A crowded world under your feet: Soil is one of the richest places for life on Earth. Its biodiversity includes tiny microorganisms, fungi and many animals.
The points to remember
- Soil holds a huge biodiversity: by one estimate, more than half of all species live in it.
- Microorganisms: bacteria (up to about a billion in a gram of fertile soil) and other microbes.
- Fungi: fine threads that spread through the soil; many decompose, some partner with roots.
- Animals: earthworms, nematodes, mites, springtails, ants, termites, beetle larvae, moles.
- Many soil species are still unknown: most soil microbes have never been named.
Remember it as: Bugs, bacteria and fungi: most of the soil's life is too small to see.
Real example: in 2023, scientists in Switzerland estimated that about 59% of all species on Earth live in the soil, from bacteria and fungi to earthworms and moles.
Name all three groups: When asked who lives in soil, cover microorganisms (bacteria), fungi and animals, and say that many species are still unknown.
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Soil supports so many species because it gives each of them a habitat and a niche. A handful of soil holds many different places to live and many different ways to make a living.
The points to remember
- Habitat = where an organism lives; niche = its role, and the conditions and resources it uses.
- Soil has pores of every size: tunnels for worms, gaps for mites, water films for bacteria.
- It offers many kinds of food: dead leaves, roots, dung, fungi and other soil animals.
- Conditions change with depth: fresh litter on top, dark, damp and steady below, so species share the soil out.
- Soil shelters organisms from light, heat, frost and drying out.
- Many niches: decomposers, grazers of fungi, predators, root partners, nitrogen-fixing bacteria.
Remember it as: A city in a handful: rooms of every size, and a job for everyone.
Real example: in Wytham Woods near Oxford, one of the most studied woods in the world, springtails graze fungi in the leaf litter, earthworms pull leaves into their burrows below, and centipedes hunt both: three niches within a few centimetres.
Habitat is not niche: The habitat is the place (the leaf litter). The niche is the role in that place (eating fungi in the litter). Questions on soil biodiversity reward both words.
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Questions give data on soil animals at two kinds of site and ask you to suggest why one has more. Read each graph, quote the values, then give the reason.
The points to remember
- More plant cover gives more litter, roots, food and shade: more habitats and niches, so more species.
- More plant cover also means less erosion, so the soil habitat is not washed away.
- More soil water supports biological activity and nutrient cycling: most soil organisms need moist soil.
- An old, stable (climax) community has more complex food webs, so it supports more species.
- A monoculture plantation or a ploughed field gives fewer kinds of food and shelter, so diversity is lower.
Real example: in Britain, surveys find fewer soil animals in ploughed farmland than in grassland and woodland, where the soil is left undisturbed and covered with plants all year.
Quote, then explain: 'The meadow sites have 22-45 mm of litter and a diversity of 1.2-1.8; more litter gives more food and habitats.' A value from the graph plus a reason scores; a reason alone may not.
The life that keeps the soil working: Soil life matters for the whole ecosystem: it recycles nutrients, builds the soil's structure and feeds animals above ground. Losing it makes soil less fertile.
The points to remember
- Soil organisms decompose dead matter and recycle nutrients for plants.
- Earthworms and burrowers mix and aerate the soil and help rain soak in.
- Fungi and bacteria glue particles into crumbs; root fungi feed plants; some bacteria fix nitrogen.
- Soil animals are food for birds, moles and hedgehogs, so they support biodiversity above ground too.
- Harmed by ploughing, pesticides, compaction, monocultures, erosion, pollution and invasive species.
Real example: in 1881 Charles Darwin published a book on earthworms. He showed that they drag leaves into the soil and bring fine soil up to the surface, slowly burying stones and mixing the whole topsoil.
Link the organism to its job: 'Earthworms' alone is a name. 'Earthworms burrow, which aerates the soil and helps water soak in' is a point.
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How this comes up: Paper 2, Section A: using two graphs, suggest why one kind of site has more soil animals [2]. One reason per mark, each tied to the data.
The figure shows soil invertebrate diversity at old meadow sites and ploughed field sites at Hollin Farm, a made-up farm.
Using the data in the figure, suggest why soil invertebrate diversity is higher at the old meadow sites than at the ploughed field sites.
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