Ecology. Michael Begon
wet winters and summer drought) in Europe, California and north‐west Mexico, and in a few small areas in Australia, Chile and South Africa. Chaparral develops in regions with less rainfall than temperate grasslands and is dominated mainly by a drought‐resistant, hard‐leaved scrub of low‐growing woody plants. Annual plants are also common in chaparral regions during the winter and early spring, when rainfall is more abundant. Chaparral is subject to periodic fires; many plants produce seeds that will only germinate after fire while others can quickly resprout because of food reserves in their fire‐resistant roots.
desert
Deserts are found in areas that experience extreme water shortage: rainfall is usually less than about 25 cm year−1, is usually very unpredictable and is considerably less than potential evaporation. The desert biome spans a very wide range of temperatures, from hot deserts, such as the Sahara, to very cold deserts, such as the Gobi in Mongolia. In their most extreme form, the hot deserts are too arid to bear any vegetation; they are as bare as the cold deserts of Antarctica. Where there is sufficient rainfall to allow plants to grow in arid deserts, its timing is always unpredictable. Desert vegetation falls into two sharply contrasted patterns of behaviour. Many species have an opportunistic lifestyle, stimulated into germination by the unpredictable rains. They grow fast and complete their life history by starting to set new seed after a few weeks. These are the species that can occasionally make a desert bloom. A different pattern of behaviour is to be long‐lived with sluggish physiological processes. Cacti and other succulents, and small shrubby species with small, thick and often hairy leaves, can close their stomata (pores through which gas exchange takes place) and tolerate long periods of physiological inactivity. The relative poverty of animal life in arid deserts reflects the low productivity of the vegetation and the indigestibility of much of it.
tropical rainforest
Tropical rainforest is the most productive of the earth’s biomes – a result of the coincidence of high solar radiation received throughout the year and regular and reliable rainfall. The productivity is achieved, overwhelmingly, high in the dense forest canopy of evergreen foliage. It is dark at ground level except where fallen trees create gaps. Often, many tree seedlings and saplings remain in a suppressed state from year to year and only leap into action if a gap forms in the canopy above them. Apart from the trees, the vegetation is largely composed of plant forms that reach up into the canopy vicariously; they either climb and then scramble in the tree canopy (vines and lianas, including many species of fig) or grow as epiphytes, rooted on the damp upper branches. Most species of both animals and plants in tropical rainforest are active throughout the year, though the plants may flower and ripen fruit in sequence. Dramatically high species richness is the norm for tropical rainforest, and communities rarely if ever become dominated by one or a few species. The diversity of rainforest trees provides for a corresponding diversity of resources for herbivores, and so on up the food chain.
aquatic biomes?
All of these biomes are terrestrial. Aquatic ecologists could also come up with a set of biomes, although the tradition has largely been a terrestrial one. We might distinguish springs, rivers, ponds, lakes, estuaries, coastal zones, coral reefs and deep oceans, among other distinctive kinds of aquatic community. For present purposes, we recognise just two aquatic biomes, marine and freshwater. The oceans cover about 71% of the earth’s surface and reach depths of more than 10 000 m. They extend from regions where precipitation exceeds evaporation to regions where the opposite is true. There are massive movements within this body of water that prevent major differences in salt concentrations developing (the average concentration is about 3%). Two main factors influence the biological activity of the oceans. Photosynthetically active radiation is absorbed in its passage through water, so photosynthesis is confined to the surface region. Mineral nutrients, especially nitrogen and phosphorus, are commonly so dilute that they limit the biomass that can develop. Shallow waters (e.g. coastal regions and estuaries) tend to have high biological activity because they receive mineral input from the land and less incident radiation is lost than in passage through deep waters. Intense biological activity also occurs where nutrient‐rich waters from the ocean depths come to the surface; this accounts for the concentration of many of the world’s fisheries in Arctic and Antarctic waters.
Freshwater biomes occur mainly on the route from land drainage to the sea. The chemical composition of the water varies enormously, depending on its source, its rate of flow and the inputs of organic matter from vegetation that is rooted in or around the aquatic environment. In water catchments where the rate of evaporation is high, salts leached from the land may accumulate and the concentrations may far exceed those present in the oceans; brine lakes or even salt pans may be formed in which little life is possible. Even in aquatic situations liquid water may be unavailable, as is the case in the polar regions.
Differentiating between biomes allows only a very crude recognition of the sorts of differences and similarities that occur between communities of organisms. Within biomes there are both small‐ and large‐scale patterns of variation in the structure of communities and in the organisms that inhabit them. Moreover, as we see next, what characterises a biome is not necessarily the particular species that live there.
1.5.2 The ‘life form spectra’ of communities
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