Q.For most groups of plants and animals, and within a single, relatively small biogeographical region, the slope (Z value) of the species-area relationship on a logarithic plot generally falls in the range

Greater solar energy in the tropics drives higher productivity, which supports greater biological diversity there — R correctly explains A.

Assertion (A) is true: tropical regions harbour far greater species diversity than temperate or polar regions (this latitudinal gradient in diversity is a well-established ecological pattern). Reason (R) is also true and is the correct explanation: tropical areas receive the most solar energy and have a relatively constant, favourable climate throughout the year, which drives higher primary productivity; this greater and more stable en …

The stable, favourable, and productive tropical climate supports far more species than temperate or polar regions, whose species must contend with seasonal extremes.

Species diversity increases markedly as one moves from the poles towards the equator, and tropical regions harbour the richest biodiversity on Earth. Several reasons account for this: tropical environments have a relatively constant and stable climate across the year — with consistently warm temperatures and abundant rainfall/sunlight — unlike temperate and polar regions, which experience harsh, seasonally fluctuating conditions (cold winters, limited growing seasons) that restrict the number of species able to survive there. Tropical regions also have a much longer evolutionary time during which species have diversified without being disrupted by glaciation, along with greater environmental predictability that favours niche specialisation, allowing more species to evolve …