Species, ecosystems and niches: the three levels of biodiversity – Introduction to Biodiversity (Senior 4 Biology) | Ganzaa

Introduction to Biodiversity

Lessons

Senior 4 · Biology

Introduction to Biodiversity

3 lessons

Lesson 1 of 3

Species, ecosystems and niches: the three levels of biodiversity

25 min

Thirteen kinds of primate in one forest

Nyungwe National Park, in the south-west of Rwanda, covers about 1,019 km² of montane rainforest, bamboo, grassland and bog. Surveys have recorded 13 primate species in it, more than 300 bird species (about 30 of them found only in the mountains of the Albertine Rift), and over 1,000 species of plant. Chimpanzees, L'Hoest's monkeys and Ruwenzori colobus share the same hillsides without one species driving the others out.

Two questions follow, and they run through this whole unit. What exactly do we mean when we say a place has "high biodiversity"? And how can so many similar animals live side by side? The answers need three precise terms (species, ecosystem and niche) that you met in S2 and S3 and now need at A-level precision.

Definition

Species (biological species concept): a group of organisms that can interbreed in nature to produce fertile offspring, and that is reproductively isolated from other such groups. Example: all domestic cattle, Bos taurus.

Population: all the organisms of one species living in one area at the same time. Example: the chimpanzees of Nyungwe.

Community: all the populations of all species living and interacting in one area.

Habitat: the place where an organism lives, described by its physical features. Example: the canopy of a montane forest.

Ecosystem: a community together with its abiotic (non-living) environment, linked by the flow of energy and the cycling of nutrients. Example: Lake Kivu with its plankton, fish, water, sediments and sunlight.

Ecological niche: the role of a species in its ecosystem: everything it needs and does, including the range of every abiotic condition it tolerates, the food it eats, when and where it feeds, and what eats it.

Biodiversity: the variety of life in an area, measured at three levels: the genes within species, the species in communities, and the ecosystems in a region.

Species: where the definition works and where it fails

The biological species concept turns on one test: can the two groups interbreed in nature and produce fertile offspring? Producing offspring is not enough. A horse ( $2 n = 64$ ) and a donkey ( $2 n = 62$ ) can mate and produce a mule, but the mule is almost always sterile, so horses and donkeys are separate species.

The concept cannot be applied everywhere:

Organisms that reproduce asexually, such as bacteria, never interbreed, so the test has no meaning for them.
Fossils cannot be tested for interbreeding.
Populations that never meet, on two separate mountains for instance, cannot be observed interbreeding in nature even if they could.

In these cases biologists use a morphological species concept (organisms that share a set of structural features) or a phylogenetic species concept (the smallest group that shares a common ancestor, shown today by DNA sequences). You will meet DNA-based classification again in Unit 2.

Worked example 1: why is a mule sterile?

A horse has $2 n = 64$ chromosomes and a donkey $2 n = 62$ . Work out the chromosome number of a mule and explain why it is almost always sterile.

Each parent passes on a haploid gamete: the horse gives $n = \frac{64}{2} = 32$ chromosomes and the donkey $n = \frac{62}{2} = 31$ .
The mule therefore has $32 + 31 = 63$ chromosomes in each body cell.
To make gametes, a cell must go through meiosis, in which every chromosome pairs with its partner (homologue). The mule has an odd number of chromosomes, and many horse and donkey chromosomes differ in shape and gene order, so they cannot pair properly.
Meiosis fails, few or no viable gametes form, and the mule cannot breed. Because the hybrid is not fertile, horse and donkey are separate species by the biological species concept.

Habitat is the address; niche is the profession

Many species share one habitat, but no two stable species share exactly the same niche. The ecologist G. Evelyn Hutchinson (1957) described a niche as a "hypervolume": imagine one axis for every condition and resource a species needs (temperature, humidity, food size, feeding height, time of activity, and so on). The region of that many-dimensional space in which the species can survive and reproduce is its niche.

The fundamental niche is the full range of conditions and resources in which a species could live if no other species interfered.
The realised niche is the part of it that the species actually occupies once competitors, predators and parasites act on it. It is always the same size as, or smaller than, the fundamental niche.
The competitive exclusion principle (G. F. Gause, 1934): two species that need exactly the same limiting resources cannot coexist in the same place indefinitely; the better competitor eliminates the other. Species that coexist must therefore differ in at least one niche dimension. Ecologists call this niche differentiation or resource partitioning.

A classic field test of these ideas was carried out by Joseph Connell on the rocky shores of Scotland, published in 1961. Two barnacles live between the tide marks. Figure 1 summarises what he found.

Worked example 2: reading a field experiment

Young Chthamalus barnacles settle on the upper, middle and lower shore, yet adults are found only on the upper shore. Where Balanus grew alongside them on the middle shore, most young Chthamalus died within a year, crushed, smothered or undercut by the faster-growing Balanus. On rocks from which Connell removed every Balanus, the Chthamalus on the middle shore survived. Explain these results, using Figure 1.

Young Chthamalus survive on the middle shore when Balanus is absent, so the middle shore is part of the fundamental niche of Chthamalus: the physical conditions there do not kill it.
Where Balanus is present, Chthamalus dies there. The difference between the two treatments is the competitor, so interspecific competition for space excludes Chthamalus from the middle shore.
Its realised niche is therefore only the upper shore, the zone that Balanus cannot occupy because it dries out when the tide is out for long periods.
The upper limit of each barnacle is set by an abiotic factor (drying); the lower limit of Chthamalus is set by a biotic factor (competition). A good answer names both kinds of factor.

Three levels of biodiversity

Level	What varies	Example from Rwanda	Why it matters
Genetic diversity	The alleles (versions of genes) carried by the individuals of one species	The many bean varieties farmers grow, differing in seed colour, cooking time and disease resistance	A population with many alleles is more likely to contain individuals that survive a new disease or a change of climate
Species diversity	The number of species in a community (richness) and how evenly the individuals are spread among them (evenness)	13 primate species in Nyungwe	More species means more links in food webs, so the loss of one species has smaller knock-on effects
Ecosystem diversity	The range of different habitats, communities and ecosystems in a region	Montane forest (Nyungwe), savanna (Akagera), wetland (Rugezi), lakes (Kivu, Ihema)	Each ecosystem supports its own species and provides its own services

Species richness alone can mislead. Two fields with the same four species are equally rich, but if one species makes up almost every plant in one field, that field is less diverse. In Lesson 3 you will turn this idea into a number, Simpson's index of diversity.

Worked example 3: richness or evenness?

A student counted the trees in two 50 m by 50 m plots (illustrative data).

Species	Plot A	Plot B
Species 1	20	74
Species 2	20	2
Species 3	20	2
Species 4	20	2
Species 5	0	0
Total	80	80

Compare the species richness and the diversity of the two plots.

Richness: each plot contains 4 species, so the richness is equal.
Evenness: in plot A each species makes up $\frac{20}{80} = 25\,\%$ of the trees; in plot B one species makes up $\frac{74}{80} \approx 93\,\%$ .
Two trees picked at random in plot B will almost always be the same species; in plot A they are usually different. Plot A is therefore more diverse, although the richness is the same.

Why biodiversity matters

Ecologists group the benefits people get from ecosystems as ecosystem services:

Type of service	Meaning	Examples
Provisioning	Products taken from ecosystems	Food, fish from Lake Kivu, timber, firewood, medicines, wild relatives of crops
Regulating	Natural processes that keep conditions stable	Pollination of crops, control of floods and erosion by forests and wetlands, water purification, carbon storage
Supporting	Processes that all the other services depend on	Soil formation, nutrient cycling, photosynthesis
Cultural	Non-material benefits	Tourism, recreation, education, spiritual and aesthetic value

Three further arguments are often made:

Medicines. Many drugs came from wild species. Quinine, used for decades against malaria, came from the bark of the Cinchona tree; artemisinin, the basis of today's malaria treatment, was isolated from the plant Artemisia annua. Each species lost is a set of chemicals never tested.
Stability. A community with many species usually has many alternative food chains. If one prey species declines, predators can switch to another, so the community resists disturbance and recovers faster.
Ethics. Many people, and many religious and cultural traditions, hold that species have a right to exist whether or not they are useful to humans.

TED-Ed: "Why is biodiversity so important?" by Kim Preshoff. Notice how the video links the three levels of biodiversity to the services ecosystems provide, and listen for its example of what happens when one species is removed from a food web.

Warning

Common mistakes

Writing "habitat" when the question asks for "niche". A habitat is a place; a niche includes what the organism does there, when and with what.
Saying two organisms belong to the same species "because they can breed". The offspring must also be fertile, and the breeding must happen in nature.
Treating species richness as if it were diversity. Diversity also takes evenness into account.
Stating that the realised niche can be larger than the fundamental niche. It cannot: other species can only remove parts of the fundamental niche.

Tip

Exam technique. When asked to explain why a species is absent from part of its range, test two kinds of cause: an abiotic limit (too dry, too cold, too acidic) and a biotic limit (competition, predation, disease). Evidence that the species survives there when a competitor is removed points to competition, and to the difference between its fundamental and realised niche.

✦

Practice: species, niches and the levels of biodiversity

Distinguish between the habitat and the niche of the mountain gorilla.
Lions ( $2 n = 38$ ) and tigers ( $2 n = 38$ ) have produced hybrids in zoos, but the two never meet in the wild today. Explain why they are still treated as separate species.
Explain why the biological species concept cannot be used to classify bacteria, and name a concept that can.
Field A has 6 species of grass with similar numbers of each; field B has 9 species, but one makes up 90 % of the plants. State which field is richer and suggest which is more diverse. Explain.
Using Figure 1, predict what would happen to the lower limit of Chthamalus if every Balanus were removed from a stretch of shore. Give a reason.
Classify each as a provisioning, regulating, supporting or cultural service: (a) bees pollinating coffee flowers; (b) bamboo cut for building; (c) decomposers releasing nitrate into soil; (d) students visiting a wetland on a field trip.
Evaluate the claim: "If a species is kept alive in a zoo, its biodiversity has been saved."

Show answersHide answers

Habitat: the place it lives, the montane forest and bamboo zone on the slopes of the Virunga volcanoes. Niche: its role there, for example a ground-living herbivore that eats leaves, stems and shoots by day, lives in family groups, builds a new nest each night, and disperses seeds; plus the range of temperature and humidity it tolerates.
The biological species concept asks whether the groups interbreed in nature. Lions and tigers are reproductively isolated in the wild by geography and behaviour, and hybrids are rare and often have low fertility, so they are separate species.
Bacteria reproduce asexually, so they never interbreed and the test has no meaning. They are classified by a phylogenetic (DNA-based) or a morphological and biochemical concept.
Field B is richer (9 species against 6). Field A is probably more diverse: its individuals are spread evenly, whereas in B two plants picked at random are very likely to be the same dominant species.
Chthamalus would extend down onto the middle shore (and possibly lower), towards the edge of its fundamental niche, because the competitor that crushes and smothers it there has gone.
(a) regulating; (b) provisioning; (c) supporting; (d) cultural.
Only partly true. A zoo keeps the species alive (species diversity) but usually holds few individuals, so most of the genetic diversity is lost; the animal no longer plays its role in its ecosystem, so ecosystem services and the other species that depend on it are not protected. Zoos are a back-up; protecting the habitat saves all three levels.

Quick check

Which statement describes the niche of a bat, not just its habitat?

Key points

A species is a group that interbreeds in nature to give fertile offspring; asexual organisms and fossils need morphological or DNA-based concepts instead.
Habitat is where an organism lives; its niche is its role, the full set of conditions, resources and interactions it needs. Competitors shrink the fundamental niche to the realised niche.
Biodiversity has three levels: genetic, species (richness and evenness) and ecosystem diversity.
Biodiversity provides provisioning, regulating, supporting and cultural services, medicines and stability, and many hold that it has value in itself.

Sources and credits

8.4.2: The Biodiversity Crisis and 8.4.3: The Importance of Biodiversity to Human Life, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the levels of biodiversity, medicines from wild species and ecosystem services. Adapted by Ganzaa.
Ecological niche and Joseph H. Connell, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for Hutchinson's niche, fundamental and realised niches, competitive exclusion and Connell's 1961 barnacle study. Adapted by Ganzaa.
Nyungwe facts (area, primate, bird and plant species): Nyungwe National Park, Wikipedia contributors, CC BY-SA 4.0. Facts only.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Photos: US National Park Service (public domain); Neil Palmer (CIAT), CC BY-SA 2.0; Alex Niragira, CC BY-SA 4.0; all via Wikimedia Commons.
Video: "Why is biodiversity so important?", TED-Ed, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Thirteen kinds of primate in one forest

2Species: where the definition works and where it fails

3Habitat is the address; niche is the profession

4Three levels of biodiversity

5Why biodiversity matters

Thirteen kinds of primate in one forest

Species: where the definition works and where it fails

Habitat is the address; niche is the profession

Three levels of biodiversity

Why biodiversity matters