When did We Start to Change Things?

As we wrote the first draft of this chapter (during early summer 2007), the potential dangers of ‘global warming’ had moved up the news agenda to a point where most major politicians were starting to take the problem seriously. Our opening quotation comes from a book published in early 2006, which seemed to coincide with the growth of this wider concern with global warming. Lovelock was not alone in trying to raise awareness of the problem; around the same time another book on climate change by the zoologist and palaeontologist Tim Flannery also attracted global attention to this issue, as did the lecture tours (and Oscar-winning film) of Al Gore—the former US presidential candidate and campaigner on the dangers of climate change. Indeed, in his role as a climate campaigner Gore won a share in the 2007 Nobel Peace Prize. It is possible that future historians will see the period 2005–2007 as the start of a crucial wider engagement with these problems. Things may not be as bad as James Lovelock suggests—in his book he deliberately emphasized the most worrying scenarios coming from computer models, and other evidence, in an attempt to draw attention to the critical nature of the problem. However, all these worst case scenarios were drawn from within the range of results that most climate scientists believed could plausibly happen—not extreme cases with little current evidence to support them. That one of the major environmental scientists of the second half of the twentieth century could write such prose as science—rather than science fiction—is clearly a case for concern about future climate change. It also raises another important question, relating to the history of human influence on our planet: when in our history did we start to have major environmental impacts on Earth as a whole? This is clearly an important issue from a historical perspective, but the answers may also have implications for some of our attempts to rectify the damage. Our discussion of this question comes with various caveats. Many of the arguments we consider in this chapter are still the subject of academic disagreement.

Why is the Sea Blue?

One answer to this chapter’s question is straightforward and based on high-school physics. The early SCUBA divers quickly discovered that if they took underwater colour photographs, even if they were only a few metres down, their pictures had a strong blue cast to them. However, if they illuminated their subjects with a flash, then a more colourful world emerged in their pictures—especially if they were photographing the rich diversity of highly coloured fish that can be found in some parts of the tropics. The reason for the blueness is that as sunlight passes through water the colours of the spectrum are absorbed at different rates, with the long wavelengths (e.g. red) absorbed first and the higher-energy shorter wavelengths (e.g. blue) penetrating deeper into the depths. It follows that underwater available light is predominantly blue and that any light reflected from within the water body is more likely to be from the bluer end of the spectrum of visible light. So, light coming from the sea to our eyes is mainly blue because these wavelengths are least absorbed; indeed oceanographers who have studied some of the cleanest waters describe them as looking ‘violet blue’. As biologists we are interested in a more ecological answer to the question, ‘Why is the sea blue’? The physics explanation only works if seawater is reasonably clear, and it is this clarity that biologists need to explain. Consider our opening quotation, which comes from Peter Matthiessen’s book describing early attempts to film the great white shark in its natural habitat. It raises an interesting ecological question—why can a SCUBA diver or snorkeler see where they are going in the ocean? Put another way, why is the sea blue rather than green? The upper layer of the ocean with enough light for photosynthesis is called the euphotic zone (defined as extending down to the point where only 1% of photosynthetically usable light is present compared with surface light levels); this is often only a few tens of metres deep, but in extremely clear water near Easter Island in the Pacific it has recently been found to extend down to 170 m depth.

Why Cooperate?

An altruistic act is one in which an individual incurs a cost that results in a benefit to others. Giving money or time to those less fortunate than ourselves is one example, as is giving up one’s seat on a bus. At first, one might consider such behaviour hopelessly naive in a world in which natural selection seemingly rewards selfishness in the competitive struggle for existence. As the saying goes, ‘nice guys finish last’. Yet examples of apparent altruism are commonplace. Meerkats will spend hours in the baking sun keeping lookout for predators that might attack their colony mates. Vampire bats will regurgitate blood to feed their starving roost fellows, while baboons will take the time and effort to groom other baboons. Some individuals, such as honeybee workers, forego their own reproduction to help their queen and will even die in her defence. The common gut bacterium Escherichia coli commits suicide when it is infected by a bacteriophage, thereby protecting its clones from being infected. If helping incurs a cost, then surely an individual that accepts a cooperative act yet gives nothing in return would do better than cooperators? What, then, allows these cases of apparent altruism to persist? In his last presidential address to the Royal Society of London in November 2005, Robert M. May argued, ‘The most important unanswered question in evolutionary biology, and more generally in the social sciences, is how cooperative behaviour evolved and can be maintained’. In this chapter, we document a number of examples of cooperation in the natural world and ask how it is maintained despite the obvious evolutionary pressure to ‘cheat’. We will see that, while it is tempting to see societies as some form of higher organism, to fully understand cooperation, it helps to take a more reductionist view of the world, frequently a gene-centred perspective. Indeed, thinking about altruism has led to one of the greatest triumphs of the ‘selfish gene’ approach, namely the theory of kin selection. Ultimately, as the quote from Mandeville indicates, we will see that cooperation frequently arises simply out of pure self-interest—it just so happens that individuals (or, more precisely, genes) in the business of helping themselves sometimes help others.

Why Do We Age?

In 2004, the amazing ‘Flying Phil’ Rabinowitz broke the world 100 m sprint record for a centenarian, setting a time of 30.86 s and beating the previous world record time by over 5 s. Despite this impressive statistic, most 20 and 30 year-olds can readily run at these speeds when dashing for a bus, and the overall world record for 100 m currently stands at 9.69 s (set by Usain Bolt at the age of 21). Age-related degeneration in bodily function is familiar to all of us, and is known as ‘senescence’, or more colloquially, as ‘ageing’. Of course, this loss of physiological functioning not only impairs our ability to run: as individuals get older they typically experience an increase in the likelihood that they will die, and also a decrease in fecundity. The incidence rates of cancers and heart attack, for example, are considerably higher in older than in younger individuals. For these reasons, ageing has been dubbed ‘the most potent of all carcinogens’, but it has also long been considered as one of the world’s worst diseases (‘senectus enim insanabilis morbus est’—a sickness for which there is no cure). . . . Live long and prosper? . . . Organisms die for all sorts of reasons. They may get run over by a bus, they may be eaten by a predator, or they may succumb to a lethal disease. However, even if individuals survive all of these ‘extrinsic’ challenges, then the odds are that they will begin to experience the signs of senescence. While being eaten by a predator is unfortunate, it is also eminently understandable as a cause of death. Natural selection will tend to act on individuals to reduce the likelihood of this extrinsic mortality (for instance, by promoting higher vigilance or the development of some form of defence) but death from accidents, predators, and parasites cannot be completely avoided. Ageing, however, poses much more of a dilemma for evolutionary biologists. In particular, one might expect that those individuals who managed to slow down the ageing process would leave more offspring, so that natural selection would favour extreme longevity.

General Conclusions

Brett Dennen is a fine musician, but listening to his lyrics, one might be tempted to think that because we ‘do [or see] it every day’, then it does not deserve an explanation. Our book has dealt with a variety of everyday phenomena such as ageing, sex, species, a green world, and a blue sea, and we hope that by now our readers will agree that there is a reason why things are this way. Indeed, the fact that these phenomena are so commonplace makes the questions all the more important. The exciting thing is that while considerable progress has been made in each of the areas we address, we still do not have a complete answer to any of the questions we have posed. We use this short concluding chapter to pull together some common threads and to discuss some of the interrelationships between our answers. First and foremost, even the most casual reader will note that there is a close interrelationship between the ecological and evolutionary explanations we have presented. Taking the perspective of evolutionary biology, almost all of the evolutionary explanations we have proposed include an important ecological component. For example, ageing is now widely seen to arise as a consequence of there being relatively weak natural selection late in an organism’s life. Yet the primary reason for this ‘selective shadow’ is that predators and parasites are likely to have killed the organism long before it reaches an advanced stage of maturity. Likewise, one explanation for the evolution of sex is that the variation it generates allows at least some of the offspring to better compete with members of the same species, or to avoid parasitism. In a similar vein, many of the ecological phenomena we have sought to explain have evolutionary origins. For example, tropical areas may have more species because rates of speciation are greater in the tropics, or because rates of extinction are greater at high latitudes, or both. Likewise, plants have evolved secondary compounds to deter herbivory, and the presence of these compounds may go some way towards understanding why the world remains green.

How will the Biosphere End?

This fictional description of the destruction of much of life on Earth comes from a novel by the astronomer Fred Hoyle, co-authored with his son Geoffrey. In the story, the formation of a quasar in the centre of our galaxy leads to the destruction of all life on Earth, except at a few fortuitously sheltered locations. Quasars—first described in 1963—are colossally energetic astronomical objects with extremely high output of radio waves. The novel built on some of Fred Hoyle’s own scientific interests because in the early 1960s, along with the astrophysicist W.A. Fowler, he had predicted that the collapse of a super-massive object could form a distinctive radio source—just before the discovery of the real thing. Although Hoyle and Fowler had the theoretical head start in explaining quasars, being busy with other work they failed to follow up on this advantage, and the current best explanation for these objects is largely due to Donald Lyndon-Bell and Martin Rees. Building on the ideas of Hoyle and Fowler, they argued that a quasar is formed by a rotating super-massive black hole, fed by a disk of in-falling matter, with jets of matter flying away from the system along its axis of rotation. Like the Hoyles’ novel, this chapter focuses on ways the biosphere could end; a fitting question for the close of a book on the ecology and evolution of Earth-based life. However, any answer to a question set in the far future can necessarily be only speculative and, of course, nobody will be around to put the theory to its ultimate test. This raises a philosophical problem namely, has such a question a place in science, or should it be left to science fiction writers? We believe that such questions count as science, not least because it would be good to know the answer (especially if something could be done to postpone the end), but also because in attempting to answer the question, we can extend our understanding of processes that are currently operating. Indeed, J.B.S. Haldane, one of the greatest scientists of the past century, wrote an early essay on much the same topic we consider here.

Why is the World Green?

Viewed from space by human eyes, the predominant colours of our planet are the blue of the oceans and the white of the clouds. The blue of the oceans forms the subject of another of our chapters. However, if one focuses on the land masses other colours dominate. On land the white colour still features prominently in the polar areas covered with snow and ice, but zoom in on lower latitudes and much of the land is a mix of the green of vegetation and the brown of more arid areas. Green dominates large areas of land, so unless you are reading this in a desert, during the high-latitude winter, or in a highly urban area, then green will probably feature prominently in your surrounding landscape. One answer to the question that heads this chapter is that the climate (often rainfall) allows some parts of the land to be green with plant life, while making other areas arid and brown. However, this green of extensive plant life is still a puzzle—plants are food for a wide range of animals, so why is so much food left unused? Swarms of locusts, destroying most plants in their path (be they biblical plagues or modern day outbreaks), are the exception not the rule. But why is this so? Why are so many parts of our world green in the face of this threat from herbivores? As we will see, if herbivores are the key to our question, then what starts as a question in plant ecology ends up being a question about factors that limit the size of herbivore populations. In effect, we need to understand why herbivore populations do not increase in density to such a level that they destroy all the available plants, giving a land that is brown rather than green. Until the middle of the twentieth century if you had put the green world question to biologists, many of them would probably have suggested that it was not in the interests of a species to consume all of its food reserves.

Is Nature Chaotic?

Centuries before King Harold of England famously received an arrow in the eye (AD 1066), Chinese officials in the T’ang dynasty (AD 618–907) began collecting annual reports on the abundance of migratory locusts. The primary aim of this initiative was to make sense of the changes over time (the dynamics) of this devastating agricultural pest, and thereby predict the timing and intensity of outbreaks. Now, despite a staggering 1,300 years of faithful recording, few patterns are evident and the data look decidedly messy. Irregular climatic fluctuations, particularly those involved in the drying up of grasslands on river deltas, may explain some of the variability. However, one might wonder whether some of this ‘messiness’ was internally driven, caused by some sort of ‘feedback’ arising within the dynamics themselves. Many long-term data sets on population dynamics have these extremely messy qualities, ranging from the daily number of damselfish reaching maturity on the Great Barrier Reef to the number of feral sheep on Scottish Islands, and it is important to know where it all comes from. The study of ‘chaos’ (easiest to define negatively as an absence of order, but we will get to a more formal definition later) has its roots in precisely the type of feedback processes referred to above, reflecting what mathematicians call ‘non-linearities’ (relationships that are not straight lines). Several mathematicians, most notably, the eminent French mathematician Henri Poincaré (1854–1912), had long noted that non-linear systems could generate some extremely unusual dynamics, such that the precise trajectory a system took was highly sensitive to the initial conditions. However, observations such as these were largely overlooked by ecologists until a new generation of researchers, notably Robert May (a physicist turned ecologist, now Lord May of Oxford), began toying with their own simple ecological models and appreciating that the behaviour of these models was not always simple. Until ecologists were made aware of the potential effects of non-linearities in the 1970s, the prevailing view was that complex dynamics must have complex causes.

Why Species?

In this chapter, we will attempt to address several interrelated questions about species and species formation. First we ask what, if anything, is a species? As we shall see, while most scientists are happy to agree on the essentials, the answer to this question is far from straightforward. We then briefly discuss the range of ways new species can evolve, and provide evidence for these different pathways. Finally, following from our opening quotations, we ask a somewhat more abstract and philosophical question that brings together many of the separate threads we have introduced: why is life not composed of a single species? . . . What is a species? . . . The classification of organisms into species is so familiar that it is easy to accept without much critical thought. On reading ‘Tiger, tiger burning bright’, or headlines such as ‘Man bites Dog’, we have no problem envisaging who the main protagonists are. Mention a tiger, and one immediately thinks of a large cat with stripes. To most people, species are simply a collection of organisms with a given set of physical traits. All classification systems include elements of personal preference as to how one chooses to classify any group of objects (e.g. by shape, size, or colour). However, there is evidence that ‘species’ represent categories that are more consistent between observers than the various ways of sorting out one’s stamp collection. The Fore, a highland people of New Guinea, are perhaps best known in the western world for the devastating prion-based disease ‘Kuru’ that afflicted their population as a result of ritualized consumption of dead family members. However, the people have close links to their natural environment and a remarkably detailed system of classifying the larger animals they see around them. In an early study to test the degree to which species assignations are consistent among peoples with different backgrounds, Jared Diamond compared the Fore nomenclature with that developed by European taxonomists. Birds found regularly in the Fore territory were divided by the Fore into 110 distinct types, and by zoologists into 120 types, with an almost exact one-to-one correspondence between Fore ‘species’ and taxonomists’ ‘species’.

Why are the Tropics so Diverse?

Our opening quotation describes Charles Darwin’s first experience of tropical forest on 29 February 1832. He had been looking forward to this moment for several years. While completing his studies at the University of Cambridge he had read Alexander von Humboldt’s accounts of tropical natural history and resolved that he too must experience the luxuriant vegetation and diversity of tropical species at first hand. Initially, Darwin planned to visit the subtropical island of Tenerife; however, this plan was superseded by the opportunity to join H.M.S. Beagle’s circumnavigation of the Earth—to his great disappointment Darwin never did get to land on Tenerife, although he saw it from the sea as the Beagle passed close by. Since Darwin’s time we have learnt much about the nature of biological diversity, both in the tropics and at higher latitudes. In this chapter, we review current knowledge of tropical diversity and how it compares with diversity at higher latitudes, before going on to discuss the various explanations that have been put forward to explain why the tropics have so many species. Here we define the tropics as the area between the Tropic of Cancer (23°28´ N) and the Tropic of Capricorn (23°28´ S) when we are discussing the modern world. In discussions of past climates, we refer to areas as ‘tropical’ if their reconstructed climates are similar to those currently experienced in the modern tropics. While we describe below how diversity changes with latitude, it is obvious that latitude itself is only part of a grid system that allows us to define the location of a point on the Earth’s surface, so it cannot itself have a direct effect on the number of species. However, many variables such as climate and land or ocean area are correlated with latitude and may provide an explanation for tropical diversity. Indeed, latitude itself is defined by the rotation of the Earth about its axis—a fundamentally abiotic (i.e. non-biological) planetary event. It follows that the ultimate cause of the gradient in diversity over latitude must be attributable to abiotic factors that are correlated with latitude, even if biological factors subsequently play a role in maintaining or promoting this diversity.