Biosphere to Ecosystems

From biosphere to ecosystem

The biosphere is the global "layer of life" — the sum of every ecosystem on Earth, made up of the atmosphere, hydrosphere (water), and lithosphere (land) all interconnected. Within the biosphere, an ecosystem is a smaller unit: a community of living organisms interacting with each other and with their non-living environment in one area.

Abiotic and biotic factors

Every ecosystem is shaped by two kinds of factors:

Factor type	Examples
Abiotic (non-living)	Temperature, light, water, soil (pH, texture, humus content), physiographic factors (slope, altitude, aspect)
Biotic (living)	All the organisms in the ecosystem, and their interactions with each other

Trophic levels — who eats whom

Organisms in an ecosystem occupy different trophic (feeding) levels:

Producers — plants and other organisms that make their own food (usually via photosynthesis).
Consumers — organisms that eat other organisms: herbivores (eat plants), carnivores (eat other animals), and omnivores (eat both).
Decomposers — break down dead organisms and waste, returning nutrients to the ecosystem.

Energy flow through an ecosystem

Energy enters an ecosystem from the sun and flows through it via food chains: producer → primary consumer → secondary consumer → tertiary consumer. At each step, a large amount of energy is lost (mostly as heat), so less energy is available at each successive trophic level. This is why food chains rarely have more than 4–5 levels, and why ecosystems can support far more producers than top predators — visualised as a food/energy pyramid.

Try the Ecosystem Energy Pyramid in the Explore tab to see how energy shrinks at each trophic level.

Answer in your exercise book.

Question 1 · (2 marks)

Distinguish between the biosphere and an ecosystem.

Question 2 · (4 marks)

Give TWO examples each of abiotic and biotic factors in an ecosystem.

Question 3 · (4 marks)

Define producer, consumer, and decomposer, and give an example of each.

Question 4 · (3 marks)

Explain why energy decreases at each successive trophic level in a food chain.

Question 5 · (2 marks)

Explain why food chains rarely have more than four or five levels.

Question 6 · (5 marks)

A field study measured the energy stored at each trophic level of a grassland ecosystem over one year:

Trophic level	Energy (kJ/m²/year)
Producers	20 000
Primary consumers	2 200
Secondary consumers	190
Tertiary consumers	18

(a) Calculate the percentage of energy transferred from producers to primary consumers. Show your working. (2)
(b) Calculate the percentage of energy transferred from secondary consumers to tertiary consumers. (2)
(c) Are the percentages you calculated exactly 10% at every step? Explain why the "10% rule" is described as an average rather than an exact law. (1)