Elevational diversity gradient

Elevational diversity gradient (EDG) is an ecological pattern where trends in biodiversity occur at different elevations. The EDG states that species richness tends to increase as elevation increases, up to a certain point, creating a "diversity bulge" at middle elevations. There have been multiple hypotheses proposed for explaining the EDG, none of which accurately describe the phenomenon in full.

A similar pattern, known as the latitudinal diversity gradient, describes an increase in biodiversity from the poles to the equator. While the EDG generally follows the LDG (i.e., high elevations in tropical regions have greater biodiversity than high elevations in temperate regions), the LDG does not account for elevational changes.

The first recorded observation of the elevational diversity gradient was by Carl Linnaeus in his treatise On the growth of the habitable earth. In this document, Linnaeus based his predictions on flood geology, assuming most of the world was at one point inundated, leaving only the highest elevations available for terrestrial life. Since, by Linnaeus’ hypothesis, all life was concentrated at high elevations, a higher species diversity would be observed there even as life re-populated lower elevations.

In 1799, Alexander von Humboldt and Aimé Bonpland described elevational changes along the Andean slopes, noting how climatic changes impacted plant and animal communities. These observations contributed to Leslie R. Holdridge’s "life zone concept" (1947). Climatic variables shaping life zones include mean potential temperature, total annual precipitation, and the ratio of mean annual evapotranspiration to mean annual precipitation. These variables, most notably precipitation and temperature, vary along an elevational gradient, resulting in the distribution of different ecosystems.

Much of the current literature correlates elevational diversity to gradients in single climactic or biotic variables including "rainfall, temperature, productivity, competition, resource abundance, habitat complexity, or habitat diversity".

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