have emphasised how the distributions of species are strongly influenced by temperature and water availability, and how many organisms are impacted by occasional extremes rather than by average conditions. Computer modelled projections imply that global climatic change will also bring greater variance in temperature, rainfall, hurricanes and so on. Hence, not only the predicted average climate changes, but also the increased frequency and severity of extremes, are certain to be accompanied by marked responses in the distribution of species and biomes.
can the biota keep up with the pace?
Global temperatures have changed naturally in the past, as we have seen. We are currently approaching the end of one of the warming periods that started around 20 000 years ago, during which global temperatures have risen by about 8°C. The greenhouse effect adds to global warming at a time when temperatures are already higher than they have been for 400 000 years. Buried pollen provides evidence that North American forest boundaries have migrated north at rates of 100–500 m year−1 since the last ice age. However, this rate of advance has not been fast enough to keep pace with postglacial warming. The rate of warming forecast to result from the greenhouse effect is 50–100 times faster than postglacial warming. Thus, of all the types of environmental pollution caused by human activities, none may have such profound effects as global warming. We must expect latitudinal and altitudinal changes to species’ distributions and widespread extinctions as floras and faunas fail to track and keep up with the rate of change in global temperatures. What is more, large tracts of land over which vegetation might advance and retreat have been fragmented in the process of civilisation, putting major barriers in the way of vegetational advance. It will be very surprising if many species do not get lost on the journey.
The ecological implications of greenhouse gas emissions are profound indeed for the spread of pest species, for future conservation and restoration management, and for the production of wild fisheries, agriculture and aquaculture. These topics will crop up throughout the book, but especially in its final chapter.
Chapter 3 Resources
3.1 Introduction
According to Tilman (1982), all things consumed by an organism are resources for it. But consumed does not simply mean ‘eaten’. Bees and squirrels do not eat holes, but a hole that is occupied is no longer available to another bee or squirrel. Similarly, females that have already mated may be unavailable to other mates. All these things have been consumed in the sense that their stock or supply can be reduced by the activities of the organisms concerned.
autotrophs and heterotrophs
There is a fundamental distinction between autotrophic and heterotrophic organisms. Autotrophs assimilate simple inorganic resources into packages of organic molecules (proteins, carbohydrates, etc.). These become the resources for the heterotrophs (decomposers, parasites, predators and grazers), which take part in a chain of events in which each consumer of a resource becomes, in turn, a resource for another consumer. At each link in this food chain, the most obvious distinction is between saprotrophs and predators. Saprotrophs – bacteria, fungi and detritivorous animals (see Chapter 11) – use other organisms as food but only after they have died, or they consume another organism’s waste or secretory products. Predators, defined broadly, feed on other living organisms, or parts of other living organisms (see Section 3.7).
photoautotrophs and chemoautotrophs
Autotrophs may themselves be divided into photoautotrophs and chemoautotrophs. The photoautotrophs – green plants and algae, and photosynthetic protists and bacteria – utilise solar radiation, carbon dioxide (CO2), water and mineral nutrients as resources. Through photosynthesis, they use the radiation as a source of energy to reduce CO2 to obtain the organic compounds and energy that they need for growth and reproduction. Directly or indirectly, photosynthesis is the source of all energy in terrestrial and most aquatic ecosystems. Its evolution has led to the current 21% levels of oxygen in the atmosphere, driving down the levels of CO2. By contrast, chemoautotrophs – certain bacteria and archaea – use chemical energy from the oxidation of inorganic substances such as hydrogen sulphide, elemental sulphur, ferrous iron or ammonia to reduce CO2 and so obtain the organic compounds and energy that they need. They typically live in ‘extreme’ environments such as hot springs and deep‐sea vents.
For both autotrophs and heterotrophs, resources, once consumed, are no longer available to another consumer. This has the important consequence that organisms may compete with each other to capture a share of a limited resource – a topic to which we turn in Chapter 5.
In this chapter we start (Sections 3.2–3.6) with the resources that fuel the growth of individual plants, and so, collectively, determine the primary productivity of whole areas of land or volumes of water: the rate, per unit area or volume, at which plants produce biomass. Broad‐scale patterns of primary productivity are examined in Chapter 20. Relatively little space in this chapter (Section 3.7) is given to food as a resource for animals, simply because a series of later chapters (Chapters 9–13) is devoted to the ecology of predators, grazers, parasites and saprotrophs (the consumers and decomposers of dead organisms). This chapter then closes with sections on two important topics, drawing on material from the present chapter and the last – one (Section 3.8) on the ecological niche and resource classification, and a second (Section 3.9) on a so‐called metabolic theory of ecology.
3.2 Radiation
Solar radiation is the only source of energy that can be used in metabolic activities by green plants and algae. It comes to the plant as a flux of radiation from the sun, either directly, or having been diffused to a greater or lesser extent by the atmosphere, or after being reflected or transmitted by other objects. The direct fraction is highest at tropical latitudes north and south of the equator, since cloud cover is typically high at the equator itself (Figure 3.1). Moreover, for much of the year in temperate climates, and for the whole of the year in arid climates, the leaf canopy in terrestrial communities does not cover the land surface, so that most of the incident radiation falls on bare branches or bare ground.
Figure 3.1 Global map of the solar radiation absorbed annually in the earth–atmosphere system: from data obtained with a radiometer on the Nimbus 3 meteorological satellite.
Source: After Laing &
