Biogeography: Why a Species Is Here and Not There
A species' range reflects environmental tolerance, dispersal, interactions and history. Species-area models estimate richness under stated assumptions. Island-equilibrium theory relates immigration and extinction to richness, without explaining every feature of real islands.
What a learner can do afterwards
- Separate a range limit set by physiology from one set by dispersal or by a competitor
- State the species-area relationship and use it to estimate loss from habitat reduction
- Explain what the equilibrium theory of island biogeography predicts and what it leaves out
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Biomes describe broad conditions; species ranges need more detail. A tolerance range describes conditions an organism can withstand. Conditions supporting survival may differ from those supporting reproduction, and life stages can differ. Physiological tolerance, dispersal, interactions and history all help explain where a species persists.
Field abundance does not directly measure tolerance: habitat availability, competitors and sampling affect it too. Conditions inside a narrow tolerance may still occur over a wide area. Responses need not be bell-shaped, and field peaks need not mark physiological optima.
An absence may reflect unsuitable conditions, failure to arrive, or interactions such as competition and predation. Past events can also matter. Compare predictions before choosing a cause. Island models add scale: greater isolation tends to reduce immigration, while smaller area often increases the risk of local extinction.
A controlled introduction can test whether an absent species can establish. If introduced organisms survive and reproduce without changing local competitors or habitat, blocked arrival becomes more plausible. This does not prove arrival was the only cause. Use permits and biosecurity safeguards; do not move organisms between habitats yourself.
Vital rates measure survival, growth and reproduction. Long-term measurements across sites can show where populations perform well or poorly. Stable rates do not by themselves prove genetic adaptation; inherited change needs separate evidence. Differences between years or sites can also reflect weather, habitat and other interacting factors.
A species-area model is S = cA^z: S is richness, A is area, and c and z are fitted constants. With unchanged constants, S_new/S_old = (A_new/A_old)^z. For illustrative z = 0.5, use the square root, the nonnegative number that squares to the area ratio. If area becomes 1/9 of its original value, predicted richness becomes 1/3, since 1/3 times 1/3 = 1/9. This is not a guaranteed or immediate extinction count; habitat quality, connectivity and time matter.
Test range-limit explanations separately. Species-area estimates depend on assumptions. Island equilibrium concerns richness, which can stay stable while species identities change.
Original lesson and exercises written for this learning dataset, including the species-area calculation.
Resource summaries draw on Khan Academy's Tolerance ranges of species and the University of Colorado Boulder Data Puzzle. The lesson discusses qualifications to the tolerance article's curve and range claims.
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.