What biodiversity is: genes, species, ecosystems and the niches that hold them
By the end of this lesson you will be able to
- Define biodiversity and distinguish genetic, species and ecosystem diversity, with an example of each.
- Describe and explain how plants are adapted to dry and to wet environments, linking each feature to water.
- Define a niche and explain how several species can share one habitat by occupying different niches.
- Explain why species-rich places such as the tropics, wetlands and hotspots hold more species than deserts or the poles.
- Outline why biodiversity matters to ecosystems and to people, using keystone species and ecosystem services.
You met this before
- A species is a group of organisms that can interbreed to produce fertile offspring; every species has a two-part (binomial) name, such as Loxodonta africana, the African savanna elephant (Year 1, Unit 4; Year 3, Unit 4).
- An adaptation is an inherited feature that helps an organism survive and reproduce in its environment. Adaptations are structural (body shape), behavioural (what it does) or physiological (how its body works).
- A population is all the organisms of one species in an area; a community is all the populations of different species in an area; an ecosystem is the community together with its abiotic (non-living) environment.
- Individuals of one species vary: variation is caused by different alleles (versions of genes), by the environment, or both (Year 3, Unit 4).
- Species are identified with a dichotomous key, and their numbers estimated by counting in randomly placed quadrats (Year 1, Unit 4).
A gemsbok (oryx) in the Kalahari can go for weeks without drinking. It feeds at night and at dawn, when plants hold more water, and it lets its body temperature rise by several degrees in the heat of the day instead of sweating. Classify each of these two features as behavioural or physiological.
Reveal
Feeding at night and at dawn is behavioural — something the animal does. Letting its body temperature rise instead of sweating is physiological — how its body works. By storing heat during the day and losing it at night, it saves the water that sweating would cost. A structural example would be its pale coat, which reflects sunlight.
Revise it in full: Year 1 · Living things · Lesson 2 — Adaptation: how organisms fit the places they live · Year 1 · Living things · Lesson 5 — Dichotomous keys and counting life · Year 3 · Classification, variation and natural selection · Lesson 3 — Variation
The puzzle
The Okavango River rises in the highlands of Angola and flows south-east into Botswana. It never reaches the sea. Instead it spreads across the sand of the Kalahari into a fan of channels, lagoons and islands that covers up to about 15 000 km² at the height of the flood, and almost all of its water evaporates or is transpired by plants.
When UNESCO listed the delta as a World Heritage Site in 2014, the listing recorded about 1 061 plant species, 89 fish, 64 reptiles, 482 birds and 130 mammals. The dry Kalahari sandveld around it, with the same rainfall, supports far fewer kinds of plants and animals.
Both places have the same sunshine, the same rain from the sky and the same Kalahari sand underneath. What is it about a swamp in a desert that lets it hold so many more kinds of living thing?
List three differences between the delta and the sandveld around it that could allow more species to live in the delta. For each one, predict whether it gives organisms more food, more places to live, or more ways of living.
Compare your answer
Water is the obvious one: permanent and seasonal water supports aquatic plants, fish, frogs, water birds and hippos that cannot live in the sandveld at all — more places to live.
Water also lets plants grow all year, so there is more food (more photosynthesis, more biomass) for the consumers above them. And the delta is a mosaic of habitats — deep channels, papyrus swamp, seasonal floodplain grassland, islands of woodland — so there are more different ways of living in a small area.
Those three ideas — energy, habitat variety and niches — are the main explanations ecologists give for why some places hold more species. This lesson builds each one.
The science — structured notes
1.1 Biodiversity at three levels
Biodiversity is the variety of life on Earth, or in a particular habitat or ecosystem. The word hides three different levels of variety, and a conservation plan that protects one level can still lose another.
Genetic diversity is the variety of alleles within a species — the reason no two elephants, or two sorghum plants, are identical. Species diversity is the variety of species in an area: how many species there are (species richness) and how evenly the individuals are spread among them (evenness). Ecosystem diversity is the variety of habitats and ecosystems in a region.
| Level | What varies | Example | How it is measured | Why it matters |
|---|---|---|---|---|
| Genetic | Alleles of genes within one species | Hundreds of local sorghum and millet varieties grown across Africa; cheetahs, by contrast, have very little | DNA sequencing; counting how many alleles of chosen genes exist in a population | Variation is the raw material of natural selection: a population with many alleles is more likely to contain individuals that survive a new disease or a drought |
| Species | Number of species and how evenly individuals are shared among them | 482 bird species recorded in the Okavango | Species richness; the Simpson index of diversity (Lesson 2) | More species means more feeding links, so the loss of one species is less likely to collapse the community |
| Ecosystem | Number and variety of habitats in a region | Permanent swamp, seasonal floodplain, islands, mopane woodland and sandveld in one delta | Mapping habitats from satellite images and field surveys | Each habitat supports a different community; losing a habitat type loses every species that depends on it |
“A population of 5 000 cheetahs has high biodiversity because it contains many individuals.”
Reveal answer
False. Biodiversity is about variety, not number. Five thousand individuals of one species is one species, and cheetahs are famous for having very little genetic diversity.
In the 1980s, skin grafts between unrelated cheetahs were not rejected — a sign that they carry almost the same alleles for the genes of the immune system, probably because the species passed through a very small population thousands of years ago.
1.2 Why biodiversity matters
People use biodiversity directly. Every crop and farm animal was domesticated from a wild species, and wild relatives still supply alleles for disease resistance. Many medicines began as chemicals from organisms: vincristine, a childhood-leukaemia drug, from the Madagascar periwinkle; artemisinin, the most important antimalarial of this century, from sweet wormwood (Tu Youyou, Nobel Prize 2015).
In southern Africa, mopane worms, devil’s claw roots (sold worldwide for joint pain) and thatching grass are harvested from the wild. Ecologists group these benefits as ecosystem services: provisioning (food, fibre, medicines, water), regulating (pollination, flood control, water purification, climate), supporting (soil formation, nutrient cycling, photosynthesis) and cultural (tourism, recreation, spiritual value).
Biodiversity also makes ecosystems stable. In a long-term grassland experiment in Minnesota, David Tilman and John Downing found that during the severe drought of 1987–1988, plots holding many species lost much less of their biomass than plots with few species (published 1994). With more species, some are likely to tolerate whatever change comes; they take over the role of those that fail.
Urban growth reduces the population of bees. Explain how this could further reduce biodiversity. [4]
Link 1: fewer bees means fewer flowers are pollinated [1].
Link 2: so fewer seeds and fruits are produced, and the plant populations fall over the following years [1].
Link 3: so less food (nectar, fruit, seeds, leaves) is available for the consumers that depend on those plants [1].
Link 4: so their populations fall and some species disappear from the area — fewer species and smaller populations, which is lower biodiversity [1].
Notice the structure: each link starts where the last one ended, and the last link uses the word biodiversity. An answer that stops at “fewer plants” earns two of the four marks.
Classify each benefit as a provisioning, regulating, supporting or cultural ecosystem service: (1) papyrus swamps trap silt and nutrients, so the water leaving the delta is clear; (2) photographic safaris in the Okavango; (3) fish caught for food in the panhandle; (4) bacteria and fungi breaking down dead plants into nutrients for new growth.
Reveal answer
(1) Regulating — water purification. (2) Cultural — tourism and recreation. (3) Provisioning — food. (4) Supporting — nutrient cycling, which every other service depends on.
Tourism alone is worth a very large share of Botswana’s income from wildlife; cultural services often have the clearest price, and supporting services the least visible one.
1.3 Adapted to dry and wet: why one species cannot live everywhere
Some organisms are better adapted to a particular environment than others, and water is the factor that separates the plants of the Kalahari from the plants of the delta. A leaf faces a trade-off: it must open its stomata to take in carbon dioxide for photosynthesis, but every open stoma lets water vapour diffuse out (transpiration).
Plants adapted to dry places — xerophytes — cut water loss and store or reach water. Plants adapted to living in water — hydrophytes — face the opposite problem: plenty of water, but little oxygen reaching their roots and little support from stiff tissue.
| Feature | Dry environment (xerophyte) | How it helps | Wet environment (hydrophyte) | How it helps |
|---|---|---|---|---|
| Cuticle | Thick and waxy (aloes, succulents) | Waterproof layer cuts water lost through the epidermis | Thin or absent (submerged pondweeds) | Water loss is no problem; gases and minerals diffuse in across the surface |
| Stomata | Few; sunken in pits or inside a rolled leaf, often among hairs (marram grass) | Traps a layer of humid air, so the water-vapour gradient is less steep and diffusion out is slower | On the upper surface only (water lilies) | The lower surface is in water; stomata on top open to the air for CO₂ |
| Leaves | Small, needle-like or reduced to spines (cacti, many acacias) | Smaller surface area loses less water; spines deter animals seeking water | Large, flat, floating pads | Large area catches light on the surface of the water |
| Tissue inside | Water-storing cells in stems or leaves (succulents, baobab trunk) | A reservoir for the dry season | Aerenchyma: large air spaces in stems and leaves (papyrus, lilies) | Buoyancy, and a pathway for oxygen to diffuse down to roots in waterlogged mud |
| Roots | Very deep taproots (shepherd’s tree in the Kalahari) or wide shallow mats | Reach groundwater, or catch rain before it evaporates | Short; some grow upwards as breathing roots (mangroves) | Waterlogged mud has almost no oxygen; the roots take it from the air |
| Timing | Stomata open at night and closed by day in some succulents (CAM plants); ephemerals grow, flower and set seed within weeks of rain | Night air is cooler and more humid, so less water is lost per CO₂ taken in | Grow all year where water is permanent | No dry season to survive |
Marram grass grows on sand dunes. Its leaves roll up in dry weather, with the stomata on the inner surface among hairs. Explain how this reduces water loss. [3]
The rolled leaf and the hairs trap a pocket of still, humid air next to the stomata [1].
So the difference in water-vapour concentration between the air spaces inside the leaf and the air outside the stomata is smaller — the concentration gradient is less steep [1].
So water vapour diffuses out of the stomata more slowly, and less water is lost by transpiration [1].
The key words are humid air, concentration gradient and diffusion. “It keeps the water in” earns nothing: the examiners want the mechanism.
Papyrus grows in the permanent swamps of the Okavango with its roots in mud that contains almost no oxygen. Its stems are full of large air spaces. Explain why the air spaces are essential for the roots to stay alive.
Reveal answer
Root cells need oxygen for aerobic respiration, to release the energy they use for growth and for active uptake of mineral ions.
Waterlogged mud has almost no oxygen, because oxygen diffuses about 10 000 times more slowly through water than through air and bacteria in the mud use up what arrives. The air spaces (aerenchyma) form a continuous low-resistance pathway from the stems in the air down to the roots, so oxygen can diffuse down to the root cells and carbon dioxide up.
A student writes: “Water lilies have stomata on the lower surface of the leaf so they can take in water. They have a thick cuticle to stop them getting waterlogged. Desert plants have more stomata so they can take in more carbon dioxide when it rains.” Find three errors and correct them.
Reveal answer
(1) Water lilies have stomata on the upper surface, which is in contact with the air; stomata exchange gases (CO₂ in, O₂ and water vapour out), they do not take in liquid water.
(2) Hydrophytes have a thin cuticle or none; there is no need to reduce water loss, and a thin surface lets gases and ions diffuse in.
(3) Xerophytes have fewer stomata (often sunken) to reduce water loss; some open theirs only at night, when less water is lost per molecule of CO₂ taken in.
1.4 Niches: many species, one habitat
A habitat is the place where an organism lives. A niche is the position or role of a species or population in an ecosystem in relation to other species: what it eats, what eats it, where and when it feeds, where it breeds, and the range of conditions it tolerates.
In a forest, the trees are producers that capture light and build the canopy; the birds are consumers that eat insects, seeds or fruit and disperse seeds; the earthworms are decomposers that break down leaf litter and mix it into the soil. Three groups, one habitat, three different niches.
In the 1930s the Russian biologist Georgy Gause grew two species of Paramecium that ate the same bacteria in the same tube. One always drove the other to extinction. This became the competitive exclusion principle: two species cannot share exactly the same niche in the same place for long; the better competitor wins.
Species that live together therefore divide resources between them (resource partitioning). In 1958 Robert MacArthur watched five species of small insect-eating warblers feeding in the same spruce trees in the north-eastern United States. Each species fed mostly in a different part of the tree — the tips of the top branches, the outer middle branches, the inside near the trunk, the lowest branches — and at slightly different times.
The same happens on an African savanna. Giraffes browse up to about 5–6 m, kudu at 1–2 m, steenbok and duiker close to the ground. Among grazers, zebra crop the tall, coarse tops of the grass, wildebeest the leafy middle and smaller antelope the short new shoots that regrow afterwards. More ways of dividing resources means more niches, and more niches means room for more species.
Two species of kingfisher live along the same Okavango channel. The pied kingfisher hovers over open water and dives for fish; the malachite kingfisher perches on reeds at the water’s edge and takes small fish and insects close to the bank. Use the idea of a niche to explain how both can live on the same stretch of water.
Reveal answer
They share a habitat but occupy different niches: they feed in different places (open water versus reed-fringed edges), with different methods (hovering and diving versus perching) and on partly different prey (larger open-water fish versus small fish and insects near the bank).
Because their use of resources overlaps only partly, competition between them is reduced, so neither excludes the other — resource partitioning, as in MacArthur’s warblers.
1.5 Why some places hold more species
Species are not spread evenly across the Earth. Richness is highest near the equator and falls towards the poles — the latitudinal gradient. A single hectare of Amazonian forest in Ecuador was found in 1994 to hold 473 species of tree; the whole of the British Isles has fewer than 40 native tree species.
Four explanations work together. Energy and water: warm, wet places have the highest rates of photosynthesis, so more food supports more consumers (the lab below tests the water half). Habitat complexity: a rainforest has several layers, from canopy to floor, each with its own niches.
Stability and time: tropical regions were less disrupted by the ice ages, so species had longer to evolve and fewer were wiped out. Area: larger areas hold more habitats and larger populations, so more species survive.
Some regions are unusually rich in endemic species — species found nowhere else. In 2000 Norman Myers and colleagues identified 25 biodiversity hotspots: regions with at least 1 500 endemic plant species that had already lost at least 70 % of their original vegetation. Together they covered 1.4 % of the land surface but held 44 % of all vascular plant species as endemics. Today 36 hotspots are recognised.
Southern Africa has three: the Cape Floristic Region (about 9 000 plant species, most found nowhere else), the Succulent Karoo, and Maputaland–Pondoland–Albany. Others include Madagascar, the Atlantic Forest of Brazil, Sundaland (Borneo and Sumatra) and the tropical Andes. The Okavango is not a hotspot — few of its species are endemic — but it is one of the richest wetlands in Africa.
A fire sweeps through an island in the Okavango and burns the woodland to the ground. Predict what happens to species richness on the island in the following year, and suggest what happens over the next 20 years if there are no more fires.
Reveal answer
In the first year richness falls sharply: trees and shrubs die, and the birds, insects and mammals that depended on them for food, nesting sites and shade leave or die. Fast-growing grasses and herbs appear first, with the insects and grazers they support.
Over the next 20 years shrubs and trees regrow, the vertical structure (layers) returns, niches multiply and richness rises again. In practice, occasional fires create a mosaic of burnt and unburnt patches, and that mosaic raises ecosystem diversity across the delta.
1.6 Interdependence and keystone species
Every species in a community depends on others: for food, pollination, seed dispersal, shelter or nutrients. Some species matter far more than their numbers suggest. A keystone species has an effect on its community much larger than its abundance, so removing it changes the whole community.
In the Okavango, hippos wade the same routes night after night, and their trails become channels that carry flood water into new areas. Fungus-growing termites (Macrotermes) build mounds on the floodplain; many of the delta’s thousands of islands began as termite mounds that raised a patch of ground above the flood, where trees could root.
Elephants push over trees and open woodland into grassland. Remove the hippos, termites or elephants and the whole mosaic of habitats — and the species it holds — slowly changes. The sea otter of the North Pacific is the textbook marine example: otters eat sea urchins, which eat kelp, so where otters were hunted out the urchins grazed away whole kelp forests.
Sea otters were hunted almost to extinction for their fur; once they were protected, their numbers rose. Pyramids of numbers from before and after show fewer sea urchins and more kelp. Explain the effect of the growing otter population on the urchins and the kelp. [4]
The number of sea urchins decreased [1] because more were eaten by the growing population of otters, their predator [1].
With fewer urchins, less kelp was eaten [1], so the kelp population increased [1].
Two changes, each with its cause. A common error is to jump from “more otters” to “more kelp” without the urchin link — which loses the two middle marks.
In the 1990s grey wolves were reintroduced to Yellowstone National Park in the United States after about 70 years of absence. Elk (a deer) numbers fell and elk stopped browsing for long periods in open river valleys. Willows and aspens along the rivers grew taller in some areas, and beaver colonies increased.
Explain, in a chain of at least four links, how a predator could increase the number of beavers, and suggest one reason why ecologists still argue about how large this effect was.
Reveal answer
Wolves prey on elk, so elk numbers fall [1]. Elk also change their behaviour and spend less time browsing where they are easily caught, such as open riverbanks [1].
So young willows and aspens along the rivers are browsed less and grow taller [1]. Beavers feed on and build dams with willow, so more willow supports more beaver colonies; their dams create ponds, which add habitat for fish, amphibians and water birds [1].
The argument: over the same years other factors also changed — human hunting of elk, drought, a rising number of bison and grizzly bears — so a correlation between wolves and willows does not prove the wolves caused it. The effect varied from place to place, and some studies found it smaller than first reported [1].
Try it — rainfall and the number of plant species
You cannot water a desert, but you can compare places that differ in rainfall. This lab lets you mark out a 10 m × 10 m plot at sites with different annual rainfall — from the dry south-west Kalahari to the wettest woodlands of the north — and count every plant species inside. It tests the energy-and-water explanation in Section 1.5.
Mean species counts at low grazing: 100 mm → 5; 300 mm → 13; 500 mm → 19; 700 mm → 23; 900 mm → 26; 1100 mm → 28; 1300 mm → 30; 1500 mm → 31. At high grazing: 100 mm → 3; 700 mm → 16; 1500 mm → 21. Describe the relationship, calculate the percentage reduction caused by grazing at 1500 mm, and explain it.
Reveal answer
The number of species rises with rainfall: by 8 species between 100 and 300 mm, but only by 1 between 1300 and 1500 mm. The rise is steep at first and then levels off, so the relationship is positive but not linear and not proportional.
Reduction = (31 − 21) ÷ 31 × 100
Reduction = 10 ÷ 31 × 100
Reduction = 32 %
Heavy grazing removes the most palatable grasses and seedlings faster than they regrow. Tough, unpalatable or fast-regrowing species survive, so the grazing — a biotic factor — lowers richness at every rainfall.
Common mistakes to avoid
- “Biodiversity means the number of animals.” It is the variety of all life — genes, species and ecosystems — and includes plants, fungi and microorganisms.
- Confusing habitat and niche. The habitat is where it lives; the niche is its role and how it uses resources in relation to other species.
- “The plant adapted to the dry weather.” Adaptations are inherited and evolve over generations; an individual does not grow a new adaptation because it needs one.
- “Stomata take in water.” Stomata exchange gases; water enters through the roots.
- Stopping a food-web chain early. Every link from the change to the final effect earns a mark; “fewer bees so less biodiversity” misses the middle two.
- Using “extinction” for a local decline. A species is extinct only when no individuals remain anywhere; the May 2021 Biology markscheme refused “extinction” for a population that falls.
- “Species richness” for the number of individuals. Richness counts species, not organisms.
Study smart: how this is examined
Study routine. Today: write the three levels of biodiversity with one example each from memory, then the table of xerophyte and hydrophyte features with a reason for each. In two days: write a four-link food-web chain for three different removals (bees, hippos, a top predator).
In a week: run the lab as a full investigation, take the quiz, and explain to someone at home the difference between a habitat and a niche using a local example.
How we found out — naming the variety of life
In 1807 the Prussian explorer Alexander von Humboldt published his Essay on the Geography of Plants with a famous cross-section of the Ecuadorian volcano Chimborazo. It showed which plants grew at each altitude and linked the zones to temperature — the first picture of how climate arranges life, and the start of the study of where species are and why.
The word niche came a century later. Joseph Grinnell, studying the California thrasher, used it in 1917 for the set of conditions a species needs. Charles Elton, in his 1927 book Animal Ecology, used it for a species’ role in the community — what it eats and what eats it.
In 1957 G. Evelyn Hutchinson combined both: a niche is the whole range of conditions and resources a species can use, which is larger than the range it actually uses once competitors are present. Gause’s Paramecium experiments (1934) and MacArthur’s warblers (1958) supplied the evidence that competition narrows niches in real communities.
The word biodiversity is younger still. Walter Rosen of the US National Research Council coined “BioDiversity” for a National Forum held in Washington in 1986; the biologist E. O. Wilson edited the book of that forum in 1988, and the word spread fast.
By 1992, at the Earth Summit in Rio de Janeiro, 150 governments signed the Convention on Biological Diversity, which defines biodiversity at the genetic, species and ecosystem levels. What changed minds was evidence of loss: rates of extinction and habitat clearing, measured in the 1970s and 1980s, far above anything in the fossil record for normal times.
Gause found that two Paramecium species could live together in one tube only when one fed on bacteria in the water and the other on yeast cells that settled at the bottom. Explain how this result supports the competitive exclusion principle.
Reveal answer
When both species used the same food in the same place — identical niches — one always outcompeted the other and drove it to extinction in the tube.
When their food and feeding places differed, their niches overlapped less, competition was reduced, and both persisted. The principle predicts exactly this: coexistence needs different niches, and removing the difference removes the coexistence.
Where it is used now
- The Okavango Delta, Botswana. The world’s 1 000th World Heritage Site (2014) and a Ramsar wetland of international importance since the 1990s. Its biodiversity underpins a photographic-safari industry that employs thousands of people in the north of the country, and the annual flood depends on rain falling in Angola — a reason for the three-country Okavango River Basin Water Commission (OKACOM).
- The Amazon basin. The largest rainforest on Earth, with about 10 % of known species. Much of its rain is recycled by the forest’s own transpiration, so clearing it can dry the rest.
- The Coral Triangle. The seas between Indonesia, the Philippines and Papua New Guinea hold about three-quarters of the world’s known reef-building coral species.
- The Cape Floristic Region, South Africa. The smallest of the world’s six floral kingdoms, with about 9 000 plant species in an area smaller than Botswana’s Central District.
- Crop genetic diversity. Seed banks and farmers’ own seed stores keep thousands of local varieties of sorghum, millet, maize and beans — genetic diversity that plant breeders use to develop drought-tolerant crops.
Science now — and the scientist you need to be
What is happening in 2026
Counting biodiversity has changed more in the last ten years than in the previous century. Environmental DNA (eDNA) — the traces of DNA that every organism sheds into water and soil — is filtered from a bottle of river water and sequenced to list the fish, frogs and mammals that live upstream, without catching any of them.
Automated sound recorders left in forests and wetlands for months pick up birds, frogs, bats and insects, and machine-learning tools such as BirdNET identify many species from their calls. Camera traps produce millions of photographs that image-recognition tools sort into species far faster than people can.
Citizen-science apps such as iNaturalist, where a computer-vision model suggests a name for each photograph and other users then check it, have added many millions of observations to the Global Biodiversity Information Facility (GBIF). The Earth BioGenome Project, launched in 2018, aims to sequence the genomes of all known eukaryotic species.
In December 2022, the governments that are parties to the Convention on Biological Diversity adopted the Kunming–Montreal Global Biodiversity Framework, whose best-known target is to conserve 30 % of land and 30 % of seas by 2030 (“30 by 30”). The next decade’s question is whether these new counts show the targets being met.
The mindshift
What you need to be able to do
Define biodiversity and niche in exact words; explain an adaptation through the process it affects (diffusion, transpiration, respiration); build a four-link food-web chain; and describe a graph in sections with values. Ecologist, conservation data scientist, eDNA technician and bioacoustics analyst are all careers that start here.
See an example
A student chose a school garden in Gaborone: a fork-tailed drongo (catches flying insects from a perch; eaten by hawks), a laughing dove (eats seeds on the ground), an earthworm (eats dead leaves in the soil; eaten by birds), a marula tree (producer; fruit eaten by people and animals) and a honeybee (nectar and pollen; pollinates flowers).
The AI assistant’s list matched, but for the drongo it wrote “lives in trees” — a habitat, not a niche. The student corrected it to “catches flying insects in the air from a perch”.
Key terms
| Biodiversity | The variety of life on Earth, or in a particular habitat or ecosystem, at the genetic, species and ecosystem levels. |
| Genetic diversity | The variety of alleles within a species or population. |
| Species diversity | The variety of species in an area: their number (richness) and how evenly individuals are shared among them (evenness). |
| Ecosystem diversity | The variety of habitats and ecosystems in a region. |
| Niche | The position or role of a species or population in an ecosystem in relation to other species. |
| Competitive exclusion | Two species cannot occupy exactly the same niche in the same place for long; one outcompetes the other. |
| Resource partitioning | The division of resources (food, space, time) among species that live together, reducing competition. |
| Xerophyte | A plant adapted to dry conditions, with features that reduce water loss or store or reach water. |
| Hydrophyte | A plant adapted to living in or on water, with air spaces, thin cuticle and upper-surface stomata. |
| Aerenchyma | Plant tissue with large air spaces that give buoyancy and let oxygen reach submerged parts. |
| Endemic species | A species found naturally in only one region and nowhere else. |
| Biodiversity hotspot | A region with at least 1 500 endemic plant species that has lost at least 70 % of its original vegetation. |
| Keystone species | A species whose effect on its community is much larger than its abundance would suggest. |
| Ecosystem services | Benefits people gain from ecosystems: provisioning, regulating, supporting and cultural. |
Think like a scientist
Criterion A A survey team studies two sites in northern Botswana in the same month. Site 1 is a seasonal floodplain on the edge of the Okavango: grassland that floods for four months a year, with water-lily lagoons, papyrus along the channels and islands of woodland.
Site 2 is Kalahari sandveld 60 km away: deep sand, scattered camel-thorn trees and grasses, no surface water for eight months of the year. The team records 212 plant species at Site 1 and 74 at Site 2. At Site 1 they see red lechwe (an antelope that feeds on grass in shallow water) and, at Site 2, steenbok (a small antelope that browses low shrubs and can live without drinking).
(a) Define biodiversity and state which level of biodiversity the numbers 212 and 74 describe. [2]
(b) Suggest two reasons why Site 1 has more plant species than Site 2. [2]
(c) Describe two features of the water lily that adapt it to Site 1 and two features of a camel-thorn tree or grass that adapt it to Site 2, explaining how each feature helps. [4]
(d) Using the idea of a niche, explain why red lechwe are not found at Site 2 and steenbok are rare at Site 1. [3]
(e) Hippos are absent from Site 2. Explain how the absence of hippos could reduce ecosystem diversity in a wetland. [3]
(f) Calculate how many times more plant species were recorded at Site 1 than at Site 2, to two significant figures. [1]
Model answer
(a) Biodiversity is the variety of life in an area, at the genetic, species and ecosystem levels [1]. The numbers are species richness — the number of plant species — so they describe species diversity [1].
(b) Water is available for more of the year at Site 1, so more photosynthesis and survival of water-dependent species (aquatic plants cannot live at Site 2) [1]. Site 1 is a mosaic of habitats — lagoons, channels, floodplain grassland, islands — so there are more niches than in uniform sandveld [1].
(c) Water lily: stomata on the upper surface, so gas exchange can happen with the air while the lower surface lies on the water [1]; large air spaces (aerenchyma) that keep the leaf afloat in the light and carry oxygen down to roots in the mud [1].
Camel-thorn: very deep roots that reach groundwater far below the dry sand [1]; small leaflets and thorns, so a small surface area loses less water by transpiration (or: a thick waxy cuticle reducing water loss) [1]. (Grass alternatives: leaves that roll in dry weather; dormancy in the dry season.)
(d) The lechwe’s niche is feeding on grasses in shallow water, and it needs to drink; Site 2 has no surface water for most of the year and no floodplain grass, so the resources of its niche are absent [1].
The steenbok’s niche is browsing low shrubs on dry ground, and it can survive without drinking; at Site 1 its browse is limited to islands, and it would compete with many other browsers there [1]. Each species occupies a niche defined by its food, feeding place and tolerance of conditions, so it is found where those resources exist [1].
(e) Hippos walk the same routes every night between water and grazing [1]; their trails are deepened into channels that carry flood water to new areas [1]; so new lagoons, channels and floodplain areas form and the mosaic of habitats is maintained — without hippos fewer channels form and the variety of habitats falls [1].
(f)
Ratio = 212 ÷ 74
Ratio = 2.86…
Ratio ≈ 2.9 times [1]
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Select the best definition of a niche: A — the place where an organism lives; B — the position or role of a species in an ecosystem in relation to other species; C — all the populations of different species in an area; D — the number of species in a habitat.
Reveal answer
B [1]. A is a habitat, C is a community, D is species richness.
Distinguish between genetic diversity, species diversity and ecosystem diversity, giving an example of each.
Reveal answer
Genetic: the variety of alleles within a species, e.g. the many local varieties of sorghum [1]. Species: the number of species and their evenness in an area, e.g. 482 bird species in the Okavango [1]. Ecosystem: the variety of habitats in a region, e.g. swamp, floodplain, islands and woodland in the delta [1].
State why pollination is important for plants and why seed dispersal is important for plants.
Reveal answer
Pollination: it is needed for (sexual) reproduction, and it increases variation in the offspring [1]. Seed dispersal: seeds carried away from the parent have a better chance of survival, because there is less competition with the parent (and with each other) for light, water and nutrients [1]. A description of pollen moving, without the reason, scores 0.
Aloes have thick, fleshy leaves with a thick waxy cuticle and few stomata. Explain how these features adapt aloes to dry environments.
Reveal answer
Fleshy leaves contain water-storing tissue, a reservoir used during the dry season [1]. A thick waxy cuticle is waterproof, reducing water loss by evaporation from the epidermis [1]. Few stomata mean fewer pores through which water vapour can diffuse out, so transpiration is reduced [1].
Honey badgers, jackals and lions all live in the same area of the Okavango. Explain how these three predators can coexist.
Reveal answer
They share a habitat but occupy different niches [1]. They take prey of different sizes — lions hunt large grazers such as buffalo and zebra; jackals small mammals, birds and carrion; honey badgers dig out rodents, reptiles, insect larvae and bee nests [1].
They hunt or forage in different ways, places and times [1]. So competition between them for the same food is reduced, and none excludes the others (competitive exclusion needs the same niche) [1].
Suggest three reasons why species richness is higher in tropical rainforest than in the Kalahari.
Reveal answer
Any three: high rainfall and temperature all year, so photosynthesis is fast and more food supports more consumers [1]; several vertical layers (canopy, understorey, floor) provide more niches [1]; a stable climate over long periods allowed more species to evolve and fewer to go extinct [1]; very large continuous areas support larger populations and more habitats [1]. (Maximum 3.)
7. Two species of Paramecium are grown separately and then together. Alone, each reaches about 200 cells per cm³ after 16 days. Together, species A reaches 180 cells per cm³ and species B falls to zero by day 18. When glass beads are added to the bottom of the mixed tube, both species persist, B living mostly among the beads. Explain all three results.
Reveal answer
Alone, each species grows until limited by food and space, reaching its carrying capacity of about 200 cells per cm³ [1].
Together, both use the same food in the same place — the same niche — so they compete; species A takes food faster or reproduces faster, so it outcompetes B and B dies out: competitive exclusion [1].
A slightly lower A population (180) shows that competition reduced A too while B was present [1].
The beads create a new microhabitat; B feeding among the beads uses a slightly different part of the resource, so the two niches overlap less and both coexist — habitat complexity creating niches and raising diversity [1].
8. A leaf loses water vapour by diffusion through its stomata at a rate proportional to the difference in water-vapour concentration between the inside of the leaf and the air just outside the pore.
Inside the leaf the concentration is 17 g m⁻³. Outside an exposed stoma on a dry afternoon it is 5 g m⁻³; inside a rolled marram leaf the air next to the stomata is at 14 g m⁻³. Calculate the percentage reduction in the rate of water loss caused by rolling, and explain one reason why the plant pays a cost for it.
Reveal answer
Exposed difference = 17 g m⁻³ − 5 g m⁻³ = 12 g m⁻³
Rolled difference = 17 g m⁻³ − 14 g m⁻³ = 3 g m⁻³ [1]
Reduction = (12 − 3) ÷ 12 × 100
Reduction = 75 % [1]
The cost: the same trapped still air also slows the diffusion of carbon dioxide into the stomata, and a rolled leaf exposes less surface to light [1], so the rate of photosynthesis — and growth — is lower while the leaf is rolled; the plant trades growth for survival, which is why marram rolls only in dry weather [1].
Summary
- Biodiversity is the variety of life on Earth or in a habitat or ecosystem, at three levels: genetic (alleles), species (richness and evenness) and ecosystem (habitats). Each level can be lost separately.
- Biodiversity provides ecosystem services — provisioning, regulating, supporting and cultural — and makes communities more stable when conditions change.
- Xerophytes reduce water loss (thick cuticle, few or sunken stomata, small leaves, rolled leaves, water storage, deep roots, night-opening stomata); hydrophytes have air spaces, a thin cuticle and stomata on the upper surface.
- A niche is the position or role of a species in relation to other species. Two species cannot share exactly the same niche for long (competitive exclusion), so coexisting species partition resources — MacArthur’s warblers, savanna browsers and grazers.
- Richness is highest where energy and water are high, habitats are complex, climates have been stable and areas are large; hotspots concentrate endemic species. Keystone species such as hippos, termites and sea otters shape whole communities.
Next time → Measuring biodiversity: how to sample a community fairly, why species richness is not enough, and how one number — the Simpson index — captures both richness and evenness.