At the end of this chapter you should:
- Appreciate some of the key ways we classify the natural world by identifying units, such as biomes.
- Understand how the natural world works through the recognition of cycles, such as the hydrological cycle.
- Appreciate the importance of scale (both time and space).
- Recognize that the state of our knowledge of the natural world is imperfect.
- Understand that our sources of data include direct and indirect methods of measurement.
- Appreciate that a scientific approach to environmental change is dominant but it is by no means the only kind of knowledge about how our planet works.
productivity, biome, food chain, biogeochemical cycle, regime shift, feedback, threshold, timelag, resistance, resilience, equilibrium, linear/non-linear system, proxy methods, palaeoenvironmental indicator
The term environment is used in many ways. This book is about issues that arise from the physical environment, which is made up of the living (biotic) and non-living (abiotic) things and conditions that characterize the world around us. While this is the central theme, the main reason for the topicality of the issues covered here is the way in which people interact with the physical environment. Hence, it is pertinent also to refer to the social, economic and political environments to describe those human conditions
characteristic of certain places at particular times, and to explain why conflict has arisen between human activity and the natural world. This chapter looks at some of the basic features of the physical environment, while Chapter 2
is concerned with the human factors that affect the ways in which the human race interacts with the physical world.
Classifying the Natural World
Geography, like other academic disciplines, classifies things in its attempt to understand how they work. The physical environment can be classified in numerous ways, but one of the most commonly used classifications is that which breaks it down into four interrelated spheres: the lithosphere, the atmosphere, the biosphere and the hydrosphere. These four basic elements of the natural world can be further subdivided. The lithosphere, for example, is made up of rocks that are typically classified according to their modes of formation (igneous, metamorphic and sedimentary); these rock types are further subdivided according to the processes that formed them and other factors such as their chemical composition. Similarly, the workings of the atmosphere are manifested at the Earth’s surface by a typical distribution of climates; the biosphere is made up of many types of flora and fauna; and the hydrosphere can be subdivided according to its chemical constituents (fresh water and saline, for example), or the condition or phase of the water: solid ice, liquid water or gaseous vapour.
These aspects of the natural world overlap and interact in many different ways. The nature of the soil in a particular place, for example, reflects the underlying rock type, the climatic conditions of the area, the plant and animal matter typical of the region, and the quantity and quality of water available. Suites of characteristics are combined in particular areas called ecosystems. These ecosystems can also be classified in many ways. One approach uses the amount of organic matter or biomass produced per year – the net production – which is simply the solar energy fixed in the biomass minus the energy used in producing it by respiration (see below). The annual net primary production of carbon, a basic component of all living organisms, by major world ecosystem types is shown in Table 1.1
. Clear differences are immediately discernible between highly productive ecosystems such as forests, marshes, estuaries and reefs, and less productive places such as deserts, tundras and the open ocean. All of the data are averaged and variability around the mean is perhaps greatest for agricultural ecosystems which, where intensively managed, can reach productivities as high as any natural ecosystem. One of the main reasons for agriculture’s low average is the fact that fields are typically bare of vegetation for significant periods between harvest and sowing.
One of the main factors determining productivity is the availability of nutrients, key substances for life on Earth: a lack of nutrients is often put forward to explain the low productivity in the open oceans, for example. Climate is another important factor. Warm, wet climates promote higher productivity than cold, dry ones. Differences in productivity may also go some way towards explaining the general trend of increasing diversity of plant and animal species from the poles to the equatorial regions. Despite many regional exceptions such as mountaintops and deserts, this latitudinal gradient of diversity is a striking characteristic of nature that fossil evidence suggests has been present in all geological epochs. The relationship with productivity is not straightforward, however, and many other hypotheses have been advanced, such as the suggestion that minor disturbances promote diversity by preventing a few species from dominating and excluding others (Connell, 1978).
Annual net primary production of carbon by major world ecosystem types
|Ecosystem type ||Mean net primary productivity (g C/m2/year) ||Total net primary production (billion tonnes/C/year) |
|Tropical rain forest ||900 ||15.3 |
|Tropical seasonal forest ||675 ||5.1 |
|Temperate evergreen forest ||585 ||2.9 |
|Temperate deciduous forest ||540 ||3.8 |
|Boreal forest ||360 ||4.3 |
|Woodland and shrubland ||270 ||2.2 |
|Savanna ||315 ||4.7 |
|Temperate grassland ||225 ||2.0 |
|Tundra and alpine ||65 ||0.5 |
|Desert scrub ||32 ||0.6 |
|Rock, ice and sand ||1.5 ||0.04 |
|Agricultural land ||290 ||4.1 |
|Swamp and marsh ||1125 ||2.2 |
|Lake and stream ||225 ||0.6 |
|Total iand ||324* ||48.3 |
|Open ocean ||57 ||18.9 |
|Upweiling zones ||225 ||0.1 |
|Continental shelf ||162 ||4.3 |
|Algal bed and reef ||900 ||0.5 |
|Estuaries ||810 ||1.1 |
|Total oceans ||69* ||24.9 |
|Total for biosphere ||144* ||73.2 |
The relationships between climate and the biosphere are also reflected on the global scale in maps of vegetation and climate, the one reflecting the other. Figure 1.1
shows the world’s morphoclimatic regions, which are a combination of both factors. Despite wide internal variations, immense continental area clearly support distinctive forms of vegetation that are adapted to a broad climatic type. Such great living systems, which also support distinctive animals and to a lesser extent distinctive soils, are called biomes, a concept seldom applied to aquatic zones. Different ecologists produce various lists of biomes and the following eight-fold classification may be considered conservative (Colinvaux, 1993):
Present-day morphoclimatic regions of the world’s land surface (Williams et al.,
- coniferous forest (also known as boreal forest or taiga)
- temperate forest
- tropical rain forest
- tropical savanna
- temperate grassland
- maquis (also known as chaparral).
A striking aspect of the tundra biome is the absence of trees. Vegetation consists largely of grasses and other herbs, mosses, lichens and some small woody plants which are adapted to a short summer growing season. The tundra is also notable for receiving relatively little precipitation and being generally poor in nutrients. The cold climate ensures that the rate of biological processes is generally slow and the shallow soils are deeply frozen (permafrost) for all or much of th...