---
title: "Biodiversity"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 30
exercises: 11
source: https://one-course.com/books/biology/1/en/chapter/30-biodiversity
---

# Chapter 30 — Biodiversity

Take one square metre of flowering meadow and start counting: a dozen kinds of [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), beetles and bugs beyond counting, spiders, snails, worms below and butterflies above. Now try the same square metre of paved schoolyard. The difference between those two counts has a name — [biodiversity](#def-g5-biodiversity-biodiversity) — and it has become one of the most important words in [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living).

## 30.1 The diversity of life

**Definition 30.1 (Species).**

A *species* is one particular kind of [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living): all the individuals built on the same model, which reproduce among themselves. The house sparrow is a species; so is the common daisy, the red fox, the garden snail. [Chapter 21](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#ch-g4-classifying-animals)’s classes are boxes of many species: “[mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)” holds thousands of them.

**Definition 30.2 (Biodiversity).**

The *biodiversity* of a place is the variety of life it holds — above all, how many different *[species](#def-g5-biodiversity-species)* live there, and how well each is doing. A flowering meadow, rich in [species](#def-g5-biodiversity-species), has high biodiversity; a paved yard, almost none.

![One square metre of meadow, mid-count: grasses, flowers, butterflies, beetles, a snail — and the count has barely begun.](https://one-course.com/images/onecourse/chapters/biology-1/g5-biodiversity/img-0de08eda9b56.jpg)

*One square metre of meadow, mid-count: grasses, [flowers](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts), butterflies, beetles, a snail — and the count has barely begun.*

**Example 30.3 (Counting the planet).**

Scientists have named about two million [species](#def-g5-biodiversity-species) so far — and are sure that is only part of the total, since new [species](#def-g5-biodiversity-species) are found on every serious expedition: beetles above all (no group has more named [species](#def-g5-biodiversity-species)), but also [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), frogs, [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), even occasional [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups). Earth’s full [species](#def-g5-biodiversity-species) count is still unknown — most estimates run into the millions beyond what is named.

**Remark 30.4 (Diversity at every scale).**

[Biodiversity](#def-g5-biodiversity-biodiversity) shows at several scales at once: many [species](#def-g5-biodiversity-species) in one [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat); many different [habitats](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) side by side (pond, hedge, meadow — each with its own [species](#def-g5-biodiversity-species), as [Example 11.2](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#ex-g2-where-animals-live-four) showed); and, within one [species](#def-g5-biodiversity-species), no two individuals quite alike — no two garden snails wear exactly the same [shell](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering). Variety within variety within variety.

## 30.2 Why biodiversity matters

**Proposition 30.5 (The web needs its threads).**

The [species](#def-g5-biodiversity-species) of a [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) feed, shelter, pollinate and clean up for one another — they are threads of one web ([Remark 19.9](https://one-course.com/books/biology/1/en/chapter/19-food-chains#rem-g3-food-chains-web)). A rich web with many threads can absorb a shock: if one prey [species](#def-g5-biodiversity-species) fails, its hunters switch to others. A poor web tears easily: each lost [species](#def-g5-biodiversity-species) [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) the survivors fewer options. [Biodiversity](#def-g5-biodiversity-biodiversity) is the web’s strength.

**Proof.** *Admitted at this level.* ∎

**Example 30.6 (The orchard test).**

An orchard with hedges, wildflower strips and old trees hosts dozens of helper [species](#def-g5-biodiversity-species): bees and hoverflies pollinate the blossom ([Remark 16.5](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#rem-g3-flowers-fruits-seeds-bees)), ladybugs eat the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) lice, [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) patrol for [caterpillars](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly), worms keep the soil open. Grub out the hedges and mow the [flowers](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts), and the helpers leave — the [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) grower must then do their jobs alone, jobs the web used to do free.

**Example 30.7 (People live from the web).**

Everything humans eat traces back to living [species](#def-g5-biodiversity-species) ([Remark 22.8](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#rem-g4-what-plants-need-everyone)); most of our medicines were first found in [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), molds and other [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living); cotton, wool, timber and leather are [species](#def-g5-biodiversity-species)’ products. The web’s threads run straight through every kitchen, pharmacy and wardrobe.

## 30.3 Biodiversity can be lost — and helped

**Proposition 30.8 (How diversity is lost).**

A [species](#def-g5-biodiversity-species) dwindles when what it depends on is taken:

- its *[habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat)* destroyed — the drained pond of [Proposition 11.7](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#prop-g2-where-animals-live-loss) , the grubbed-out hedge, the paved meadow;
- its [environment](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#def-g2-caring-for-nature-environment) poisoned — litter and chemicals ( [Proposition 13.4](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#prop-g2-caring-for-nature-harm) );
- too many of it taken — overfished, overpicked, overhunted.

A [species](#def-g5-biodiversity-species) entirely gone — everywhere, forever — is *extinct*. Extinction is not like a lost game: there is no next round.

**Proof.** *Admitted at this level.* ∎

**Method 30.9 (Helping biodiversity, starting small).**

The recipes of [Method 13.6](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#met-g2-caring-for-nature-rules) scale up:

1. keep and [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) variety — a hedge of five bushes beats a fence, a [flower](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) strip beats bare gravel;
2. leave wild corners: dead wood, [leaf](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) piles, unmowed grass are [habitats](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) ( [Example 13.2](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#ex-g2-caring-for-nature-home) );
3. help the helpers: nest boxes, insect shelters, a pond however small ( [Example 11.8](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#ex-g2-where-animals-live-schoolpond) );
4. count what lives around you, year after year — what is counted is noticed, and what is noticed gets protected.

**Example 30.10 (The return of the storks).**

Where wet meadows were restored and nest platforms raised, storks — once nearly gone from whole regions — came back to breed, and with them frogs, dragonflies and meadow [flowers](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts). [Species](#def-g5-biodiversity-species) return when their threads are retied: protection works, and its results can be watched from the village square.

## 30.4 Exercises

**Exercise 30.1 ★.**

What is a [species](#def-g5-biodiversity-species)? Give three examples from this chapter.

**Solution of Exercise 30.1.**

One particular kind of [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) — individuals built on the same model, reproducing among themselves. Examples: the house sparrow, the common daisy, the red fox, the garden snail.

**Exercise 30.2 ★.**

What is [biodiversity](#def-g5-biodiversity-biodiversity)? Compare the meadow and the paved yard.

**Solution of Exercise 30.2.**

The variety of life a place holds — above all its number of [species](#def-g5-biodiversity-species) and their health. The flowering meadow holds dozens of [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) and [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [species](#def-g5-biodiversity-species) in one square metre: high [biodiversity](#def-g5-biodiversity-biodiversity). The paved yard holds almost none.

**Exercise 30.3 ★.**

About how many [species](#def-g5-biodiversity-species) are named so far? Why is the true number certainly higher?

**Solution of Exercise 30.3.**

About two million. Because every serious expedition still finds unnamed [species](#def-g5-biodiversity-species) — beetles especially — so millions more must exist beyond the catalogue.

**Exercise 30.4 ★.**

Name the three scales of diversity in [Remark 30.4](#rem-g5-biodiversity-scales), each with its example.

**Solution of Exercise 30.4.**

Many [species](#def-g5-biodiversity-species) in one [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) (the meadow’s square metre); many [habitats](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) side by side (pond, hedge, meadow); variety within a [species](#def-g5-biodiversity-species) (no two garden snails’ [shells](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) quite alike).

**Exercise 30.5 ★.**

Why does a species-rich web survive shocks better than a poor one? Use the hunters-switch-prey argument.

**Solution of Exercise 30.5.**

In a rich web, a hunter whose prey fails can switch to other prey, and every role has understudies; the shock is absorbed. In a poor web each [species](#def-g5-biodiversity-species) has few options, so one loss drags others after it.

**Exercise 30.6 ★.**

List four helper [species](#def-g5-biodiversity-species) of the orchard and the free job each does.

**Solution of Exercise 30.6.**

Bees and hoverflies: pollinating the blossom. Ladybugs: eating the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) lice. [Birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups): patrolling for [caterpillars](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly). Worms: keeping the soil open.

**Exercise 30.7 ★.**

Give the three ways a [species](#def-g5-biodiversity-species) dwindles, with one example each. What does *[extinct](#prop-g5-biodiversity-loss)* mean?

**Solution of Exercise 30.7.**

[Habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) destroyed (the drained pond); [environment](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#def-g2-caring-for-nature-environment) poisoned (litter and chemicals); too many taken (overfishing). [Extinct](#prop-g5-biodiversity-loss) means gone entirely — everywhere, forever.

**Exercise 30.8 ★.**

Try it: count the [species](#def-g5-biodiversity-species) — [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) and [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), named or described — in one small square you choose outdoors. Then count a paved square for comparison. Your two numbers are a [biodiversity](#def-g5-biodiversity-biodiversity) measurement.

**Solution of Exercise 30.8.**

Open answer. A grassy or planted square typically yields ten or more [species](#def-g5-biodiversity-species); the paved square, close to zero — the difference is the measurement.

**Exercise 30.9 ★★.**

“Losing one beetle [species](#def-g5-biodiversity-species) out of thousands cannot matter.” Answer with [Proposition 30.5](#prop-g5-biodiversity-web) — and with the fact that nobody knows all the threads a [species](#def-g5-biodiversity-species) holds.

**Solution of Exercise 30.9.**

Each [species](#def-g5-biodiversity-species) is a thread: its loss [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) every neighbor — whatever ate it, whatever it pollinated or cleaned up — with fewer options, and weakens the web’s power to absorb the next shock. And since no one knows all the threads a [species](#def-g5-biodiversity-species) holds, no one can certify a loss harmless in advance; extinction being forever, the mistake could never be unmade.

**Exercise 30.10 ★★.**

The stork story is called a success of *[habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat)* protection rather than of protecting storks one by one. Explain the difference, using [Proposition 11.7](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#prop-g2-where-animals-live-loss) and what returned alongside the storks.

**Solution of Exercise 30.10.**

Protecting storks one by one would mean guarding [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups); restoring wet meadows rebuilt what the storks *depend on* — and a [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) feeds all its inhabitants at once, which is why frogs, dragonflies and meadow [flowers](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) returned with the storks. No [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat), no inhabitants; [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) restored, inhabitants return.

**Exercise 30.11 ★★★.**

An orchard keeper replaces hedges and [flower](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) strips with fences and gravel, then must rent beehives for pollination and spray against [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) lice — spending money on jobs once done free. Use [Example 30.6](#ex-g5-biodiversity-orchard) to explain what was really removed — and why the accounts of nature’s free work usually surface only after it stops.

**Solution of Exercise 30.11.**

What was removed was not decoration but a workforce: the hedge and [flower](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#def-g3-flowers-fruits-seeds-parts) [species](#def-g5-biodiversity-species) housed the pollinators and pest eaters, whose services appeared in nobody’s accounts while free. [Nature](https://one-course.com/books/biology/1/en/chapter/13-caring-for-nature#def-g2-caring-for-nature-environment)’s work is invisible bookkeeping — constant, unpaid, unnoticed — so its price is discovered exactly when the workers are evicted and the jobs come back as bills.