Another, more recent, idea of how species divide up resources, devised by Mark Ritchie and Han Olff, resuscitates the MacArthurian idea that biodiversity is a question of how many species you can pack into ecological space and combines it with our old friends scaling and fractals.

Imagine a group of species all fighting over a pot of resources, such as grasses for light, water, and nutrients, or grazers competing for grasses, big cats for grazers, and parasites for hosts. What each species needs is not spread evenly across the world. It comes in clumps, such as a moist, well-lit piece of bare ground, or some juicy young shoots, or a carcass. To survive, organisms must find these patches amid the barren areas that cannot feed them—grass seeds must disperse over areas of woodland; wildebeest must follow the rains; lions must find the wilde-beest. Frogs must find ponds; blackbirds, worms; fleas, dogs.

Just as not all places are equally hospitable, neither are all patches. They will vary in size and quality. A grass seed could find itself on a prairie or in a crack between some paving stones; a flea could find itself on a glossy young pup or a mangy old mutt. Whether a patch of habitat can keep an animal or a plant going depends on its size and quality. It also depends on the properties of the organism seeking to exploit that patch—principally its body size, because this property controls how much food an animal needs and how long it can go without starving. Big animals need more food, but they can go for longer periods between meals, and so can use large but thinly spread patches. Smaller ones need less food but in higher concentrations. Such a multiplicity of factors makes working out how the environment might control the diversity of organisms seem horribly complicated. A glance at any map shows that each landscape is a unique mishmash of wet, dry, rocky, fertile, high, low, windy, and sheltered places. How can we hope to predict what sort of patches it contains and what sort of life these patches can support? We can look for regularity hidden amid the chaos.

Take a map and scan it into a computer. Get some software to look for all the patches of one sort of habitat, such as woodland, and work out how big each patch is. Then get the computer to divide the patches into size classes, such as 0.1 to 1 hectares, 1.1 to 10 hectares, and 10.1 to 100 hectares, and plot how many patches fall into each size class. The



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