Abstract

Abstract The biodiversity level (number of species) of the Earth declines from the tropics to the poles and is strongly correlated with temperature and water availability. ‘Energy theories’ provide a simple explanation: more energy=more organisms=more species, but the question is more complex, and the empirical evidence equivocal. Theoretical models try to show how energy/climate can influence species birth (speciation), species death (extinction) and species migration; but there is only limited understanding of what controls these parameters, which may further be influenced by planetary area (smaller towards the poles) and the intricate processes of adaptive evolution, which build highly structured communities. The distribution of life is further deeply influenced by long‐ and short‐term climatic change. The three main explanations of the biodiversity gradient are therefore (1) energy theories, (2) area theories, (3) climate change theories and (4) community‐building theories. Key Concepts: The latitudinal diversity gradient: Biodiversity (the number of species) declines from the tropics to the poles, and is strongly correlated with climatic temperature and rainfall. Energy theories attempt to explain how climate influences the rates of speciation, species extinction and species migration. The water–energy (interim general) model and the metabolic theory of ecology show how energy (and water) could affect biodiversity. Dante's principle (species are adapted to their local climate) and the favourableness hypothesis propose that there are more species adapted to warmer climates than to cooler climates. The privileged tropics: Most clades originate in the tropics which are a passport‐free zone for species and have an effectively much greater area and volume than any other part of the planet. Niche conservatism: Species can adapt to novel conditions and expand their geographic ranges, but seem usually to be unable to do so. The ‘more individuals hypothesis’ successfully explains how larger populations will produce more species, but is challenged by empirical evidence. Community structure: Adaptive evolution produces intricate competitive and mutualistic relationships within ecological communities, whose species composition is not simply a random sample from the wider geographical community. Biodiversity gradients will differ according to the grain of sampling (the quadrat size). The biological volume of a geographic region is not measured by the simple area on the map, because from the organisms' point of view the area is fractally folded across the surface of the ground and the vegetation.

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