Lesson 5 of 12 · 10 min
Species-area relationship
NCERT §13.1.2
The group surveys a 10 m square of the grove, then a 100 m square, then the whole grove. Each larger area adds species, but fewer each time.
The lesson in notes
In short
Exploring South American jungles, Alexander von Humboldt saw that within a region, species richness rose as the explored area grew, but only up to a limit.
For many taxa (angiosperm plants, birds, bats, freshwater fishes) species richness against area is a rectangular hyperbola.
On logarithmic scales the relationship becomes a straight line: log S = log C + Z log A, where S is species richness, A is area, Z is the slope (regression coefficient) and C is the Y-intercept.
Z lies between 0.1 and 0.2 whatever the taxonomic group or region: plants in Britain, birds in California and molluscs in New York state give remarkably similar slopes.
For very large areas such as whole continents the line is much steeper, with Z between 0.6 and 1.2.
For frugivorous (fruit-eating) birds and mammals in tropical forests of different continents, Z is 1.15.
A steeper slope means species richness climbs much faster with area, so losing area costs more species.