The Equilibrium and the Gradient: A Narrative Review of Island Biogeography and the Latitudinal Diversity Gradient, from Wallace's Tropics to the Synthetic Era
Year: 2026
Type: review
Abstract: Two patterns organize the geography of biodiversity: the latitudinal diversity gradient—species multiplying toward the tropics, on land and in the sea—and the island equilibrium—species number set by immigration against extinction, area against isolation. This article presents a narrative review of the primary literature that built the two patterns' science, from Alfred Russel Wallace's 1878 Tropical Nature, which stated the gradient's primacy, through Fischer's 1960 latitudinal variations, Pianka's 1966 review of the gradient's proposed causes, MacArthur and Wilson's 1963 equilibrium paper and 1967 The Theory of Island Biogeography, which made islands models, Brown's 1981 Santa Rosalia assessment, Rosenzweig's 1995 Species Diversity in Space and Time, Heaney's 2000 dynamic disequilibrium, Willig, Kaufman, and Stevens's 2003 synthesis of the gradient's pattern, process, and scale, Hillebrand's 2004 demonstration of the gradient's generality, Mittelbach and colleagues' 2007 speciation-extinction analysis, and Whittaker and Fernández-Palacios's 2007 codifying Island Biogeography. The synthesis is organized around three themes: the gradient's statement, in which the tropics' richness became a pattern demanding explanation; the equilibrium's model, in which islands turned biogeography into mechanism; and the synthetic settlement, in which the gradient's causes were tested across scales and the island model itself made dynamic. It is concluded that the two literatures converged—islands measure what the gradient states—and that the modern synthesis reads the tropics' richness as history and ecology at once, speciation's tropics the gradient's modern center.
Authors: Revista, Zen, GEOGRAPHY, 10
Institution: Faculdades Guarulhos
Language: English
Subfield: Nature and Landscape Conservation
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204635227