Community and Ecosystem

Building up the key terms

1. Habitat, the specific place where an organism normally lives.
2. Population, all the individuals of one species living in a habitat at a given time.
3. Community, all the different populations of different species living and interacting in a habitat.
4. Ecosystem, a community together with the non-living (abiotic) components of its environment, such as light, water, and soil.
5. Niche, the specific role an organism plays within its habitat, including what it feeds on and how it interacts with other organisms.

Biotic and abiotic components

The biotic components of an ecosystem are all the living organisms present, producers, consumers, and decomposers. The abiotic components are the non-living physical and chemical factors, such as temperature, light intensity, water, pH, and soil type.

Biotic and abiotic factors constantly interact, for example when abiotic light intensity limits how much a producer can photosynthesise, which in turn limits the food available to consumers.

Feeding relationships

Energy flows through a community along food chains, each starting with a producer and passing through a series of consumers at different trophic levels. Because most organisms eat more than one type of food, real communities are better represented by a food web, which links many overlapping food chains together.

Decomposers break down dead organisms and waste at every trophic level, returning nutrients to the ecosystem.

Interactions between organisms and how this is examined

• Predation, one organism (the predator) hunts and kills another (the prey) for food.
• Competition, organisms compete for a limited resource such as food, space, or light.
• Mutualism, both organisms in the relationship benefit, for example bees and flowering plants.
• Commensalism, one organism benefits while the other is neither helped nor harmed, for example an epiphyte growing on a tree.
• Parasitism, one organism (the parasite) benefits at the expense of the other (the host).

Energy flow and pyramids of number

Energy enters most ecosystems as sunlight, which producers convert into chemical energy stored in food. This energy then flows along the food chain as one organism is eaten by another.

At each transfer a large part of the energy is used for respiration, movement and body heat, or is lost in undigested waste, so less energy reaches each successive level.

Because energy and numbers usually decrease along a chain, the feeding relationship can be drawn as a pyramid of numbers, with a broad base of producers and few top consumers at the tip. This falling energy supply explains why producers are the most abundant organisms and why top predators are comparatively rare.

Key terms

• Ecosystem, a community of organisms together with the abiotic factors of its environment.
• Food chain, a sequence showing how energy passes from a producer through a series of consumers.
• Food web, the interconnected food chains within a community.
• Producer, an organism, usually a green plant, that makes its own food by photosynthesis.
• Decomposer, an organism that breaks down dead matter and returns nutrients to the ecosystem.

Worked exam-style question

Question. A grassland community contains grass, grasshoppers, frogs, snakes and hawks. One of its food chains is grass → grasshopper → frog → snake → hawk.

(a) Name the producer and state how it obtains its energy. (b) State the trophic level of the frog in this food chain.

(c) Explain what would happen to the grasshopper population if all the frogs were removed. (d) Suggest why the total mass of hawks in this community is much smaller than the total mass of grasshoppers.

Model answer. (a) The producer is the grass; it makes its own food by photosynthesis, trapping light energy to build glucose. (b) The frog is at the third trophic level and is a secondary consumer.

(c) With no frogs, fewer grasshoppers are eaten, so the grasshopper population would increase at first; the larger population would then eat more grass, and competition for food would eventually limit their numbers. (d) Energy is lost at each trophic level through respiration, heat, movement and waste, so less energy is available higher up the chain; the community can support only a small mass of hawks compared with grasshoppers.

Practice question

In a pond ecosystem, dead leaves and dead fish collect at the bottom. (a) Name the type of organism that breaks down this dead material.

(b) Explain why these organisms are important for the pond ecosystem.

Exam tip

Source:SRC-DSKP-EN

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