Introduction

Charles Darwin provided the essential elements of the explanation for how species originated and thus how life has evolved on earth. This work has changed forever the way educated people see themselves in relation to the rest of the natural world.

Although correct in essentials, Darwin's ideas had some major technical problems in their own time. For one thing, in the absence of the then-undiscovered nuclear forces, it can be shown that neither the sun nor (by a separate argument) the earth can be more than a few tens of millions of years old. For another thing, heritable variation is roughly halved in each generation if inheritance blends the characteristics of mother and father (as was thought to be the case in Darwin's day), making it hard to understand how such variability – the raw stuff on which selection can act – is maintained. A widespread recognition that genetic inheritance operates in a discrete, particulate way, tending to conserve variability, had to await the rediscovery of Mendel's work some 50 years later. Many other questions, including the mode and tempo of evolutionary change, the role of ‘neutral selection’ as gene frequencies drift under random statistical fluctuations, the selective advantage of sex, and other topics, remain active areas of research today. But all this work takes place within the sturdy framework erected by Darwin.

Given a basic understanding of how species originate, the next question would seem to be how we use this understanding to estimate – from first principles – how many species are likely to be found in a given region.

Climatic changes in the past

The climate on earth has changed throughout the evolution of our planetary system over about 4500 million years. Sometimes these changes have been rapid. During other periods they have been slow, hardly noticeable over millions of years, as far as we can judge from the evidence that is found to-day in rocks and sediments. To be able to discuss possible future climates we must understand the mechanisms behind past changes. The climatic system is too complex for us to rely merely on simple extrapolation of observed trends and variations or theoretical models that have not been tested against past data.

We know the gross features of climatic change on earth during the last major geological epochs, i.e. during the last few hundred million years. These have been determined by analyses of the abundance of the oxygen-18 isotope in remains of planktonic micro-organisms (foraminifera). The ratio of oxygen-18 to the normal isotope, oxygen-16, reflects water temperature and global ice volume. High proportions of oxygen-18 correspond to glacial periods when more of the lighter isotope oxygen-16 was locked up in ice. These analyses reveal that water temperatures in the deep layers of the oceans have been close to zero degrees only during the last few million years, i.e. the Quaternary period, and also that ocean surface temperatures were probably higher during these earlier periods than they are today (Fig. 7.1). Presumably the climate on land was also significantly different from to-day.

A much more detailed record of the proportion of oxygen present as oxygen-18 for the Quaternary period shows marked variations on much shorter timescales.

Introduction

Much of our knowledge of the history of human relationships with the environment has resulted from close, long-standing links between the disciplines of geography and archaeology. Traditionally, such studies have centred on a desire to explain in terms of environmental influences either the distribution of human activities in space or the changing fortunes of human groups through time. For example, Huntington and his co-workers interpreted migration and the decline of cities in Central Asia in terms of environmental changes and saw dry phases as the driving force of the Pulse of Asia (1907). The archaeologist Gordon Childe saw the adoption of domestication and cultivation as a revolutionary response to desiccation in post-glacial times. More recently, many of the great collaborative ventures in environmental archaeology have sought to place archaeological sites in their environmental context. A new discipline – Geoarchaeology – has emerged.

In this chapter, however, I wish to approach the subject from a different perspective, seeking to outline some of the ways in which prehistoric groups have caused environmental changes. I shall concentrate on the use of fire, hunting, agriculture and settlement, with particular reference to the post-glacial peoples of Britain, and also on the exploitation of minerals. My hypothesis is that the changes brought about by these activities were rather substantial, though I recognise that the impacts will have varied from area to area, according to length of human occupation, technological level and population numbers.

Human life probably first made its appearance on our planet some three or so million years ago, before the start of the Ice Age (see the timescale in the Appendix to this chapter).

Stories told by distant travellers have long provoked the imaginations of placebound listeners. One such account telling about the adventures of a young former Cambridge student during his five-year circumnavigation of the earth was published in 1839. In The Voyage of the Beagle, Charles Darwin gives us his first narrative exploration of experiences and reflections that eventually created a lasting revolution in the sciences.

Today we live in an age when travellers can circle the earth in approximately 90 minutes. Their stories and photographs, when coupled with the data returned by unmanned satellite sensor systems, can be the guide to new discoveries about our environment. In this chapter I propose to take the reader on an orbital photographic journey around the earth and to demonstrate that many of the issues discussed by the other contributors to this series can be examined through evidence seen by unaided vision from the altitude of the Space Shuttle orbiters, averaging about 280 km. Since in no way can one attempt to do justice to all NASA programmes of terrestrial observation and the many investigations currently pursued by other agencies and university research groups, I shall choose to focus on several topics of personal interest. These include the fate of tropical forests, the nature of the African drought environment, the factors changing global climate, the geological hazards posed by particular volcanoes and the dynamics and biology of the world's oceans.

Exploring the earth from orbit

Before beginning to consider environmental topics, it is useful to recall the historical development of earth surveillance from space.

In recent years the subject of famine has been greatly sensationalised by the media. This chapter nevertheless attempts to approach the subject in an objective, academic manner. It stresses the importance of never under-estimating the nutritional complexities of the Third World and the long-term political, social, economic and technical developments that will be needed to avert the threat of famine which is always just over the horizon in the great majority of Third World countries.

The acute crisis

I shall not attempt to discuss modern famines within a historical perspective. It would have been intellectually attractive to ponder upon, say, the Biblical Famine, now ascribed the date of 1708 BC, in Genesis, Chapter 41, or perhaps on the West Bengal famine of AD 1770, or the more recent one of 1943. The potato famine of 1845 in Ireland would have provided equally good academic mileage, though the Africa famines in Nigeria of 1968 associated with the Biafran Civil War, or those of Ethiopia and the Sahelian region as a whole in 1972, 1978 and 1984–5 might have been of more immediate interest, but I am not going to discuss any of these famines.

My primary reason for not concentrating on such crisis situations is that it might reinforce the belief that the single catastrophic events which occurred on these dates, for example seven years of crop failure starting in 1708 BC in the Middle East or the peak of the drought in AD 1984 in Ethiopia, were the primary causes of these famines.

Introduction

All human activity is ultimately based on resources found in nature. Whether it is consumption, production, or exchange, the commodities which are involved can always be traced to constituents provided by nature.

A tractor requires for its manufacture iron and steel, rubber, plastics, nonferrous metals, labour of various skills, factory-machinery, water, and so on. Of these, to take an example, the steel requires for its production iron-ore, coal, furnaces, water, labour, and so forth. The furnaces in turn require for their manufacture, among other things, iron ore, brickworks, and labour. One can thus break down any produced good into the inputs involved in its manufacture and one can, if one has sufficient patience, trace them ultimately to a combination of labour and natural resources. Of course, labour too is produced and sustained by natural resources. So ultimately all commodities and services can be traced to natural resources.

My purpose in reminding you of the morphology of produced goods and services is not to prepare a chapter on the resource-theory of value. Like Marx's labour theory, such a construct quickly runs into analytical difficulties. My purpose, rather, is to set a materialistic tone so that one may in an unhampered way view natural resources in the light of their use to us in running our lives. I am not suggesting that this is the only defendable perspective; I am merely indicating that this is the attitude I shall strike in this chapter. I need to declare my position at the outset, because if you scratch a resource economist you are likely to find a nature-worshipper trying to get out.

Historical Introduction

Aqueous solutions of alcohols have been for many years, and are still today, of considerable interest to a wide range of scientists and technologists. A low cost and miscibility with water of the lower members of the homologous series renders such mixtures useful as industrial solvent media for a variety of chemical reactions and for small and large scale separation processes. In particular, aqueous solutions of alcohols are often employed in the extraction and manipulation of labile materials such as proteins.

Physical chemists have long been intrigued by the eccentric properties of the mixtures, especially in the low concentration range. A review, published in 1966, and entitled ‘The structural properties of alcohol–water mixtures’ highlighted and tried to explain these peculiar features in the light of the then current ideas about the intermolecular nature of liquid water and solute–solvent hydrogen bonding. Emphasis was on thermodynamic properties with some excursions into spectroscopic, dielectric and transport phenomena. The general conclusion was that the predominant interaction, at least in dilute solutions, is not hydrogen bonding between the two species, but is determined primarily by the alkyl residue. The review posed more questions than it was able to answer but it stimulated considerable interest and activity among organic, physical and biochemists, as confirmed by the number of citations it achieved and the rapid growth in the number of publications describing investigations into alcohol–water mixtures.

In the light of developments over the past two decades, the time is ripe for a reappraisal of binary aqueous solutions of alcohols.

Introduction

Water is a common substance. Its basic properties are widely known yet it represents a considerable challenge to the scientist who wishes to understand its behaviour on a molecular scale. The apparent familiarity with water in its bulk liquid phase creates a deceptive illusion about the simplicity of the molecular interactions which govern these microscopic properties since it is found that water is an extremely complex material. Although a considerable amount of information has been gathered over several decades of research investigation and presented in numerous individual reviews a clear picture of the detailed behaviour has not yet been unambiguously determined. In this context the interest remains as strong as ever and the development of new techniques for putting together the ‘final’ pieces of the jigsaw remains as a tantalizing challenge to a wide range of the scientific community. The series of articles comprising Water: A Comprehensive Treatise provides a clear indication of the way the subject has developed in recent years, but it is also apparent that some of the work described in the earlier volumes has now been superseded by new measurements and new ideas.

In this article, the most recent developments in neutron diffraction techniques will be described. The wide range of experiments undertaken by various research groups in many countries has been initiated in an attempt to provide a more complete picture of the spatial correlations that exist between molecules in the liquid. The overriding feature affecting the interaction between water molecules in the condensed state centres on the phenomenon of hydrogen-bonding, in which strongly orientation-dependent forces are known to influence the structural configuration. The exact nature of the time-averaged molecular correlations remains to be established.

Water is probably the most eccentric chemical known to man. It is the only substance which exists in all three states of matter on this planet and it is also the only inorganic liquid which occurs naturally. On the other hand it is the natural substrate for all in vivo processes and the lifelong environment for many species.

It has long been a source of wonder for philosophers, painters, poets, composers and, much more recently, for physicists, chemists, biologists, and even astronomers. Although there is now general agreement that its remarkable properties derive from hydrogen bonding, we are still at a loss how to explain the bulk physical properties in terms of the molecular structure of the H2O molecule or, indeed, the water dimer. Intensive study of water dates from the 1960s, after the foundation had been laid by Bernal and Fowler in their classic paper of 1933. The realization that water plays a central role in maintaining native biopolymer structures was slow in coming, but since 1970 the importance of hydration figures largely in the protein literature.

Over a period of thirteen years I was involved in the publication of the seven-volume work Water – A Comprehensive Treatise. For various reasons both of a personal and practical nature, I decided not to continue with this project. On the other hand, there are still many topics where water takes a central position and which are due for a review. There are also other topics which featured in the Comprehensive Treatise but where recent progress has been so rapid that an update is opportune.

In Memoriam: George Amer (1891–1985)

Introduction

The thermodynamics, dynamics and structure of any condensed phase depends ultimately on the interatomic or intermolecular interactions; an adequate theory of the liquid state would be able to calculate structure and properties at a given thermodynamic state point using only a knowledge of the elementary potential function. The structure of ‘simple’ liquids such as inert gases is determined essentially by the repulsive core of the potential function; the details of the structure can be considered as perturbations from an ‘ideal’ structure, which to zeroth order approximation relates to Bernal's random packing of hard spheres.

The (relative) simplicity of ‘simple’ liquids is due mainly to the isotropic nature of the potential function Φ. Neglecting three-body effects – which for these systems can be considered as a perturbation – the potential energy of a pair of particles is determined solely by their separation r, so that

Φ = Φ (r only). (1)

This spherical symmetry, together with the sufficient hardness of its repulsive core, means that the structures of the condensed phases (solid and liquid) are determined largely by packing considerations. Thus, we can immediately explain qualitatively such ‘normal’ liquid behaviour as contraction on freezing, expansion on heating, and the increase in viscosity with pressure.

For molecular liquids, this simplicity is lost, as the potential function becomes a more complex function of relative position and orientation of a pair of molecules. The departure of the behaviour of molecular liquids from ‘normality’ will depend upon the nature of the intermolecular interactions. Most still behave ‘normally’ in terms of volume changes on freezing and expansion on heating.

This chapter mainly considers areas of major ignorance to which allometric arguments might make a valuable contribution. Possible shortcomings of the allometric approach used throughout this book are also voiced.

Allometry in plants

With a few notable exceptions, remarkably little work has been done on the allometry of plant growth and reproduction. Despite some early allometric approaches (Pearsall, 1927; Turrell, 1961) more recent papers and books on these topics (e.g. Hunt & Parsons, 1974; Hurd, 1977; Hunt, 1978; Venus & Causton, 1979) fail to include allometric analyses and instead abound with detailed calculations of plant growth curves, in particular; yet the plethora of equations and statistical techniques used fail to provide a functional framework within which to consider plant growth and reproduction.

A first approach would be to see whether the allometric growth equation of Chapter 5, Equation (5.4), provides a useful fit to plant growth curves.

A very great deal more could also be done on the allometry of plant reproduction along the lines of Whittaker & Woodwell (1968) and Hubbell (1980). Hubbell found that a doubling of circumference in the tropical tree Bursera simaruba produced nearly a 50-fold increase in mean seed crop (cf. Chapter 4). Such studies, whether intraspecific or interspecific, are easy and relatively quick to carry out, produce invaluable data and may be of commercial value too.

Optimal organ size

As one might expect, the weights of many organs scale both intraspecifically and interspecifically on body weight with exponents of close to one. There are, however, some interesting and notable exceptions.

Surface area/volume arguments have been extensively used in biology. They are valid provided, first, that one of the properties being considered scales as the surface area of an organism and the other as the volume of an organism and, secondly, given that this is the case, that the surface area/volume ratio represents an important evolutionary constraint pertinent to the problem under consideration.

This chapter looks at some instances where surface area/volume arguments may have been misappropriated or misapplied. It also considers when they may be valid.

Why cannot large animals rely solely on diffusion for gaseous exchange?

The universal answer given in school (Mackean, 1973; Revised Nuffield Biology, 1975; Soper & Tyrell Smith, 1979; Rowlinson & Jenkins, 1982; Jones & Jones, 1984; Green, Stout & Taylor, 1984; Roberts, 1986a, b; Hill & Holman, 1986) and university (Marshall & Hughes, 1965; Barrington, 1967; Alexander, 1971, 1979; Barnes, 1974; Bligh, Cloudsley-Thompson & MacDonald, 1976; Schmidt-Nielsen, 1983, 1984) text-books to the question ‘Why do large species of animals have lungs, gills or other structures specialized for gaseous exchange while smaller species manage by diffusion’ is that the larger an organism, the smaller its surface area to volume ratio. This answer makes two implicit assumptions. First, that the oxygen requirements of an animal are proportional to its volume or weight. Secondly, that diffusion can only provide oxygen at a rate proportional to an animal's surface area. The former of these assumptions is incorrect (Reiss, 1987b). The oxygen requirements of animals scale on body weight with exponents of less than one.

In this chapter existing quantitative models of body size are critically reviewed. Special attention is paid to Belovsky's (1978) model for moose, as this is the only attempt yet to predict numerically how males and females should differ in weight. The criticisms here of nearly all the other quantitative models of body size may appear rather negative. The purpose of this chapter is not, however, solely to condemn most previous work. Evidence is presented that the energy assimilated per unit time scales at about body weight to the two-thirds power in ruminants, and it is pointed out why quantitative predictions of optimal female adult body weight from the model of Chapter 3 are not yet feasible either.

A review of existing models

Pearson (1948)

Pearson (1948) argued that in a plot of metabolic rate (measured as cubic centimetres of O2 per hour per gram of body weight) on body weight for mammals, the extrapolated curve for shrews becomes asymptotic at about 2.5 g. Consequently, he maintained, no (adult) mammals are lighter than this, because such lighter animals would be unable to gather enough food to support their ‘infinitely rapid metabolism’. This argument is unconvincing. Smaller animals also eat more, relative to their body weight (Chapter 2).

If Pearson's theory was valid, it might be predicted that smaller species should spend more time feeding. Smaller birds do feed for longer (Hinde, 1952; Gibb, 1954; Pearson, 1968). However, the interspecific dependence of feeding time on size differs between taxa. Larger primates, for example, feed for longer (Clutton-Brock & Harvey, 1977), and the same may be true for ungulates (Eltringham, 1979).

Many authors, from Darwin (1871) onwards, have considered factors that might affect the degree of sexual dimorphism. Two generalizations are frequently made. First, that dimorphism increases with the degree of polygyny (e.g. Crook, 1962; Lack, 1968; Clutton-Brock & Harvey, 1976; Clutton-Brock et al., 1977; Alexander et al., 1979) and, secondly, that larger species are more dimorphic (e.g. Rensch, 1950; Wiley, 1974; Rails, 1976a; Leutenegger, 1978; Harvey & Mace, 1982). Intrasexual selection acting on males is usually given as the reason why males are larger than females in polygynous species (Selander, 1972; Belovsky, 1978). With increased polygyny increased sexual selection is expected, with the predicted consequence that the sexes should differ more in their optimal sizes. More recently, however, the role of selection for optimal female size has been emphasized, and advantages for smaller females have been postulated in polygynous species (Willner & Martin, 1985; Robinson, 1986).

The reasons, however, why larger species should exhibit greater sexual dimorphism in body size are less apparent than the reasons why more polygynous species should be more dimorphic. This chapter first reviews the evidence for the association between size and sexual dimorphism, and then considers those theories that predict that larger species should show greater dimorphism in body size.

A review of the evidence

Rensch (1950) and Rensch (1959) are frequently cited as evidence that sexual dimorphism is greater in larger species, but the data are unconvincing. No data are presented in Rensch (1959). In his 1950 paper, Rensch presented information from birds, mammals and carabid beetles.