Biodiversity & Ecosystem Services · Grade 12 Science
Chapter: Ecology, Populations & BiodiversityBiodiversity & Ecosystem Services
Learning goals
- I can distinguish genetic, species, and ecosystem diversity and use a diversity index to compare communities.
- I can explain how biodiversity supports ecosystem services and resilience, and why keystone species matter.
Readings for this unit
Before this unit
Species/genetic/ecosystem diversity and fractions.
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Start with something to explain
Why can equal species counts hide very different communities?
Illustrative evidence case. Two 100-individual samples each contain four species but have different dominance.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can equal species counts hide very different communities?
Your first model
Retain species counts and calculate proportions before summarizing.
Build the language from the idea
- biodiversity
- The total variety of life in a place, measured at three levels: genetic, species, and ecosystem diversity.
- genetic diversity
- The variety of genes and alleles within a population or species, which supplies the raw material for adapting to change.
- species diversity
- A measure combining species richness (how many species) with evenness (how equally abundant they are).
- ecosystem diversity
- The variety of different ecosystems, habitats, and ecological communities across a region.
- ecosystem services
- The benefits people get from functioning ecosystems, grouped as provisioning, regulating, supporting, and cultural services.
- keystone species
- A species whose effect on its ecosystem is far larger than its abundance would suggest, so its loss triggers disproportionate change.
Biodiversity is the variety of life, and ecologists measure it at three nested scales. Genetic diversity is the variation in genes and alleles within a single population. Species diversity is the variety of species living together in a community. Ecosystem diversity is the range of distinct habitats and ecological communities spread across a landscape or region. A place can be rich at one level and poor at another, so healthy biodiversity really means variety at all three at once.
Genetic diversity is the quietest of the three but arguably the most fundamental, because it is the raw material evolution works with. A population carrying many different alleles is far more likely to contain a few individuals that can survive a new disease, a heat wave, or a novel predator — some will happen to have resistant traits, and the population persists. A genetically uniform population, by contrast, is fragile: the classic cautionary tale is a monoculture crop in which every plant is nearly identical, so a single pest or blight can wipe out the entire field at once. Low genetic diversity is also why very small or inbred populations struggle to recover.
Species diversity has two ingredients that students often blur together. Richness is simply the count of species present. Evenness describes how balanced their abundances are. Two forests can both have four tree species (equal richness), yet one where each species is common is far more diverse than one dominated by a single species with three rare stragglers. To capture both at once, ecologists use a diversity index such as Simpson's Diversity Index, D = 1 - Σ(n/N)², where n is the number of individuals of a species and N is the total; D runs from 0 (one species dominates completely) toward 1 (many species, evenly spread). The index lets scientists put a single, comparable number on how diverse two communities are.
Why should anyone outside biology care? Because functioning ecosystems quietly perform work that human economies depend on, called ecosystem services. These fall into four groups: *provisioning* services supply tangible goods like food, timber, and fresh water; *regulating* services stabilize conditions by purifying water, pollinating crops, controlling pests, and buffering the climate; *supporting* services underpin the rest through soil formation, nutrient cycling, and photosynthesis; and *cultural* services provide recreation, beauty, and scientific and spiritual value. Economists estimate these services are worth tens of trillions of dollars a year — and generally, more diverse ecosystems deliver them more reliably.
Not every species contributes equally to holding a system together. A keystone species exerts an influence out of all proportion to its numbers. Sea otters are the textbook case: by eating sea urchins, a modest otter population protects entire kelp forests that dozens of other species need. Remove the otters and urchins explode, the kelp is grazed to bare rock, and diversity collapses. Wolves in Yellowstone and beavers that engineer wetlands play similar oversized roles. Losing a keystone species can unravel an ecosystem far faster than losing a more abundant but ordinary one.
Biodiversity can support resilience when organisms have differing responses or overlapping functional roles, but recovery also depends on species identity, disturbance, habitat, and time. More species does not guarantee faster recovery from every event. A resilience claim should specify the function being maintained and the disturbance being tested. Conservation reasoning needs measured ecological outcomes and mechanisms rather than treating one diversity count as a universal performance guarantee.
Model check: A service value or diversity index does not represent every ecological or ethical criterion. D=.5. State this version of the index and the sampling effort before comparing sites.
Build your understanding
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Part 1 · Distinguish levels of biodiversity
Biodiversity includes variation within species, among species and among ecosystems. Genetic diversity concerns heritable variation, while species diversity includes identity and relative abundance. A landscape with several habitat types adds another scale. A high species count does not guarantee high genetic variation in each population, and one diversity index cannot summarize all dimensions. Define the target and sampling extent before comparing places or claiming that one number captures ecological value.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Can two habitats have the same species count but different species diversity?
Visual reasoning — after your attempt
Yes. Relative abundances, identities and other diversity dimensions can differ.
Biodiversity can describe genetic variation, species diversity, and ecosystem variety. A high species count does not establish high genetic variation within every population or all ecological functions. Specify the scale and question before choosing a measure.
Two sites can have the same number of species but different population sizes, habitats, and genetic patterns. One index cannot answer every conservation question.
Pause and explain
Why can equal species richness conceal meaningful differences?
Check your reasoning after answering
The abundances, identities, genetic variation, and habitat contexts can differ despite the same count of species.
Part 2 · Separate richness from evenness
Species richness is the number of species observed within a defined sampling extent. Evenness describes how evenly individuals are distributed among those species. Communities with counts 25,25,25,25 and 97,1,1,1 both have richness 4, but the second is dominated by one species. Comparable methods and effort are essential because a longer or more sensitive search can increase observed richness without a real ecological increase. Preserve counts rather than keeping only a summary label.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Which community is more even and what evidence supports that conclusion?
Visual reasoning — after your attempt
X is more even because all four observed species have equal counts.
Richness counts observed species; evenness describes how individuals are distributed among them. Sampling effort affects apparent richness, and rare species can be missed. A diversity measure must name its definition rather than assuming every index has the same direction or interpretation.
A community dominated by one species differs from an evenly distributed community even when each contains three observed species.
Pause and explain
Does greater sample size necessarily mean a more diverse habitat?
Check your reasoning after answering
No. Greater sampling effort can reveal more species in the same habitat; comparisons need matched effort and method.
Part 3 · Calculate a defined diversity measure
For this course, the selected Simpson diversity convention is D=1−Σpi², with pi the observed fraction in species i. Larger values mean greater diversity under this convention. Other publications use the sum or its reciprocal, so the name alone is ambiguous. For four equally common species D=0.75. The result is an estimate from sampled counts and does not identify habitat quality, extinction risk or causal mechanisms by itself.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Find D for two species with fractions 0.8 and 0.2.
Visual reasoning — after your attempt
D=1−(0.64+0.04)=0.32 under the stated convention.
One introductory Simpson diversity measure is 1−Σpi², where pi is each observed species’ fraction. A larger value in this convention reflects a lower chance that two independent draws with replacement select the same species. Related Simpson conventions differ, so state the formula explicitly.
This sample-based index describes observed composition. It is not a direct count of ecosystem services or a full estimate of unsampled species.
Pause and explain
Why must a Simpson value name its convention?
Check your reasoning after answering
Some versions report Σpi² or its reciprocal, changing the scale and direction of interpretation.
Part 4 · Link functions to evidence rather than slogans
An ecosystem service is a benefit people receive from ecological processes, such as pollination, water regulation or culturally valued experiences. Explain the mechanism and name the beneficiary rather than assuming any larger index automatically improves every outcome. Organisms’ functional traits, timing and environmental context matter. A comparison should measure the selected function with matched conditions and recognize tradeoffs among services and among people with different needs or priorities.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
What evidence would strengthen a claim that a planting improves pollination service?
Visual reasoning — after your attempt
Comparable measurements of pollinator activity and relevant plant reproduction, with timing, habitat and alternative explanations controlled or documented.
Pollination, nutrient processing, water regulation, and other services depend on organisms and environmental context. Diversity can support resilience through differing responses, but more species does not guarantee every function improves in every situation. Compare the actual function and disturbance being studied.
A pollinator count and measured pollination success answer related but distinct questions. Species identity and season can also matter.
Pause and explain
What would support a claimed improvement in a service?
Check your reasoning after answering
Comparable measurements of the service itself, along with composition, conditions, and repeated observations, would test whether the proposed mechanism holds.
Apply the whole unit · Compare richness and abundance diversity
Illustrative matched samples each contain 100 individuals across three species. Site A counts are 90,5,5; site B counts are 34,33,33. Use D=1−Σpi².
Your task
Calculate richness and D for both sites, explain their difference, and state what additional evidence is needed to compare ecosystem function or total biodiversity.
What your response should show
- Define the system, quantities, units, and relevant conditions before selecting a model.
- Show intermediate calculations or a labeled evidence account, including signs or denominators.
- Explain how the supplied evidence supports the conclusion rather than reporting only an answer.
- Identify a meaningful uncertainty, model limit, or measurement needed to test the conclusion.
Response guidance — after your attempt
Both samples have observed richness 3.
A: D=1−(0.90²+0.05²+0.05²)=0.185.
B: D=1−(0.34²+0.33²+0.33²)=0.6666.
The second sample is more even and has the larger value under the supplied convention. Neither result establishes higher genetic diversity or better performance of every ecosystem service. Species identity, sampling representativeness, seasonal variation, and direct functional measurements remain needed.
Scientific references and source-model foundations
Parent / teacher teaching notes
Check that fractions sum to one and the index convention is written. Reward a precise observed-sample claim rather than an unsupported ranking of entire ecosystems. Ask the learner to explain each symbol, axis, unit and arrow before calculating. Calculated examples, supplied illustrative records and actual historical observations are labeled separately. Selected public OpenSciEd middle-school source figures are foundation examples, not complete high-school editions or an endorsement; all advanced explanations are supplied locally.
The key idea
Biodiversity spans genetic, species, and ecosystem levels; it powers the ecosystem services human societies depend on and gives ecosystems the resilience to recover from disturbance — and keystone species can shape a whole community out of proportion to their numbers.
Simpson's Diversity Index
D = 1 - Σ(n/N)²
n is the number of individuals of a given species and N is the total number of individuals; D ranges from 0 (one species dominates) toward 1 (many species, evenly spread), combining richness and evenness.
Worked examples
Example 1
- Find the total: N = 40 + 40 + 20 = 100.
- Compute each proportion squared: (40/100)² = 0.16, (40/100)² = 0.16, (20/100)² = 0.04.
- Sum them: 0.16 + 0.16 + 0.04 = 0.36. Then D = 1 - 0.36 = 0.64.
Example 2 · Worked application
- Richness counts species, not each species' functional contribution.
- Service depends on abundances, traits and interactions.
- A loss can change function without an immediate large richness change.
Use a service-specific measure and sampling design.
Example 3
- Forest A: Σ(n/N)² = 4 × (0.25)² = 4 × 0.0625 = 0.25, so D = 1 - 0.25 = 0.75.
- Forest B: Σ(n/N)² = (0.91)² + 3 × (0.03)² = 0.8281 + 0.0027 = 0.8308, so D = 1 - 0.8308 ≈ 0.17.
- Both forests have the same richness (4 species), but Forest A is far more even, while Forest B is dominated by one species.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: Use a service-specific measure and sampling design.
- Functions and interactions differ; redundancy may exist but cannot be assumed without evidence.
Use a service-specific measure and sampling design.
Example 5 · Reason from the visual model
- For this course, the selected Simpson diversity convention is D=1−Σpi², with pi the observed fraction in species i. Larger values mean greater diversity under this convention. Other publications use the sum or its reciprocal, so the name alone is ambiguous. For four equally common species D=0.75. The result is an estimate from sampled counts and does not identify habitat quality, extinction risk or causal mechanisms by itself.
- D=1−(0.64+0.04)=0.32 under the stated convention.
Original index calculation from illustrative proportions, with explicit formula and no finite-sample correction.
Avoid these mistakes
- A service value or diversity index does not represent every ecological or ethical criterion.
Check your understanding
Two species have fractions.5,.5. UseD=1−Σpi².
Discuss the answer
D=.5. State this version of the index and the sampling effort before comparing sites.
Return to the phenomenon
Return to the initial case. Why can equal species counts hide very different communities? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Richness ignores abundance distribution; evenness and a specified diversity index reveal additional patterns without proving ecological cause.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, calculate and revise
Keep a first model and an evidence-supported revision. Attempt each task before opening guidance; all required explanations and supplied-data routes are available locally.
Activity 1 · Return to the opening case
Use the opening phenomenon, unit definitions and explained Parts 1–4.
Your task
Return to the initial case. Why can equal species counts hide very different communities? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use the stated observations and appropriate units.
- Connect a labeled model with the physical/environmental mechanism.
- State assumptions and an evidence limitation.
- Explain a substantive revision or next test.
Hint
Keep object/material identity, material transfer and energy account distinct.
Response guidance — discuss after your attempt
Richness ignores abundance distribution; evenness and a specified diversity index reveal additional patterns without proving ecological cause.
Activity 2 · Transfer to a changed case
Cover the worked answer and use the explained models, relationships and notebook.
Your task
Counts are 6,3,1 for three species. Find richness and D=1−Σpi², then state a sampling limitation.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the physical or environmental mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name known quantities and conditions, show intermediate steps and check conservation.
Response guidance — discuss after your attempt
Richness=3. Fractions 0.6,0.3,0.1 give D=1−(0.36+0.09+0.01)=0.54. The sample may miss rare or hard-to-detect species; matched effort and replication matter.
Words to know
- biodiversity
- The total variety of life in a place, measured at three levels: genetic, species, and ecosystem diversity.
- genetic diversity
- The variety of genes and alleles within a population or species, which supplies the raw material for adapting to change.
- species diversity
- A measure combining species richness (how many species) with evenness (how equally abundant they are).
- ecosystem diversity
- The variety of different ecosystems, habitats, and ecological communities across a region.
- ecosystem services
- The benefits people get from functioning ecosystems, grouped as provisioning, regulating, supporting, and cultural services.
- keystone species
- A species whose effect on its ecosystem is far larger than its abundance would suggest, so its loss triggers disproportionate change.
Two ways to put a number on diversity
Simpson's index is not the only diversity metric. Simpson's Diversity Index emphasizes dominance — it answers 'if I grab two individuals at random, how likely are they to be different species?' The Shannon index instead weighs how much uncertainty there is in predicting the species of the next individual, giving rarer species relatively more influence. Both combine richness and evenness into one comparable number, and ecologists pick between them depending on whether they care more about common or rare species — a reminder that 'biodiversity' only becomes measurable once you define exactly what you mean.
Biodiversity as biological insurance
Why does diversity make ecosystems more stable? The leading answer is the insurance hypothesis (or portfolio effect). When many species can perform a similar role, the failure of any one is cushioned because others carry on — much like a diversified investment portfolio weathers a single stock's collapse. Experiments in grasslands show that more diverse plots resist drought better and rebound faster than species-poor plots. Redundancy that looks wasteful in good years becomes the safety margin that keeps the system functioning in bad ones.
Trophic cascades and the Yellowstone wolves
When wolves were returned to Yellowstone in 1995 after a 70-year absence, they thinned and moved the overabundant elk, which allowed willows and aspen along streams to recover, which in turn benefited beavers and songbirds — a chain reaction called a trophic cascade. It is one of the most famous demonstrations of a keystone species reshaping a whole landscape from the top down. (Popular accounts sometimes overstate it — claiming wolves literally 'changed the rivers' — so scientists urge care in separating the well-documented effects from the poetic ones.)
HIPPO: the drivers of biodiversity loss
Biologist E.O. Wilson summarized the main threats to biodiversity with the acronym HIPPO: Habitat destruction, Invasive species, Pollution, human Population growth, and Overharvesting — with habitat loss the single largest driver. Add accelerating climate change, and current extinction rates run an estimated hundreds to thousands of times above the natural background rate, leading many scientists to argue we have entered a human-caused 'sixth mass extinction.' The unsettling part is that these drivers compound one another — a fragmented habitat is far more vulnerable to an invasive species or a warming climate.
When a species runs out of genes: cheetahs and the banana
Genetic diversity can be quietly lost even while individuals remain plentiful. Cheetahs passed through a severe population bottleneck thousands of years ago and are now so genetically uniform that unrelated individuals can accept skin grafts from one another — leaving the species dangerously exposed to any new disease. The commercial Cavendish banana is a living cautionary tale: it is a single clone, genetically identical worldwide, and a spreading fungal disease now threatens it exactly as an earlier blight wiped out its predecessor. Abundance is not the same as safety.
In the real world
When economists try to price the world's ecosystem services — pollination, water purification, flood control, climate regulation — the estimates run into the tens of trillions of dollars a year, benefits that would be enormously expensive to replace with technology.
Exit check
Two species have fractions.5,.5. UseD=1−Σpi².
Check your explanation after answering
D=.5. State this version of the index and the sampling effort before comparing sites.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.