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We pointed out earlier the crucial importance of geographic isolation in allowing populations to diverge under selection. The geographic distributions of species, genera, families and even higher taxonomic categories of plants and animals often reflect this geographic divergence. All species of lemurs, for example, are found on the island of Madagascar and nowhere else. Similarly, 230 species in the genus Eucalyptus (gum tree) occur naturally in Australia (and two or three in Indonesia and Malaysia). The lemurs and the gum trees occur where they do because they evolved there – not because these are the only places where they could survive and prosper. Indeed, many Eucalyptus species grow with great success and spread rapidly when they have been introduced to, for example, California, Spain and Kenya. A map of the natural world distribution of lemurs tells us quite a lot about the evolutionary history of this group. But as far as its relationship with a biome is concerned, the most we can say is that lemurs happen to be one of the constituents of the tropical rainforest biome in Madagascar.

Similarly, particular biomes in Australia include certain marsupial mammals, while the same biomes in other parts of the world are home to their placental counterparts. A map of biomes, then, is not usually a map of the distribution of species. Instead, we recognise different biomes and different types of aquatic community from the types of organisms that live in them. How can we describe their similarities so that we can classify, compare and map them? In addressing this question, the Danish biogeographer Raunkiaer developed, in 1934, his idea of ‘life forms’, a deep insight into the ecological significance of plant forms (Figure 1.24). He then used the spectrum of life forms present in different types of vegetation as a means of describing their ecological character.

Figure 1.24 Raunkiaer’s life forms. The drawings above depict the variety of plant forms distinguished by Raunkiaer on the basis of where they bear their buds (shown as green circles). Below are life form spectrums for five different biomes. The green bars show the percentage of the total flora that is composed of species with each of the five different life forms. The grey bars are the proportions of the various life forms in the world flora for comparison.

Source: From Crawley (1986).

Raunkiaer’s plant classification

Plants grow by developing new shoots from the buds that lie at the apices (tips) of existing shoots and in the leaf axils. Within the buds, the meristematic cells are the most sensitive part of the whole shoot – the ‘Achilles’ heel’ of plants. Raunkiaer argued that the ways in which these buds are protected in different plants are powerful indicators of the hazards in their environments and may be used to define the different plant forms (Figure 1.24). Thus, trees expose their buds high in the air, fully exposed to the wind, cold and drought; Raunkiaer called them phanerophytes (Greek phanero, ‘visible’; phyte, ‘plant’). By contrast, many perennial herbs form cushions or tussocks in which buds are borne above ground but are protected from drought and cold in the dense mass of old leaves and shoots (chamaephytes: ‘on the ground plants’). Buds are even better protected when they are formed at or in the soil surface (hemicryptophytes: ‘half hidden plants’) or on buried dormant storage organs (bulbs, corms and rhizomes – cryptophytes: ‘hidden plants’; or geophytes: ‘earth plants’). These allow the plants to make rapid growth and to flower before they die back to a dormant state. A final major category consists of annual plants that depend wholly on dormant seeds to carry their populations through seasons of drought and cold (therophytes: ‘summer plants’). Therophytes are the plants of deserts (they make up nearly 50% of the flora of Death Valley, USA), sand dunes and repeatedly disturbed habitats. They also include the annual weeds of arable lands, gardens and urban wastelands.

But there is, of course, no vegetation that consists entirely of one growth form. All vegetation contains a mixture, a spectrum, of Raunkiaer’s life forms. The composition of the spectrum in any particular habitat is as good a shorthand description of its vegetation as ecologists have yet managed to devise. Raunkiaer compared these with a ‘global spectrum’ obtained by sampling from a compendium of all species known and described in his time (the Index Kewensis), biased by the fact that the tropics were, and still are, relatively unexplored. Thus, for example, we recognise a chaparral type of vegetation when we see it in Chile, Australia, California or Crete because the life form spectrums are similar. Their detailed taxonomies would only emphasise how different they are.