Biogeography: Why a Species Is Here and Not There
A species range is set by tolerance, by dispersal and by history, and the three explanations make different predictions. Island patterns, where richness tracks area and isolation, are the cleanest place to test them.
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
1 · Read
Every species survives and breeds only inside its tolerance range, from sweet spot to edge limits. Graph residents against oxygen or heat and you get a bell: most cluster at the peak, few linger at the edges, none live past them. Narrow curves, like the shiner's oxygen band, mean restricted ground; wide ones, like the topminnow's, mean broad ground.
An absence has three usual suspects: physiology cannot take it, a barrier blocks arrival, or a rival or predator holds the ground. Transplant past the edge to test them: if newcomers thrive, tolerance was never the wall.
Warming moves each plant's zone, and moss campion from New Mexico to Alaska is the test case. Researchers track vital rates, survival, growth, and reproduction, across sites and elevations over many years against the changing climate. Holding rates mean the plant adapts in place; failing rates mean it must shift ground.
Islands show the pattern cleanest: richness tracks area and isolation. Shrink a habitat and the species-area link forecasts the loss. Equilibrium theory says arrivals balance losses on each island, though real islands always leave parts of the story out.
Tolerance, barriers, and rivals each predict a different response, so test absences before blaming physiology.
2 · Watch
Take it off screen
Where it sits
Learn first
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.