Do We Need Pandas?

The Uncomfortable Truth About Biodiversity

Ken Thompson

• Publisher: Green Books
• Year: 2011
• Print ISBN: 9781900322867
• eBook ISBN: 9780857840059
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

A fascinating popular science book that reveals how much we really know - and don't know - about the natural world. Explains why we need to be more concerned about ecosystems than individual iconic species such as the giant panda and gorilla.

Chapter 1

What is biodiversity?

O wonder! How many goodly creatures are there here!

William Shakespeare, The Tempest

How many species are there?

Since this book is about biodiversity, I suppose a good place to start would be: what exactly is biodiversity? The Rio Convention on Biological Diversity says:

Biological diversity – or biodiversity – is the term given to the variety of life on Earth and the natural patterns it forms. The biodiversity we see today is the fruit of billions of years of evolution, shaped by natural processes and, increasingly, by the influence of humans. It forms the web of life of which we are an integral part and upon which we so fully depend. This diversity is often understood in terms of the wide variety of plants, animals and microorganisms.

In other words, on one level (the simplest), biodiversity is the number of different kinds of living things on planet Earth.

So, how many living things are there? Well, one place to start estimating is those species we have described, classified and named. This amounts to about one-and-three-quarter million, over half of them insects and less than a quarter of them plants in the strict sense of the word. How close is this figure to the true number? It would be unduly pessimistic of me to say we haven’t a clue, but the truth is not much better than that. However, there is no shortage of guesses. One of the most celebrated was made by Terry Erwin in 1982, in what must surely be the most famous paper ever published (or ever likely to be) in The Coleopterists Bulletin.¹ Erwin was (and is) an expert on the beetles of tropical tree canopies, and in particular the genus Agra, in which he has named many new species; Agra vation and Agra cadabra to name but two. Erwin’s research led him to suspect that there is a lot more biodiversity in the canopies of tropical trees than anyone realised. Using an insecticide ‘fog’, he extracted all the creatures from the canopies of 19 individuals of one particular kind of tropical tree in Panamá. Most of these animals were insects and most were new to science. Around 1,200 of the species that fell out of his sampled trees were beetles, and Erwin estimated that 163 of them live on only this one tree species. We know there are around 50,000 tropical tree species, so if we assume that our target tree is typical (in other words, if all tropical tree species have about the same number of unique beetles), then there must be just over eight million different beetles in the canopies of tropical trees. About 40 per cent of all arthropods (animals with jointed exterior skeletons: insects, spiders, etc.) are beetles, so that means there must be about 20 million tropical-canopy arthropods. Around half as many species again live in or on the ground, making 30 million tropical arthropods in all, most of them insects.

It’s not difficult to find fault with Erwin’s method, and indeed biologists have been arguing about it ever since. Supposing Erwin’s method missed some of the beetles that live on the trees he sampled? In which case his estimate would be too low. What if he overestimated the proportion of specialists among the beetles he caught? In which case it would be too high. In short, it’s easy to see why hardly anyone agrees with Erwin’s estimate, and it may indeed be too high (or too low), but to argue about the exact number is to miss the point: there are very many more species than we have named, and we haven’t much idea how many.

Of course, you might argue that tropical rainforest, hugely diverse and largely unexplored, is where you would expect to find most of the Earth’s unknown biodiversity. And maybe you’d be right, but there are unknown and unrecognised species everywhere. For 15 years college lecturer Jennifer Owen trapped, identified and counted the animals in her suburban garden in Leicester, an average-sized city in the middle of England. Simply by looking at one family of parasitic wasps, a group that very few people care about and fewer still are qualified to identify, she found 15 species not previously recorded in Britain and four species completely new to science.² So great was the effort required that she and her wasp expert kept up the sampling and identification of this one family of insects for only three years. There’s every reason to expect that more effort would have yielded more discoveries.

But insects, by and large, are rather small. Surely there’s nothing large out there waiting to be discovered? Wrong again. Even today, a new large terrestrial mammal is discovered roughly every three years, while the ocean yields a new large fish or mammal every five years. Altogether, 408 new mammal species have been described since 1993, including a pygmy sloth from Panama, a ‘giant’ muntjac deer from Vietnam, a white titi monkey from Brazil, and the Solomon Islands’ monkey-faced bat. Nor is it just new animals that continue to be found. An expedition in 2002 to northern Vietnam discovered a species belonging to a completely new genus of conifers, the golden Vietnamese cypress. In fact the locals had known about this tree all along, and were busy cutting it down for its valuable timber. The same expedition discovered more than a hundred other new plant species, including several shrubs and about two dozen orchids.

In 2008 botanists were startled by the discovery of an enormous new palm on the island of Madagascar. The palm, originally spotted by Xavier Metz, a Frenchman who manages a cashew plantation nearby, has a huge trunk 18m in height, topped by fan-shaped leaves 5m in diameter. It’s the most massive palm ever found on the island and one of the largest known flowering plants. Ironically, once you know they are there, the trees can plainly be seen on Google Earth. Across the whole of life, around 300 new species are described every day, a rate that shows no sign of slowing.

Of course, you might say, we’re back to the tropics again, which is surely where all the large undiscovered plants are? Wrong again. In 1994, David Noble, from the New South Wales National Parks and Wildlife Service, Australia, was walking in the Wollemi National Park, about 150km north of Sydney. He saw some trees he didn’t recognise, took a sample home, and found he had stumbled on a completely new conifer – and no ordinary conifer either. The nearest relatives of the Wollemi pine (Wollemia nobilis) are the Norfolk Island pine, the monkey puzzle and other members of the Araucariaceae family. In truth, however, it doesn’t have any really close living relatives. Its real relatives are fossils dating back 150 million years to the age of the dinosaurs. The Wollemi pine grows 35m tall, with elegant foliage and bark like no other tree, best described as looking like gently simmering chocolate. Despite being one of the world’s rarest plants, numbering as few as 43 adult trees at the time of its discovery, it looks like following in the footsteps of that other great survivor, the ginkgo (Ginkgo biloba), and enjoying a completely new lease of life in cultivation.

The diversity of the very small

But all this diversity, impressive as it is, is merely the tip of the iceberg. As organisms become smaller they become both more numerous and more diverse, and few things are as small or as numerous as bacteria. There is a tendency to think that because of their importance to medicine and molecular biology, bacteria are reasonably well known. Nothing could be further from the truth. One hundred grams of ordinary garden soil contains 100 billion bacteria – as many as there are stars in the galaxy. Yet how many species of bacteria have been described and named? About 4,500. The reason is simple: the first step in studying bacteria is to isolate single species by growing colonies from individual cells on a culture medium, but far less than one per cent of bacteria can easily be cultured in this way. The rest just refuse to cooperate and therefore cannot be studied by conventional techniques. Biologists have begun to get round this problem by looking directly at bacterial DNA. The results are startling: one gram of soil contains 4,000-5,000 different kinds of bacterial DNA, and thus the same number of different bacteria, while one gram of marine sediment contains another, quite different 4,000-5,000 bacteria. (Don’t think, by the way, that DNA is only of use for sorting out the taxonomy and diversity of the very small. Searching for elusive large creatures is much simpler now that an animal can be identified by a DNA profile from a single hair. If the yeti exists, you can bet we will become aware of its DNA before anyone catches the animal itself.)

There are other, even more cunning ways around the refusal of bacteria to stand up and be counted. Ecologists have discovered that communities of living organisms obey certain ‘laws’, or highly repeatable patterns of abundance of the species within them. Essentially, a few species are common and almost all are rare. If you know how many individuals are present, and how common the top few species are, a combination of some reasonable assumptions and some fancy mathematics allows you to make a stab at how many species there might be. It’s a bit like guessing how many postmen, teachers and bus drivers live in a country or city, knowing only the total population and the proportion of bank managers and doctors; it should work as long as different professions occur in reasonably fixed proportions. If you do this for bacteria, one gram of soil probably contains between 6,400 and 38,000 species; in other words, the DNA estimate is in the right ballpark. Scientists are always reassured when two completely different methods of estimating an unknown quantity turn up with similar answers: there’s always the possibility that one method might be completely up the creek, but two different methods are unlikely to be wrong in exactly the same way. Astonishingly, however, if you do something that can’t be done in the lab and ask how many different kinds of bacteria are in a tonne of soil, the answer is around four million. How is that possible? The key to such huge diversity is the enormous physical diversity of the soil environment at the scale experienced by bacteria. A fragment of decaying leaf harbours one bacterial community, the clay particles next door have a quite different set, while yet another community inhabits the gut of a nearby earthworm. The total surface area of all the tiny pores and channels in two tablespoons of soil is about the same as a typical city block: for bacteria and other creatures that live on the surface of soil crumbs, there’s an awful lot of room in soil. This is in sharp contrast with the world’s oceans, which are essentially structureless and may contain ‘only’ two million species of bacteria altogether.

Even though bacterial diversity beggars belief, this is only part of the story. C. Northcote Parkinson once observed that since the British Civil Service (in defence of its policy of employing only classics graduates) was unable to decide whether one man was better in geology than another man in physics, it was convenient to rule them both out as useless. We have long had a similar attitude to bacteria. Since they are very small, prokaryotes (lacking a true nucleus), and for the most part cannot be grown in the laboratory, it has been convenient to lump them together as a single kingdom, in the same way that all plants (or animals) constitute a single kingdom. In reality, there may be ten or fifteen bacterial kingdoms, each at least as different from the others as animals are from plants. A new bacterial phylum (a phylum is the level of classification just below a kingdom, so arthropods – insects, spiders, crustaceans etc. – are a phylum) is discovered every month, and the further we look, the more we discover that there seems no limit to where bacteria can live. In 1993 living bacteria were found in rocks over two kilometres below the Earth’s surface, where they may have survived for millions of years without any contact with the surface. Nor is all this hidden diversity confined to the bacterial realm. The microscopic single-celled animals that were once erroneously lumped together as protozoa may be equally diverse. And the diversity of some other living organisms is turning out to be much higher than anyone thought, simply because many of these species live in places nobody thought to look in until recently. We’ve known for a long time that plants form symbiotic partnerships with various fungi that live in or on their roots. This mycorrhizal symbiosis seems to be crucial for most plants to obtain enough phosphorus from soils. It’s also very ancient: fossils of the earliest land plants show clear signs of mycorrhizal symbiosis.

But what no one realised until recently is that there are many other fungi associated with plants. Every plant species so far examined (although not every individual) plays host to fungi that live inside its leaves. These fungal endophytes may be remarkably diverse; one estimate being that there are as many different fungal endophytes as all other fungi combined. Like many other groups, the pinnacle of their diversity seems to be in tropical forests. In Puerto Rico, up to 17 species of fungal endophyte were found inside a single leaf, each occupying an area of only a few square millimetres. The obvious question is: What are all these fungi doing? For nearly all of them the answer is: we’ve no idea. Many may have no obvious effects on their hosts at all, but we know that some contain toxins that help to defend the plant against predators and pathogens, while others seem to increase tolerance of salinity or drought. One American grass can grow at soil temperatures up to 65°C next to hot springs in Yellowstone National Park, but only if infected with a particular endophyte. What’s more, it turns out that the endophyte can only confer this remarkable heat tolerance if it is itself infected with a certain virus. Turf and grass merchants have not been slow to appreciate the commercial possibilities, and are already marketing ‘endophyte-enhanced’ grass cultivars for extreme environments.

A lot more than fish in the sea

By now many readers will have noticed that so far, with the arrogance typical of a terrestrial primate, I have concentrated almost exclusively on organisms that live on dry land. The simple, pragmatic reason is that we still know relatively little about life in the sea. More people have walked on the moon than have visited the deepest oceans, and we have better maps of the surface of Mars than of the ocean floor. In 2005 a US submarine crashed into a 2,000-metre submarine mountain that rises to within 50 metres of the surface near Guam. No one knew it was there. It’s easy to forget that almost three-quarters of the area available for life, and about 99 per cent of the actual space, is in the sea. If all dry land down to sea level were bulldozed into the ocean, it would occupy just one-twenty-third of the ocean volume. To a mere landlubber most of the sea looks the same, but the oceans harbour habitats as distinct as are tropical rainforests, dry deserts and the polar ice caps. The diversity and sheer variety of life in the sea is correspondingly enormous. Many animal phyla (the plural of phylum) are entirely marine, and it’s no accident that the first port of call for anyone wanting to appreciate animal diversity is the nearest rocky shore. Dry land, totally dominated by insects, is exceedingly dull in comparison. And yet, especially in the case of the deep ocean, marine biodiversity remains almost unknown.

Among the many startling animals down there is the siphonophore Praya dubia, the largest invertebrate in the world (and indeed the longest animal of any kind, at up to 40m). Trailing a curtain of tentacles bearing powerful stings, this is truly one of the planet’s great killing machines. In fact the deep sea quickly teaches you that you can forget almost everything you thought you knew about what it takes to be a predator. In the deep ocean ‘jellyfish’ (in the sense of organisms made of jelly) rule: animals made of 95 per cent seawater and just enough skin, muscle and nerves to stop them falling apart. So fragile that most are impossible to catch in one piece; often quite transparent, without teeth, bones or brains; they are nevertheless the dominant predators of the open ocean. Of course there are plenty of vertebrates too, some of them extraordinarily abundant. The deep-sea benttooth bristlemouth fish (Cyclothone acclinidens) may be the commonest vertebrate in the world, yet only a handful of marine biologists have even heard of it, and fewer still have seen one (they are only 3-4cm long, and largely transparent). For the last 25 years a new deep-sea species has been discovered about once every two weeks, and the number yet to be discovered is estimated at anywhere between 10 and 30 million. So if you think I’m not paying enough attention to life in the oceans, then I apologise, but that’s just a reflection of how little we know.

Why are there so many species?

Counting the Earth’s species is only the start. What are they all doing, and how do they manage to coexist? Of course, the basic answer to this question is natural selection. It was Charles Darwin’s genius to recognise that the unique profligacy of the natural world makes it also uniquely creative. All over the world, in every generation, trillions of animals and plants produce many more offspring than can possibly survive. Mostly these offspring are not identical, although the differences between them may be only subtle. Some of these differences are merely skin deep, and are not heritable, but many are the result of tiny genetic mutations and as such can be inherited. At the same time, all genes are constantly being mixed and shuffled into new combinations by sex, massively accelerating the changes that could occur by mutation alone. Living organisms are therefore constantly probing the possibilities of making a living from the environment, by the simple expedient of keeping those that do this best and killing off those (the overwhelming majority) that aren’t quite as good. Since this environment itself consists largely of other species, all doing the same, it’s not hard to see how natural selection continuously creates both new opportunities and the organisms to exploit them. For the most part, evolution by natural selection proceeds at what appears, to human senses, to be a snail’s pace, but every now and then it takes sudden and rather dramatic strides. Sometimes this is because some barrier is removed, as when the dinosaurs disappeared and left the field clear for the rapid evolution of the mammals. Sometimes evolution itself gives rise to an innovation that allows a new group to power ahead, as when the previously dominant cycads and conifers gave way before the proliferating flowering plants.

But natural selection alone is not enough. If all the world’s biodiversity were like one homogeneous, frequently stirred pot, the power of natural selection to create new species would be seriously compromised. Populations at the edges of species’ ranges are often exposed to rather atypical conditions, leading to a tendency to evolve characteristics that differ from those in the centre of the range. As long as these different populations are in contact with the main bulk of the species, however, it’s very hard for them to evolve into new species, because their new features keep being swamped by the flow of genes from the centre. In reality speciation (i.e. splitting into two or more daughter species) works best, or perhaps only works at all, when populations of animals and plants are free to evolve in isolation. In fact, so effective is isolation at promoting speciation that it can occur even when the conditions experienced by the isolated population are identical to those experienced by the parent species. Isolation of just a small part of the original gene pool (the ‘founder effect’), or the loss of key genes by chance (‘genetic drift’) can lead quickly to the evolution of a new species.

The results can be seen on many levels. On a geological timescale, the northern hemisphere was for a long time one single land mass, which broke up into two chunks (Eurasia and North America) relatively recently, so it has essentially a single flora. If you are familiar with the plants of Europe, you won’t find too many surprises among the floras of North America or Japan. Each region has different but related oaks, beeches and...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Acknowledgements
• Foreword by Tony Kendle
• Introduction
• 1 What is biodiversity?
• 2 Biodiversity: where and why?
• 3 Inside species
• 4 What is biodiversity worth?
• 5 Threats to biodiversity
• 6 Are species necessary?
• 7 Reasons to be cheerful?
• Glossary
• References
• Index