy.
- Producers are green plants and algae. They are at the start of every food chain because they trap light energy in photosynthesis and store it as chemical energy in food.
- Consumers are animals that eat other organisms. A primary consumer (herbivore) eats producers; a secondary consumer eats primary consumers; a tertiary consumer eats secondary consumers.
- Decomposers (bacteria and fungi) break down dead material and release the nutrients back into the ecosystem.
Each feeding level is a trophic level: producers are the first trophic level, primary consumers the second, and so on.
Energy flow and why food chains are short
Energy enters an ecosystem as light and is captured by producers. As it passes up the food chain, most is lost at each step.
Food webs and interdependence
Real ecosystems are not single chains. A food web is many food chains linked together, showing that most animals eat more than one kind of food and most organisms are eaten by more than one kind of predator.
Because the species in a web are linked, removing or reducing one population sends effects through the whole web. If a prey species declines, its predators have less food and may decline too, while the plants that prey grazed may increase. This is why a food web is the better model: it shows how interdependent a community is.
Examples in context
Example 1. Light as a limiting abiotic factor. In a woodland, bluebells flower early in spring before the trees come into full leaf, timing their growth to when light at ground level is highest. This shows how an abiotic factor (light) shapes where and when a species grows.
Example 2. Niche separation in shorebirds. On an estuary, wading birds feed in the same habitat but probe to different mud depths with bills of different lengths. By taking different prey they occupy different niches, avoid direct competition, and so share one area.
Try this
Q1. State the difference between a biotic and an abiotic factor, giving one example of each. [2 marks]
- Cue. Biotic = a living influence such as predation or competition; abiotic = a non-living influence such as light intensity or temperature.
Q2. Explain why a food chain rarely has more than four or five links. [1 mark]
- Cue. Only about 10 per cent of energy passes to the next level (most lost as heat from respiration), so too little is left to support further levels.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksUsing the terms producer, consumer and trophic level, describe how energy passes through a food chain and explain why food chains are usually short.Show worked answer →
A 4-mark describe-and-explain answer needs the flow of energy plus the reason for short chains, so plan two description marks and two explanation marks.
Energy enters the food chain when a producer, such as grass, traps light energy in photosynthesis and stores it as chemical energy. A primary consumer (herbivore) eats the producer and gains some of that stored energy, and a secondary consumer eats the primary consumer, so energy passes from one trophic level to the next.
Food chains are usually short because only about 10 per cent of the energy at one trophic level is passed on to the next. The rest is lost as heat from respiration, in movement, and in undigested material. After three or four links there is too little energy left to support another level, so the chain stops.
Markers reward naming the roles (producer, consumer), describing the transfer between trophic levels, and giving energy loss as the reason for short chains.
SQA N5 style3 marksA food web is shown. Explain what would happen to two other populations if the number of rabbits in the web fell sharply.Show worked answer →
This is an interdependence question, so each effect must name a population and link it back to the change in rabbits.
Effect 1. A predator that eats rabbits, such as foxes, would have less food, so the fox population would fall.
Effect 2. A plant that rabbits graze, such as grass, would be eaten less, so the grass population would rise.
A third mark could come from a knock-on effect: with fewer rabbits, foxes may eat more of an alternative prey such as voles, so the vole population would also fall.
Markers reward each correctly reasoned change. A food web shows interdependence, so a change in one species spreads through the connected species. Stating an effect without linking it to the fall in rabbits scores nothing.
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