Latitudinal and Spatial Diversity Gradients

The large-scale geographic patterns of biodiversity, above all the rise in species richness from the poles to the tropics, and the explanations proposed for them.

Definition

A diversity gradient is a systematic change in the number or composition of species along a geographic or environmental axis — most prominently the increase in richness from high latitudes toward the equator, but also gradients with elevation, area, and energy availability.

Scope

Covers macroecological patterns in the distribution of biodiversity: the latitudinal diversity gradient, elevational and depth gradients, the species-area relationship, and gradients with productivity and climate. Includes the leading hypotheses for these patterns and their conservation implications. Excludes index calculation (sibling topic) and the mechanisms of local community assembly (treated in ecology).

Core questions

• Why are the tropics so much richer in species than temperate and polar regions?
• How does species richness scale with area?
• What roles do climate, energy, and evolutionary time play in setting diversity?
• How do these patterns guide where conservation effort is concentrated?

Key concepts

• Latitudinal diversity gradient
• Species-area relationship
• Energy-diversity hypothesis
• Elevational and bathymetric gradients
• Endemism and range size
• Evolutionary time hypothesis

Key theories

Latitudinal diversity gradient hypotheses

The poleward decline in richness has been attributed to greater solar energy and productivity, climatic stability, larger tropical area, and longer evolutionary time for diversification; these explanations are complementary rather than mutually exclusive.

Species-area relationship

The number of species rises with sampled area in a predictable, typically power-law fashion, a regularity rooted in island biogeography that underlies both diversity estimation and predictions of extinction from habitat loss.

Clinical relevance

Because biodiversity is concentrated in the tropics and scales with area, these patterns explain why tropical habitat loss is so consequential and why reserve size matters. The species-area relationship is also the basis for estimating how many species are committed to extinction when habitat area shrinks.

History

Naturalists from Humboldt and Wallace onward noted the tropics' extraordinary richness. The species-area relationship was formalized in the early twentieth century and given theoretical grounding by MacArthur and Wilson's island biogeography in 1967. The causes of the latitudinal gradient have been debated since, with dozens of competing hypotheses catalogued by the late twentieth century.

Debates

What causes the latitudinal diversity gradient?

Energy and productivity, climatic stability, area, and evolutionary time have all been proposed; disentangling their relative contributions is difficult because they covary across latitude, and no single explanation has achieved consensus.

Key figures

• Alfred Russel Wallace
• Robert H. MacArthur
• Edward O. Wilson
• Klaus Rohde

Seminal works

• primack2014
• groom2006
• pimm2014

Frequently asked questions

Why are there more species near the equator?

There is no single agreed cause. Leading explanations include greater solar energy and plant productivity, more stable climates, the large area of tropical biomes, and the longer time tropical lineages have had to diversify. These factors likely act together.

What is the species-area relationship?

A consistent pattern in which larger areas hold more species, usually following a power law. It helps estimate total diversity and predict how many species may be lost as habitat area declines.