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2026-09-23

AP Environmental Science Unit 2 — The Three Scales of Diversity: Richness, Evenness, and What a Drought Measured

AP Environmental Science Unit 2
AP Environmental Science Unit 2@Jennifer
AP Environmental Science Unit 2 — The Three Scales of Diversity: Richness, Evenness, and What a Drought Measured

In nineteen sixty-six, two biologists in Florida Bay set out to kill every insect and spider on six small islands — on purpose.

0:00 / 10:46
Unit 2 uses one word, biodiversity, for three different things, and most of the marks lost on this topic come from answering the question about one of them with the answer to another. This episode takes the three scales apart — genetic diversity inside a species, species diversity across a community, habitat diversity across a region — and then asks what the count is worth when a disturbance arrives.
The evidence runs through two experiments. In 1969 two biologists published an experiment in which they had fumigated six mangrove islets in Florida Bay, killed every arthropod on them, and then watched the species count come back within a year. In 2015 a team pooled forty-six grassland experiments and put numbers on what plant diversity does to production during a drought. From there the episode goes to the rule that decides how many species a place can hold in the first place — bigger areas hold more species, and not in proportion — and to what happens to that count when a forest is cut into fragments. It closes with five new practice questions worked through out loud, answers and reasoning included.

The short version

Diversity is counted at three scales, and they nest: a habitat contains species, and a species contains genes.
  • Genetic diversity is the variation inside one species. It is the raw material a population adapts with, and a population that has lost it adapts slowly, if at all.
  • Species diversity is measured with two numbers: species richness (how many species) and species evenness (how evenly the individuals are spread across those species). Same species list, different evenness, different diversity.
  • Habitat diversity is the variety of places a region holds. This unit's guide uses that name; some textbooks call it ecosystem diversity.
Species diversity pays off through functional redundancy — several species doing the same job, so losing one does not stop the work. The result is that a diverse community resists a disturbance better: it changes less while the disturbance is running. Do not confuse that with resilience, how fast it recovers afterwards.
The study guide frames this section as high species diversity → high resilience. The measured result in the largest test anyone has run shows up as resistance: low-diversity grassland plots changed production by about 50% during climate events, high-diversity plots by about 25%, and both had recovered a year later. Use whichever word the question asks about — resistance for how much it changed, resilience for how fast it came back.
How many species a place can hold at all is set by area and isolation. Multiply the area by ten and the species count rises by a fixed factor, not by ten times — in one surveyed island system, ten times the area multiplied the bird count by about 1.3, and the slope is steeper for habitat specialists than for generalists.

The cases behind the numbers

CaseWhat was measured
Six mangrove islets, Florida Bay, published 1969Daniel Simberloff and Edward O. Wilson fumigated the entire arthropod fauna of six small mangrove islands — each a few red mangroves in shallow water — and censused them for a year. By 250 days after defaunation, every island except the most distant one had regained species numbers and a composition similar to untreated islands, while population densities were still low. The authors concluded that a dynamic equilibrium number of species exists for any island 1
Forty-six grassland experiments, 2015Experiments that manipulated grassland plant diversity from one species to thirty-two, tested through wet and dry, moderate and extreme, brief and prolonged climate events. Plots with 1–2 species changed productivity by about 50% during the event; plots with 16–32 species changed by only about 25%. A year later production had recovered or overshot in both, so the study found no detectable dependence of recovery speed on diversity 2
Thousand Island Lake, ChinaA dam on the Xin'an River flooded the valley in 1959. The lake holds 1,078 islands larger than 0.25 hectares; bird surveys on 41 of them between 2006 and 2009 fitted a species-area slope in which ten times the area multiplied the bird count by about 1.3 3
Amazon forest fragments, 2003In the Biological Dynamics of Forest Fragments Project north of Manaus, understory birds were followed for 13 years in 11 plots of 1, 10 and 100 hectares. A 100-hectare fragment loses half its species in under 15 years, and slowing that loss tenfold would take a thousand times more area 4
Specialists versus generalists, 2014Across 23 bird habitat-island datasets, the species-area slope was steeper for habitat specialists than for generalists in the majority of datasets, and specialist slopes were often steeper than the value commonly used to predict extinctions from habitat loss 5

Vocabulary you should be able to use in a sentence

  • Genetic diversity — the total variation in the genetic makeup of a species.
  • Species diversity — the number of species in a community together with how evenly the individuals are spread across them.
  • Species richness — the number of different species present, on its own.
  • Species evenness — how close the species are to equal in abundance.
  • Habitat diversity — the variety of terrestrial or aquatic habitats in a region; some textbooks call it ecosystem diversity.
  • Specialist species — a species that needs a particular habitat or food, or a large territory, and copes badly with change.
  • Generalist species — a species that tolerates a wide range of conditions and foods.
  • Functional redundancy — several species performing the same ecological role, so the role survives the loss of one of them.
  • Resistance — how little a system changes while a disturbance is running.
  • Ecosystem resilience — how quickly a system recovers after a disturbance.
  • Species-area relationship — the pattern that a larger area holds more species, rising as a power law rather than in proportion.
  • Habitat fragmentation — the breaking of continuous habitat into isolated patches, which behave like islands in an inhospitable sea.

The five practice questions

1. Three measurements, three scales. A student records: the number of plant species in a square metre of marsh; the number of different versions of one gene in the marsh snail population; the number of habitat types in the marsh (open water, mudflat, cordgrass, upland edge).
Species richness, genetic diversity, habitat diversity — in that order. The common error is filing every count as "biodiversity" without naming the scale.
2. Two ponds, eight species each. Pond A: one species accounts for 900 of 1,000 fish counted. Pond B: the eight species run between 100 and 160 each. Which pond has the higher species diversity, and what does that mean if disease hits the dominant species?
Pond B. Richness is identical, so the difference is evenness — and evenness counts toward species diversity. In pond A the community's functions rest on one species; its collapse takes most of the fish and the jobs they did. In pond B the same loss leaves seven species working.
3. Species-area arithmetic. Three forest blocks: 10 hectares held 40 species, 100 hectares held 68, and each tenfold step in area multiplied the count by about 1.7. Predict the count for 1,000 hectares, and say what is wrong with claiming a 1,000-hectare block should hold ten times the species of a 100-hectare block.
68 × 1.7 ≈ 115 species. The ten-times claim assumes the relationship is proportional; it is a power law. Ten times the area, about 1.7 times the species.
4. Same size, different distance. Two 50-hectare fragments: X is 1 km from continuous forest, Y is 20 km away. Which holds fewer species, and why?
Fragment Y, because immigration is lower: fewer colonists reach a fragment 20 km out, while extinction inside small populations continues. Area is held constant, so the difference is isolation.
5. A drought graph. Plot 1 (two species): production falls from 800 g/m² to 400. Plot 2 (twenty-four species): production falls from 800 to 600. Which is more resistant, by how much, and what is wrong with saying the diverse plot recovered faster?
Plot 2 — a 25% drop against a 50% drop. Less change during the event is resistance. Recovery speed is a different measurement that these numbers do not describe, and in the 46-experiment study recovery a year later was similar in rich and poor plots. Check the axis too: production per square metre is not species count.

Four ways this part of the unit gets missed

Richness is not diversity. A species list gives you richness only. If a question hands you counts per species, it is testing evenness — and the community where one species dominates is the less diverse one.
Area and isolation are separate variables. Two fragments can differ in size, in distance, or in both. Only the variable the question moved explains the change in species count.
Resistance is not resilience. Resistance is how much the system changed during the disturbance; resilience is how fast it recovered afterwards. This unit's guide poses the resilience question, and the measured evidence behind it is a resistance effect — answer the term the question uses.
Genetic diversity is not species diversity. A single population can carry a great deal of genetic variation and still be the only species in its habitat. The scales nest, but they are not substitutes for one another.

References

  1. 1
    Simberloff and Wilson, Ecology, 1969

    jstor.org

  2. 2
    Isbell et al., Nature, 2015

    doi.org

  3. 3
    Ding et al., Zoological Research, 2015

    pmc.ncbi.nlm.nih.gov

  4. 4
    Ferraz et al., PNAS, 2003

    europepmc.org

  5. 5
    Matthews, Cottee-Jones and Whittaker, Diversity and Distributions, 2014

    doi.org

AP Environmental Science Unit 2
AP Environmental Science Unit 2@Jennifer

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