The species area relationship, as proposed by Alexander von Humboldt,…

The correct answer is Option (3) → The species richness increases with increase in explored area, but only up to a limit

The species-area relationship was described by the German naturalist Alexander von Humboldt. He found that within a region, species richness rises as a larger area is explored, but only up to a limit. A graph of species richness against area for many groups of organisms is a rectangular hyperbola. On a logarithmic scale it becomes a straight line, log S = log C + Z log A, where Z is the slope of the line.

(1) The species diversity decreases with increase in explored area — Incorrect. A larger explored area gives more species, not fewer.

(2) The relationship is in form of sigmoid curve — Incorrect. A sigmoid (S-shaped) curve is the logistic growth curve of a population. The species-area graph is a rectangular hyperbola.

(3) The species richness increases with increase in explored area, but only up to a limit — Correct. This is Humboldt’s observation.

(4) Slope of the line is identical for all taxonomic groups — Incorrect. Z lies between 0.1 and 0.2 for different groups and regions, so the slopes are similar, not identical. For very large areas such as whole continents, the slope is much steeper.

NCERT Extract:
Patterns of Biodiversity – “… (ii) Species-Area relationships: During his pioneering and extensive explorations in the wilderness of South American jungles, the great German naturalist and geographer Alexander von Humboldt observed that within a region species richness increased with increasing explored area, but only up to a limit. In fact, the relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) turns out to be a rectangular hyperbola (Figure13.2).”