Biodiversity, Variation & Ecosystem Resilience · Grade 7 Science

Chapter: Ecological Evidence & Environmental Design

Biodiversity, Variation & Ecosystem Resilience

Learning goals

Before this unit

Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.

Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.

Two woods, each with a hundred trees of three species. In the first, ninety-eight are beech and there is one oak and one birch. In the second there are thirty-four beech, thirty-three oak and thirty-three birch. Both have three species and a hundred trees, and almost nobody would call them equally diverse. Capturing that difference needs more than one number, and learning which number answers which question is most of this unit.

Biodiversity is the variety of life in a place, and it is measured in at least three ways that can disagree. Species richness is simply how many different species are present — three in both woods above. Abundance is how many individuals there are, of each species or in total — a hundred in both. Evenness is how the individuals are distributed among the species, and that is the one that separates the two woods: the second is even, the first is dominated by one species. A place can be rich and uneven, even and poor, or any combination, so a claim about diversity should say which measure it means.

Diversity matters beyond counting because of resilience — the ability of a community to absorb a disturbance and return to something like its former state. More variety often means more resilience, because alternatives exist: if one species fails, something else can fill its role, which is the food web argument from the last chapter. But the connection is conditional rather than automatic. A community of three hundred species that all depend on one plant is fragile despite its richness, and diversity measured at one place and time cannot by itself establish that a community will withstand whatever comes next.

Build your understanding

Where this fits

The last unit showed effects travelling through a network of relationships. This unit asks what makes such a network more or less able to absorb a change — which means being precise about what diversity is, since the word covers at least three different measurements that can point in different directions.

Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.

Part 1 · Richness, abundance and evenness are three questions

Richness counts species and ignores numbers: a wood with three species is richer than one with two, whatever the individual counts. Abundance counts individuals and ignores identity. Evenness describes the shape of the distribution — whether individuals are spread across the species or concentrated in one.

Work the comparison. Sample A detects three species with counts of 8, 1 and 1. Sample B detects three species with counts of 4, 3 and 3. Both have richness of three and total abundance of ten. They differ only in evenness: A is dominated by one species that makes up eighty per cent of the individuals; B is close to even.

That difference has consequences. In A, losing the dominant species removes most of the community's individuals and whatever function they performed, while losing one of the singletons removes almost nothing measurable. In B, no single loss is as damaging, and each species is common enough to persist. Two samples with identical richness and abundance describe quite different situations, which is exactly why one number is not enough.

Two samples with identical richness and different evennessSame richness, same abundance, different evennessSample A8sp 11sp 21sp 33 species · 10 individualsSample B4sp 13sp 23sp 33 species · 10 individualsLosing A's dominant species removes 80% of the community; no single loss does that in B
Why one number cannot describe diversity. Richness and abundance are identical in both samples and almost nobody would call them equally diverse, which is what the third measure captures.

Pause and explain

Samples A and B both detect three species among ten individuals, with counts of 8,1,1 and 4,3,3. How do they differ?

Check your reasoning after answering

In evenness. Richness is three in both and total abundance is ten in both, so those measures cannot distinguish them. A is dominated by one species holding eight of the ten individuals, while B spreads them almost equally. Losing the dominant species would remove most of A's community and no single loss would do that in B.

Part 2 · Diversity within species and across habitats

Counting species is not the only level that matters. Genetic diversity within a species — the range of alleles present, produced by the meiosis and recombination of Chapter 5 — determines whether a population contains individuals able to cope with a new disease or a changed climate. A population of genetically near-identical individuals is vulnerable in exactly the way a monoculture is, however many of them there are.

Habitat diversity is a third level. A landscape with woodland, grassland, wetland and bare ground supports species that need each, and species that need more than one during their life cycle — amphibians breeding in water and living on land, insects whose larvae and adults use different places entirely. Simplifying a landscape reduces diversity even if no species is removed directly.

The three levels interact. Genetic diversity within a species supports its persistence; species diversity supports the community's function; habitat diversity supports both by providing the conditions each needs. A survey counting species in one habitat type measures one level and should say so.

Pause and explain

Why can a population of thousands of genetically near-identical individuals still be vulnerable?

Check your reasoning after answering

Because numbers do not supply variation. If the individuals are genetically near-identical, a disease or change that defeats one defeats all of them, since none carries a different combination that might cope. Genetic diversity within a species is what makes it likely that some individuals survive a new pressure, which is the same argument as for species diversity at the community level.

Part 3 · Resilience is conditional, not guaranteed

The reason diversity is linked to resilience is redundancy. If several species can pollinate a plant, or several can decompose leaf litter, then losing one leaves the function intact. That argument is sound, and it is about functional overlap rather than about the number of species as such.

So richness alone does not guarantee resilience. A community of many species that all depend on a single plant, or all require the same narrow conditions, has little redundancy where it matters. Conversely a community of few species, each robust and able to cover several roles, may absorb a disturbance well. The useful question is whether the functions a community performs have backups.

This also means resilience is specific to what is being resisted. A community can be highly resilient to drought and collapse under a new disease, because the overlaps that cover one do not cover the other. A claim that a diverse community is stable needs to say stable against what, measured how, over what period — otherwise it is a slogan rather than a finding.

Pause and explain

Why does high species richness not guarantee that a community will be resilient?

Check your reasoning after answering

Because resilience comes from functional overlap rather than from species number. Many species that all depend on one plant, or all need the same narrow conditions, provide no backup where it matters. Resilience is also specific: a community can absorb a drought and collapse under a new disease, so a claim needs to say resilient against what.

Part 4 · Comparing interventions with stated criteria

Decisions about land usually involve a choice between options, and diversity measures are one input rather than the answer. The honest way to compare is to state the criteria first — what outcome matters, measured how, over what period and at what cost — and then assess each option against them. Choosing the criteria afterwards is how a preferred option gets justified rather than tested.

Different criteria select different options, legitimately. Maximising species richness, maximising the population of one species of concern, maximising evenness, and minimising cost are four different goals that will rank the same options differently. Saying which you are optimising is part of the proposal, not a detail.

And the measurements have to be comparable. Equal-effort sampling, the same season, the same methods, the same area — otherwise a difference in the numbers may be a difference in how hard anyone looked. The next unit is entirely about that problem, which is the practical foundation under everything in this one.

Pause and explain

Why should the criteria for comparing two land-management options be stated before the options are assessed?

Check your reasoning after answering

Because different criteria rank the options differently — maximum species richness, the population of one species, evenness and cost can each favour a different plan. Choosing criteria after seeing the results allows a preferred option to be justified rather than tested. Stating them first makes the comparison checkable by someone who disagrees.

Apply the whole unit · Compare communities

Illustrative equal-effort samples, not surveyed data: sample A has species counts of 8, 1 and 1; sample B has counts of 4, 3 and 3. Both samples contain ten individuals and three detected species.

Your task

Compare the samples on richness, total abundance and distribution, without claiming that any one number establishes which community is more stable.

What your response should show

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt

  1. Compare richness: three species detected in each sample, so richness is identical.

  2. Compare total abundance: ten individuals in each, so abundance is identical too.

  3. Compare the distribution: A has one species holding 8 of 10 individuals, while B spreads them 4, 3 and 3 — B is far more even.

  4. State what this cannot establish: evenness at one place and time does not show which community would withstand a particular disturbance.

The two samples are identical on the first two measures and differ on the third. Richness is three species in each, and total abundance is ten individuals in each, so neither of those numbers distinguishes them at all. The difference is in distribution: sample A is dominated by one species holding 8 of the 10 individuals, with two species represented by a single individual each, while sample B spreads its individuals 4, 3 and 3 and is close to even. That difference matters for what a loss would do. In A, losing the dominant species removes 80% of the individuals and whatever function they perform, while losing either singleton removes almost nothing measurable — and a species represented by one individual in a sample may be close to absent in the community anyway. In B, no single loss removes more than 40%, and each species is common enough that the sample is reasonable evidence it is established. What these figures cannot establish is which community is more stable. Resilience depends on functional overlap — whether the remaining species could cover the roles of one that was lost — and on what disturbance is being resisted, neither of which a count at one moment reveals. A community can absorb a drought and collapse under a disease. The samples are also single equal-effort snapshots, so they say nothing about seasonal change, nothing about the genetic diversity within each species, and nothing about species present but not detected.

Scientific references for this unit

Parent / teacher teaching notes

Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.

The key idea

Biodiversity includes variety within species, among species, and across ecosystems. Species richness is the number of species detected within a defined survey; abundance describes individual counts. Two communities can have equal richness yet different abundance patterns. Detection method and survey effort influence both measures.

Worked examples

Example 1 · Choose the measure that answers the question

A manager wants to know whether a meadow is dominated by one grass. Which diversity measure answers that, and which would mislead?
  1. Identify what dominance means: one species holding a large share of the individuals.
  2. Match it to a measure: evenness describes how individuals are distributed among species, so it answers directly.
  3. Check the others: richness counts species and would be the same whether one dominates or not, and total abundance counts individuals regardless of species.
Answer: Evenness answers it, since dominance is precisely an uneven distribution. Richness would mislead — a dominated meadow and a balanced one can have identical species counts. Total abundance would mislead too, counting individuals without regard to which species they belong to. The question determines the measure.
Check the reasoning

Three measures, three questions. Name the question before choosing the number.

Example 2 · Test a resilience claim

A report states that a wood is resilient because it contains forty species. What is missing, and what would strengthen the claim?
  1. Identify the mechanism resilience depends on: functional overlap, so that losing one species leaves its role covered.
  2. Ask whether richness establishes that: it does not, since forty species could all depend on one plant or all need the same conditions.
  3. Name what resilient against what requires: a specified disturbance, a measured outcome and a period.
Answer: The claim gives a species count and no information about overlap — whether other species could cover the roles of one that was lost — and does not say resilient against what. Strengthening it would mean identifying which functions have backups, naming the disturbance in question, and ideally pointing to a recorded recovery from a comparable event.
Check the reasoning

Resilience is a claim about a response to something. Ask: to what, measured how?

Avoid these mistakes

Check your understanding

Can equal individual totals have different richness?

Discuss the answer

Yes. Individuals may belong to different numbers of species.

Does counting species directly measure all genetic diversity?

Discuss the answer

No. Genetic variation within and among populations requires other evidence.

Words to know

biodiversity
The variety of life in a place, measured at the level of genes within species, species within communities, and habitats within landscapes.
species richness
How many different species are present, regardless of how many individuals each has.
abundance
How many individuals there are, in total or of each species; identical to richness in telling you nothing about distribution.
evenness
How individuals are spread among the species; 8,1,1 and 4,3,3 have the same richness and abundance and very different evenness.
resilience
The ability to absorb a disturbance and return to a similar state; it comes from functional overlap and is specific to what is being resisted.

Extend your scientific explanation

Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.

In the real world

Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.

Exit check

Why distinguish evidence from a decision criterion?

Check your explanation after answering

Evidence describes conditions or effects; a criterion states what the decision seeks to achieve or prioritize.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

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