Читать книгу Ecology - Michael Begon - Страница 30

Before we examine the differences and similarities between communities, we need to consider the larger groupings, ‘biomes’, in which biogeographers recognise marked differences in the flora and fauna of different parts of the world. The number of terrestrial biomes that are distinguished is a matter of taste. They certainly grade into one another, and sharp boundaries are a convenience for cartographers rather than a reality of nature. We describe eight terrestrial biomes and illustrate their global distribution in Figure 1.22, and show how they may be related to annual temperature and precipitation (Figure 1.23). Apart from anything else, understanding the terminology that describes and distinguishes these biomes is necessary when we come to consider key questions later in the book. Why are there more species in some communities than in others? Are some communities more stable in their composition than others, and if so why? Do more productive environments support more diverse communities? Or do more diverse communities make more productive use of the resources available to them?

Figure 1.22 World distribution of the major biomes of vegetation.

Source: From http://www.zo.utexas.edu/faculty/sjasper/images/50.24.gif.

Figure 1.23 Biomes in relation to rainfall and temperature. The variety of environmental conditions experienced in terrestrial biomes can be described in terms of their annual rainfall and mean annual temperatures.

Source: After Woodward & Lomas (2004).

tundra

Tundra occurs around the Arctic Circle, beyond the tree line. Small areas also occur on sub‐Antarctic islands in the southern hemisphere. ‘Alpine’ tundra is found under similar conditions but at high altitude. The environment is characterised by the presence of permafrost – water permanently frozen in the soil – while liquid water is present for only short periods of the year. The typical flora includes lichens, mosses, grasses, sedges and dwarf trees. Insects are extremely seasonal in their activity, and the native bird and mammal fauna is enriched by species that migrate from warmer latitudes in the summer. In the colder areas, grasses and sedges disappear, leaving nothing rooted in the permafrost. Ultimately, vegetation that consists only of lichens and mosses gives way, in its turn, to the polar desert. The number of species of higher plants (i.e. excluding mosses and lichens) decreases from the Low Arctic (around 600 species in North America) to the High Arctic (north of 83°, e.g. around 100 species in Greenland and Ellesmere Island). In contrast, the flora of Antarctica contains only two native species of vascular plant and some lichens and mosses that support a few small invertebrates. The biological productivity and diversity of Antarctica are concentrated at the coast and depend almost entirely on resources harvested from the sea.

taiga

Taiga or northern coniferous forest occupies a broad belt across North America and Eurasia. Liquid water is unavailable for much of the winter, and plants and many of the animals have a conspicuous winter dormancy in which metabolism is very slow. Generally, the tree flora is very limited. In areas with less severe winters, the forests may be dominated by pines (Pinus species, which are all evergreens) and deciduous trees such as larch (Larix), birch (Betula) or aspens (Populus), often as mixtures of species. Farther north, these species give way to single‐species forests of spruce (Picea) covering immense areas. The overriding environmental constraint in northern spruce forests is the presence of permafrost, creating drought except when the sun warms the surface. The root system of spruce can develop in the superficial soil layer, from which the trees derive all their water during the short growing season.

temperate forests

Temperate forests range from the mixed conifer and broad‐leaved forests of much of North America and northern Central Europe (where there may be 6 months of freezing temperatures), to the moist, dripping forests of broad‐leaved evergreen trees found at the biome’s low‐latitude limits in, for example, Florida and New Zealand. In most temperate forests, however, there are periods of the year when liquid water is in short supply, because potential evaporation exceeds the sum of precipitation and water available from the soil. Deciduous trees, which dominate in most temperate forests, lose their leaves in the autumn and become dormant. On the forest floor, diverse floras of perennial herbs often occur, particularly those that grow quickly in the spring before the new tree foliage has developed. Temperate forests also provide food resources for animals that are usually very seasonal in their occurrence. Many of the birds of temperate forests are migrants that return in spring but spend the remainder of the year in warmer biomes.

grassland

Grassland occupies the drier parts of temperate and tropical regions. Temperate grassland has many local names: the steppes of Asia, the prairies of North America, the pampas of South America and the veldt of South Africa. Tropical grassland or savanna is the name applied to tropical vegetation ranging from pure grassland to some trees with much grass. Almost all of these temperate and tropical grasslands experience seasonal drought, but the role of climate in determining their vegetation is almost completely overridden by the effects of grazing animals that limit the species present to those that can recover from frequent defoliation. In the savanna, fire is also a common hazard in the dry season and, like grazing animals, it tips the balance in the vegetation against trees and towards grassland. Nonetheless, there is typically a seasonal glut of food, alternating with shortage, and as a consequence the larger grazing animals suffer extreme famine (and mortality) in drier years. A seasonal abundance of seeds and insects supports large populations of migrating birds, but only a few species can find sufficiently reliable resources to be resident year‐round.

Many of these natural grasslands have been cultivated and replaced by arable annual ‘grasslands’ of wheat, oats, barley, rye and corn. Such annual grasses of temperate regions, together with rice in the tropics, provide the staple food of human populations worldwide. At the drier margins of the biome, many of the grasslands are ‘managed’ for meat or milk production, sometimes requiring a nomadic human lifestyle. The natural populations of grazing animals have been driven back in favour of cattle, sheep and goats. Of all the biomes, this is the one most coveted, used and transformed by humans.

chaparral

Chaparral or maquis occurs in Mediterranean‐type climates (mild, 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.