Energy and Biodiversity

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.

Keywords: favourableness; productivity; species richness; speciation; extinction; neutral theory of biodiversity; tropics; grain; climate change; latitudinal‐biodiversity‐gradient

Figure 1.

Biodiversity in time and space (from Turner and Hawkins, ). (a) The LDG in birds in the Americas, Africa, Northern Eurasia and Australia. The pecked lines are the tropics (BA Hawkins). (b) The change of butterfly diversity over time at two places with similar latitudes in England (Huddersfield and Doncaster) (JRG Turner). The trough in diversity corresponds with a period, from approximately 1870 to 1910 when the summers became detectably cooler, and the subsequent increase corresponds with the recent warming trend. Reproduced with permission from Sinauer Associates Inc. © Sinauer Associates Inc.

Figure 2.

The correlation of bird species richness (from Figure a) with productivity (annual AET). Reproduced from Turner and Hawkins with permission of Sinuaer Associates Inc. © Sinauer Associates Inc.

Figure 5.

(a) The greater area of the tropics, compared with belts of similar ‘depth at higher latitudes’, is guaranteed by the two tropical belts (tan) being back to back, whereas equivalent high‐latidude belts (blue) are split between the two hemispheres. The belts also decrease in circumference with increasing latitude. (b) However, the decrease in area will affect biodiversity only if species spread (arrows) more readily east‐west than they do north‐south. (c) If the spread is isotropic (the same in all compass directions) every point on the earth's surface is surrounded by a ‘migration area’ which is the same over the whole planet, in which case (d) it would be possible to argue that the band on either side of the Greenwich meridian, or any other great circle, had a greater area than the bands nearer to Ecuador and Indonesia, which is a reductio ad absurdum. (e) However, if the rate/distance of isotropic migration were less at higher latitudes than at the equator, this would tend to generate the LDG. The real situation is probably more like (f): the tropics are a passport‐free zone with isotropic dispersal; outside the tropics north‐south dispersal is narrower and dispersal is broader east‐west than north‐south, so that the ‘area’ does become more restricted at very high latitudes.

Figure 6.

The tropics are a passport‐free zone for species. The planetary gradient in temperature is roughly linear on either side of the tropical belt, but almost level across the tropics. This means that a species with a temperature tolerance of R degrees can occupy a relatively narrow latitudinal range L if it is adapted to a climate outside the tropics; however, a species with the same range of tolerance, but adapted to the equatorial climate can spread, without evolving any further temperature adaptations, right across the intertropical belt and for L degrees in both the subtropics. Reproduced from Turner , based on work by Terborgh and Rosenzweig . With permission from Sinuaer Associates Inc. © Sinauer Associates Inc.

Figure 7.

Theories of the fitting of species into the ecological volume of the tropics. The boxes represent the hyperdimensional ‘ecological space’ whose axes are all the environmental factors encountered by organisms. The spheres (circles) are the ecological requirements of individual species. Habitats in (a) the temperate zones differ from the tropics, which might (b) have species with narrow ecological requirements, so that more pack into the space, and/or (c) species which share some of the ecological space by mutualism, and/or (d) a larger ecological space, because the range of variation in the environmental variables is greater. Reproduced from Turner and Hawkins , with permission from Sinuaer Associates Inc. © Sinauer Associates Inc

Figure 3.

A simple demonstration of the effect of grain size on estimates of diversity. There are two extreme types of (circular) island: on type A islands, all the species are allopatric, with nonoverlapping geographical ranges (represented by different colours); on type B islands all the species are sympatric, occurring together over the whole island. On islands of type A, a small quadrat (white square) will typically show a constant species richness of one, no matter how many species are on the island; but if whole islands are sampled there will be considerable differences in species richness (between 4 and N in this case). On type B islands this same difference, between 4 and N, is shown also in small quadrats. In general therefore, as it is not possible for all species to be sympatric, there will be a mixture of type A and type B situations, and diversity gradients will become steeper as the sampling grain (quadrat size) increases.

Figure 4.

The graphical representation of the IGM (water–energy model). Species richness is a combined function of the energy level (parabolic) and the water supply (linear, third dimension). Adapted from O'Brien et al., with permission from Ecography. © Nordic Society Oikos.

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Wang Zh, Tang Zh and Fang J (2009) The species–energy hypothesis as a mechanism for species richness patterns [in Mandarin]. Biodiversity Science 17: 613–624.

Whittaker RJ and Triantis KA (2012) The species‐area relationship: an exploration of that ‘most general, yet protean pattern’. Journal of Biogeography 38: 623–626.

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Turner, John RG(Sep 2013) Energy and Biodiversity. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022551]