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One of the first ecological ideas that many readers of this book will have come across at high school is that of the simple food chain. Green plants fix solar energy and then are eaten by a herbivore that is later consumed by a carnivore, and some of the energy is passed up the food chain; so in one sense, lions can be described as solar-powered animals. This simple idea suggests that it might be useful to ask questions about the amount of light available for photosynthesis in the tropics compared to higher latitudes—after all, holiday brochures suggest that you should go to tropical beaches if you want lots of sunshine.

Initially, this does not appear to be a very promising idea as everywhere on Earth gets the same amount of light—half a year's worth. This is because while higher latitudes have long winter nights, they make up for it with long summer days. However, the Earth is roughly spherical which means that a fixed amount of incoming solar energy is spread over a greater amount of ground surface at higher latitudes.78 The classic way to demonstrate this is with a globe and a flashlight in a darkened room. Shine the light beam directly at the equator and note the amount of surface of the globe that is illuminated. If you raise the flashlight vertically (keeping it horizontal) so it is now shining on the Arctic the light will be spread over more of the globe as the curve of the sphere bends away from the incoming light beam. So, low latitudes acquire more solar energy per unit area of ground than higher latitudes—this is why the tropics are warmer than high latitudes. With more energy flowing into the base of food chains, it might seem reasonable that the tropics should support more species; however, it is not immediately apparent why more energy should give rise to more species rather than just more biomass—with a few superabundant species using up all this extra energy.79 Increased productivity in the tropics was one of the potential mechanisms for tropical diversity identified by Eric Pianka in his influential 1960s review paper5 and was also championed by G.E. Hutchinson80 around the same time in a highly influential paper on animal diversity published in American Naturalist.

This idea is often called the 'energy richness hypothesis' and has been formally described in the following way: 'Species richness varies as a function of the total number of individuals in an area. Net primary productivity (NPP) limits the number of individuals, and climate strongly affects NPP'.63 Excluding arid areas, where water shortage limits net primary productivity (NPP) (i.e. the amount of biological production by autotrophs—such as green plants—after the effects of respiration have been subtracted), then NPP will often be strongly correlated with temperature—for the reasons outlined earlier about latitudinal gradients in solar energy. This idea looks promising, in that there is a widely described relationship between measures of productivity and species richness for many groups of organisms.18,63 However, more detailed consideration suggests that the situation may be rather more complex than one might at first assume. One of the explicit assumptions of the energy richness hypothesis is that the density of individuals should be positively correlated with productivity—so that the density of individuals should be greatest in warm, wet places. However, analysis of several large-scale data sets (of forest trees from tropical America, North American breeding birds, and North American butterflies) show at best a weak relationship between productivity and density of individuals.63 In addition, the causal relationships in the energy richness hypothesis suggest that NPP affects the number of individuals (I), which in turn affects the number of species (S), so that NPP ^ I ^ S. It follows from this that the correlations between NPP and I or between I and S should be stronger than those between NPP and S, because other variables not relevant to the energy richness hypothesis will be affecting both links in the chain so reducing the overall correlation between NPP and S. However, in general, correlations between NPP and S are found to be stronger than those between NPP and I; this undermines the causal relationships suggested by the energy richness hypothesis.63 The above-mentioned tighter correlation between energy and species richness may come as something as a surprise: for example, our previous chapter on speciation dealt with the variety of ways in which new species form, but at no point did we suggest that higher temperatures, or energy input to an ecosystem, facilitated the speciation process. In this case, it is probably worth reminding oneself of the well-worn maxim repeated in a multitude of statistics textbooks that correlations do not demonstrate causality.

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