Measuring Biodiversity, and Why the Measure Matters
- Distinguish species richness from species evenness and from genetic diversity
- Explain how biodiversity confers ecosystem resilience
- Apply island biogeography to habitat fragments and reserve design
Three kinds of diversity, three different questions
Species richness is simply how many species are present — a count. Species evenness is how equally the individuals are distributed among those species; a forest with 100 trees split 25/25/25/25 across four species is far more even than one split 97/1/1/1. Together they determine species diversity, and two communities with identical richness can differ enormously in evenness. Genetic diversity operates within a species and is what allows a population to adapt when conditions change — which is why a population reduced to a small number of individuals is vulnerable even after its numbers recover. On the exam, a scenario describing a monoculture crop is asking about genetic diversity, while one describing a reef survey is asking about richness and evenness.
Why diversity produces resilience
The mechanism is redundancy of function. If several species perform a similar ecological role, the loss or failure of one does not remove that function from the system — another expands to fill it. A diverse system therefore absorbs disturbance and recovers, while a simplified one can lose a function outright. The clearest cases are agricultural: the Irish potato famine followed from a crop with almost no genetic variation meeting a pathogen it had no resistance to, and modern banana cultivation faces the same structural risk for the same reason. The general statement worth carrying into an essay is that diversity is insurance, and its value shows up only when conditions change.
Island biogeography, and why fragments behave like islands
The theory has two variables and both are examinable. Size: larger islands support more species, because they hold larger populations that are less likely to go extinct by chance. Distance from the mainland: nearer islands receive more immigrants, so species lost are replaced faster. Species number settles at an equilibrium between immigration and extinction. The reason this sits in an AP course rather than a biogeography one is that habitat fragments behave like islands: a woodland surrounded by farmland is an island in a sea of crops. That gives reserve design its rules — bigger is better, closer together is better, connected by corridors is better than isolated, and a round reserve beats a long thin one because it has less edge. Edge habitat differs in light, temperature and wind, and it favors generalists and predators over interior specialists.
Two forest fragments each hold 40 hectares. One is a single square block; the other is four separate 10-hectare patches. Explain which supports more interior species, and why.
- 1.Both have the same total area, so total area alone does not distinguish them.
- 2.Apply the edge principle: perimeter grows relative to area as a shape is subdivided, so four small patches have far more total edge than one block of the same area.
- 3.Edge habitat has different light, temperature and wind, and favors generalists and predators over interior specialists.
- 4.Apply the island-size principle: each 10-hectare patch supports smaller populations, which are more vulnerable to chance extinction than the larger populations in one 40-hectare block.
Reserve-design questions come down to four comparisons: bigger beats smaller, closer beats farther apart, connected beats isolated, round beats elongated. Each has a one-sentence reason, and giving the reason rather than the preference is what earns the point.
Two communities each contain four species. Community A has 25 individuals of each; Community B has 97, 1, 1 and 1. Which statement is correct?
Wildlife corridors between habitat fragments are valuable primarily because they
Answer the 2 checkpoints as you read.
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