Biogeography and Species Distributions

Species are not spread evenly over the Earth; biogeography asks where organisms live, why their ranges fall where they do, and what historical and ecological processes generate the great geographic patterns of diversity.

Definition

Biogeography and species distributions is the study of the spatial patterns of organisms and biological diversity across the Earth and of the ecological, historical, and evolutionary processes that determine where species occur.

Scope

This topic covers the geographic distribution of species and diversity: the determinants of range limits, island biogeography and species-area relationships, latitudinal and elevational diversity gradients, the influence of history, dispersal, and climate on distributions, and the use of species distribution models to predict where organisms can occur. It bridges ecology, evolution, and Earth history at regional to global scales.

Core questions

• What determines the geographic range limits of a species?
• Why do larger and less isolated areas hold more species?
• Why does diversity tend to increase toward the tropics?
• How can the distribution of a species be predicted from environmental conditions?

Key theories

Equilibrium theory of island biogeography

The number of species on an island settles at a dynamic equilibrium between immigration, which declines with isolation, and extinction, which declines with area, producing predictable species-area and species-isolation relationships.

Niche-based distribution and the latitudinal gradient

Species distributions reflect the match between environmental conditions and a species' niche, and the broad increase of diversity toward the tropics is explained by hypotheses invoking energy, climatic stability, area, and evolutionary time.

Mechanisms

A species' range reflects the intersection of its physiological tolerances, the availability of suitable habitat and resources, biotic interactions, and the dispersal and historical processes that have allowed it to reach and persist in an area. Species distribution models formalise this by relating recorded occurrences to environmental predictors to estimate suitability across space. At broad scales, the species-area relationship and island equilibrium arise because larger and better-connected areas receive more colonists and lose fewer species, while latitudinal gradients emerge from the combined effects of greater energy, climatic history, and accumulated evolutionary time in the tropics.

Clinical relevance

Biogeographic analysis underlies the identification of biodiversity hotspots, the prediction of range shifts under climate change, the assessment of invasion risk, and conservation prioritisation. This is educational context, not management prescription.

History

Wallace founded zoogeography in the nineteenth century by mapping faunal regions, building on Humboldt's earlier work on plant geography. MacArthur and Wilson's 1967 equilibrium theory gave island biogeography a quantitative basis, and macroecology and computational species distribution modelling expanded the field through the 1990s and 2000s.

Debates

Explaining the latitudinal diversity gradient

The increase of species richness toward the equator is among ecology's most general patterns, yet its cause remains contested, with competing energy, area, climatic-stability, and evolutionary-time hypotheses that are difficult to disentangle.

Key figures

• Alfred Russel Wallace
• Robert MacArthur
• Edward O. Wilson
• Robert Whittaker

Seminal works

• lomolino2017
• macarthur1967
• gaston2000

Frequently asked questions

What is biogeography?

Biogeography is the study of the distribution of species and ecosystems across geographic space and through geological time, and of the processes that produce those patterns.

What is a species distribution model?

A species distribution model is a statistical or machine-learning tool that relates known occurrences of a species to environmental variables in order to estimate where else the species could occur.