Island Biogeography, explained.
Equilibrium island biogeography describes species richness as a balance of immigration and local extinction from a larger source pool.
Why it happens
A pool of one hundred species supplies immigration I(1−S/100), while extinction is ES/100. The richness equilibrium is 100I/(I+E) when either rate is positive, with nonzero turnover possible there.
Equilibrium island biogeography describes species richness as a balance of immigration and local extinction from a larger source pool.
Read the result
Richness is an expected count index. The equilibrium turnover rate can remain positive even when the richness trajectory has stopped changing visibly.
A worked example
Stable richness with ongoing replacement
Set maximum immigration six and maximum extinction three with a source pool of one hundred.
Run successive time blocks toward equilibrium richness about 66.67.
Inspect turnover two species per model time unit at equilibrium without inferring which species arrive or leave.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
A pool of one hundred species supplies immigration I(1−S/100), while extinction is ES/100. The richness equilibrium is 100I/(I+E) when either rate is positive, with nonzero turnover possible there.
Where this idea is useful
A practical use
Avoid interpreting a stable species count as evidence that the same species remain present.
A common misconception
“An unchanged species count proves that species identities are unchanged.”
Immigration and extinction can balance in number while replacing identities. This model reports richness and expected turnover separately.
What this explanation leaves out
- The model tracks expected richness, not species identities, speciation, habitat differences or a calibrated island forecast. Both zero rates preserve the initial richness.
Why does the zero-rate case not display a unique equilibrium?
If neither immigration nor extinction occurs, every initial richness remains unchanged; division by zero would invent a unique answer.
Would a stable total hide substantial replacement within the system?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.