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    Ecology and Natural History
    Ecology and Natural History
    Ecology and Natural History
    Ebook656 pages7 hours

    Ecology and Natural History

    By David Wilkinson

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    Ecology is the science of ecosystems, of habitats, of our world and its future. In the latest New Naturalist, ecologist David M. Wilkinson explains key ideas of this crucial branch of science, using Britain’s ecosystems to illustrate each point.

    The science of ecology underlies most of the key issues facing humanity, from the loss of biodiversity to sustainable agriculture, to the effects of climate change and the spread of pandemics. In this accessible and timely addition to the New Naturalist series, ecologist David M. Wilkinson introduces some of the key ideas of this science, using examples from British natural history. Extensively illustrated with photographs of the species and habitats that can be seen in the British countryside, this book shows how the observations of field naturalists link into our wider understanding of the working of the natural world.

    Investigating ecosystems across the British Isles, from the Scottish and Welsh mountains to the woodlands of southern England and the fens of East Anglia, Wilkinson describes the relationships between organisms and their environments. Factors such as climate and chemistry influence populations of every kind of organism, and the interactions between these organisms determine the makeup of ecological communities. Using examples from the full range of organisms on Earth – from bacteria to badgers – Wilkinson introduces the crucial ecological processes that support life, addressing how these
    ideas can be applied to understand our effect on the environment not just of Britain, but of the whole planet.

    LanguageEnglish
    PublisherHarperCollins UK
    Release dateJun 24, 2021
    ISBN9780008293642
    Ecology and Natural History

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      Book preview

      Ecology and Natural History - David Wilkinson

      Copyright

      William Collins

      An imprint of HarperCollinsPublishers

      1 London Bridge Street

      London SE1 9GF

      WilliamCollinsBooks.com

      This eBook edition published by William Collins in 2021

      Copyright © David M. Wilkinson 2021

      Photographs © David M. Wilkinson unless otherwise credited

      David M. Wilkinson asserts his moral right to be identified as the author of this work

      Cover art by Robert Gillmor

      A catalogue record for this book is available from the British Library.

      All rights reserved under International and Pan-American Copyright Conventions. By payment of the required fees, you have been granted the non-exclusive, non-transferable right to access and read the text of this eBook on-screen. No part of this text may be reproduced, transmitted, downloaded, decompiled, reverse engineered, or stored in or introduced into any information storage and retrieval system, in any form or by any means, whether electronic or mechanical, now known or hereinafter invented, without the express written permission of HarperCollins Publishers.

      Source ISBN: 9780008293635

      Ebook Edition © May 2021 ISBN: 9780008293642

      Version: 2020-06-07

      EDITORS

      SARAH A. CORBET, SCD

      DAVID STREETER, MBE, FRSB

      JIM FLEGG, OBE, FIHORT

      PROF. JONATHAN SILVERTOWN

      PROF. BRIAN SHORT

      *

      The aim of this series is to interest the general

      reader in the wildlife of Britain by recapturing

      the enquiring spirit of the old naturalists.

      The editors believe that the natural pride of

      the British public in the native flora and fauna,

      to which must be added concern for their

      conservation, is best fostered by maintaining

      a high standard of accuracy combined with

      clarity of exposition in presenting the results

      of modern scientific research.

      Contents

      Cover

      Title Page

      Copyright

      About the Editors

      Editors’ Preface

      Author’s Foreword and Acknowledgements

      1 The Entangled Bank

      2 Cwm Idwal and the Nature of the Environment

      3 Wytham: Questions about Life in a Deciduous Woodland

      4 Moor House: Thinking Big While Looking at the Very Small

      5 Windermere: An Introduction to the Nature of Ecosystems

      6 Competition on the Isle of Cumbrae

      7 Cooperation in the Cairngorms

      8 Can We Explain Selborne’s Swifts?

      9 Succeeding in Wicken Fen

      10 Wytham Revisited: Exploring the Ecological Niche

      11 Park Grass and the Hay Meadow Conundrum

      12 The View from Ringinglow Bog: Britain as a Microcosm of the Planet

      References

      Species Index

      General Index

      The New Naturalist Library

      About the Author

      About the Publisher

      Editors’ Preface

      This is the 143rd volume in the New Naturalist Library, every one of which has been in some degree ecological, and yet this is the first book in the series to take ecology itself as its subject matter. As the author says in his foreword, the book was in part inspired by the late Sam (R. J.) Berry’s Inheritance and Natural History (New Naturalist 61, 1977), which used examples from the British Isles to illustrate the science of genetics and evolution. Here, in this present volume of the series, Professor Dave Wilkinson uses British habitats to explicate the science of ecology. One could of course just as easily draw ecological and evolutionary object lessons from anywhere on the planet, and yet where else than in Britain can one reference and explore the very entangled bank that Charles Darwin wrote about in the Origin of Species , and see the very orchids that appear in his book on how these flowers are pollinated?

      Darwin’s Downe Bank is just one of the many ecological locations that you will visit in this book in the company of Professor Wilkinson. His evocative prose, ecological expertise and wonderful photographs combine to produce a fascinating tour that takes the reader simultaneously through the metaphorical and the actual landscapes of ecology. The tour begins at Downe Bank because here we see the ecological complexity that Darwin so clearly understood, and that provides the ecological theatre in which evolution performs its play. Each location visited in this book serves three functions. First, as at Downe Bank, there is its historical significance in the development of science; second, all are habitats chosen because they help us understand a facet of ecology particularly well (interactions among species in the case of Downe Bank); and third, every location has been studied for many decades, providing a clear record of ecological change, which is key to both understanding and conserving our environment. Many of the species that Darwin knew are still present in the environs of Downe – although, as Dave Wilkinson observes, today ‘the parakeets would have surprised him.’

      Ecology is the science of the relationships between organisms and their environment, including other organisms. It is in extreme environments such as at Cwm Idwal in Snowdonia (Chapter 2) that the physical constraints of temperature and soil become clearly visible, for example in the distribution of arctic–alpine plants. Next stop is Wytham Woods on the outskirts of Oxford, where the ecology of the woodland community has been intensively studied for three-quarters of a century. Many volumes in the New Naturalist Library would be the skinnier but for these studies at Wytham. The focus here is on what the wood tells us about the carbon cycle.

      At Moor House National Nature Reserve, we learn of the large significance of the very small. Decades of study at this upland site inform our knowledge of soil invertebrates and microbes. The concept of ecosystems is developed at Windermere, competition between species is revealed on the shores of the Isle of Cumbrae, and cooperation is uncovered in the Cairngorms. Gilbert White’s observations at Selborne in Hampshire are the starting point for studying long-term trends in animal populations, but this extends much further afield to the Bass Rock in Scotland and then to a nationwide, nine-decade record of the Grey Heron population in England and Wales. Successional changes in vegetation are seen at Wicken Fen in Cambridgeshire. There is much more yet, but this sketch of the breadth and depth of Ecology and Natural History should by now have made you impatient to embark at Chapter 1. Whether you are naturalist, ecologist or no -ist at all, this book will provide pleasure and a new perspective to anyone interested in natural history and its modern scientific scion – ecology.

      Author’s Foreword and Acknowledgements

      ‘W hat?’ and ‘which?’ are probably the most common starting questions in natural history. What species was that? Which species are found at this location? However, the obvious follow-on question is ‘why?’ Why is this particular species found here? Why are some sites more species-rich than others? It’s always been ideas – the ‘why’ questions – that particularly appeal to me, and my aim in this book is to introduce some of the basic ideas of the science of ecology, using examples from British natural history. Ecology is a science where natural history still plays a major role – not just in answering the ‘what’ questions such as identifying the insects in your sweep-net sample or moth trap, but in generating and answering the ‘why’ questions too. As Mary Willson and Juan Armesto (2006) put it, ‘Very commonly, it is simple natural history observations, planned or unplanned, that tweak the imagination into challenging existing dogma, asking novel questions, and seeing natural phenomena from a different perspective.’

      The contingencies of history make Britain a good country in which to base a book of this type. Ecology started to develop here earlier than in many other places, so many classic studies that helped develop the basic ideas of the subject were carried out at British sites. For example, some of the longest-running field experiments in ecology – such as the Park Grass Experiment at Rothamsted (started 1856) or the Godwin Plots at Wicken Fen (started 1927) – are found in Britain. Wytham Woods is arguably one of the best-studied deciduous woodlands in the world, after being acquired by the University of Oxford as a research site in the mid-twentieth century, and there has been almost a century of ecological research based at Windermere in the Lake District, making it one of the most intensively studied lakes around (Fig. 1). Each chapter in this book takes such a classic location as its starting point to introduce a range of ecological ideas – from nutrient cycling, to population ecology, to ecosystems and the workings of the whole Earth system. Although this is a book about the underlying science, the final chapter discusses a number of more applied issues, concerning the effects our species has been having on the wider environment.

      Over four decades ago R. J. (Sam) Berry wrote a similar book in this series on genetics called Inheritance and Natural History. Sam’s idea was similar to mine, to use examples from British natural history to illustrate the basic ideas of his science, in his case genetics; however, the complexities of his subject led to something that looked rather like a textbook (indeed, he said that the book was effectively his final-year lectures, with most of the maths removed). I have tried for something more accessible, while maintaining scientific rigour. So I have no mathematical equations, and I have adopted a policy of using graphs and tables sparingly, mainly illustrating the points with photographs of the species and habitats under discussion – the things you can see in the field. The references should allow the reader in need of more detailed technical background to know where to look. However, this book is similar to Sam Berry’s in another way as well. He wrote in his own foreword that, as a reasonably popular account, ‘it does not have to be as inclusive and balanced as a textbook’ in its choice of topics and examples. The same is true here. For example, neither predation nor parasitism has its own chapter (as would almost certainly be the case in a textbook), although they are covered in several of the chapters discussing other ecological ideas. I have focused mainly on questions in population and ecosystem ecology; a similar book could be written concentrating on ideas in behavioural and evolutionary ecology.

      This book has another key theme, namely the diversity of life involved in ecological processes. Indeed, Chapter 4 is entirely devoted to the importance of the microorganisms that make up most of the diversity of life on Earth, when viewed from either a genetic or a biochemical perspective. All naturalists are members of just one species – Homo sapiens, ourselves. Think about the implications of this for potential biases in understanding life on Earth. Our taxonomic chauvinism is easily seen if you look at TV natural history programmes, popular natural history books or even university-level ecology textbooks – they are all preferentially populated with organisms that are rather like us, such as mammals and birds (Kokko 2017). And yet in most ecological processes, microbes, fungi and plants are far more important than birds or mammals.

      FIG 1. Key sites discussed in the book. Sites used as chapter openings are shown in red, and other sites covered at some length (or used in multiple chapters) are shown in blue. At the scale of this map two other Peak District sites (Winnats Pass and the village of Eyam) are in an almost identical location to Ringinglow Bog. Most of these sites are public access – or can be seen from public paths and roads. Access to Wytham Woods is by permit, but these are readily available (a web search should find the current details). The Godwin Plots at Wicken Fen are on a part of the National Trust reserve with a visitor fee for non-members.

      I have tried to take examples from across the full range of biological diversity, as restricting the range of organisms that we use to think about ecology will necessarily restrict the range of answers we are likely to come up with. For example, we usually partition life into organisms that can photosynthesise and those that eat other organisms (incorrectly equating this split with the terms ‘plants’ and ‘animals’). However, many microorganisms do both, a lifestyle called mixotrophy. Imagine if a mixotrophic amoeba could write an ecology book – would it consider organisms that are so specialised that they can only get their energy by just one method strange exceptions to the more usual way of things? That amoeba is mixotrophic because it has algae living inside its cell, something that seems alien to us but is again common in many organisms, and deep in geological history gave rise to the evolution of plants. In that imaginary book authored by a protozoan, there might well be only one or two illustrations of vertebrates – creatures so far away from what the author considers normal that they are only discussed in passing as interesting oddities.

      The fact that all science is carried out by people not only affects how we view biodiversity, but necessarily means that it is ‘a socially embedded activity’ too (e.g. Gould 1981). Society’s norms cannot help but affect the way science and academia function. While this must be the case – scientists are people too – exactly how you interpret this insight has been something of a Pandora’s box, and the subject of much academic debate and misunderstanding (Latour 1999). However, this socially embedded insight has implications for a book such as this one which concentrates on introducing the basic ideas – for these were often developed decades ago, at a time when the sex ratio of most areas of science was strongly skewed towards males. In this respect, science was reflecting wider British society at the time. So many of the key studies I write about were carried out by men. This skewed sex ratio is no longer as apparent in academic ecology, and indeed the acknowledgements from this book provide a nice illustration of these changes. Friends and colleagues thanked for providing technical advice on draft chapters or helping with fieldwork form a population that is close to a 50:50 sex ratio – while those, mainly retired, thanked for helping with points of history by supplying memories of when they were young scientists are almost all male. However, even today if you stand before a group of British university students attending an ecology lecture it’s still likely that the audience will not match the wider country in its ethnic origins – suggesting that we are still missing out on much potential talent in ecology. This matters for all sorts of reasons, including that we need all the talent we can find. For ecology is a science that underpins our attempts to address many of the most challenging global problems.

      ACKNOWLEDGEMENTS

      Many people have helped with the process of writing this book. Jonathan Silvertown, Hannah O’Regan and Graeme Ruxton have read and commented on all the chapters. All three have had a significant role in improving the text. Jonathan (my editor as well as a plant ecologist and writer) was very influential in early discussions on how to structure such a book. Hannah is my wife and has provided support and encouragement through the multi-year writing process. Graeme read all the chapters purely out of interest and friendship – his enthusiasm for this book is greatly appreciated. The following commented on chapters in their particular area of expertise: Tom Barker, Sarah Dalrymple, Jane Fisher, Stuart Humphries, Edward Mitchell and other members of the soil ecology group at the University of Neuchâtel, Tom Sherratt and Sandra Varga. Several people provided comments on various points of detail: Richard Betts, Martha Crockett, Stefan Geisen and Becky Yahr. Help and/or advice with fieldwork came from Charles Deeming, Jenny Dunn, Paul Edy, Sarah Elton and Anne-Marie Nuttall. Historical information (recollections of when they were younger) came from Bill Block, John Coulson, Alastair Fitter, Ian Hodkinson, Brian Huntley, John Lawton, Penny Oakland, Donald Pigott and Richard West. It takes many talented people to turn a manuscript into a finished book – including Myles Archibald and Hazel Eriksson at HarperCollins, and Namrita and David Price-Goodfellow of D & N Publishing. Lisa Footit did a great job compiling the index, while Hugh Brazier’s contribution merits particular acknowledgement, as it went well beyond the grammatical tweaks normally associated with the role of copy-editor. The photos are my own, except for three by the late Bryan Nelson (thanks to June Nelson for supplying these), two by Becky Yahr, one by Angela Creevy (from our work at Mere Sands Wood nature reserve), and one by my father Lionel Wilkinson – who sadly didn’t live to see the completed book. This book is dedicated to Hannah with love, and in memory of Lionel.

      CHAPTER 1

      The Entangled Bank

      It is an afternoon of uncertain weather in early May. The clouds repeatedly thickening, promising rain, then breaking to reveal patches of blue – allowing the sun to briefly warm the ant hills on the grassy slope in front of me. I am sitting on a small fragment of chalk grassland on the side of the Cudham valley in Kent. This remnant of a previously much more diverse countryside is now just a thin sliver of open habitat on an otherwise wooded slope ( Fig. 2 ). The valley bottom below me illustrates the twentieth-century fate of much of this species-rich vegetation – converted to bright green fertilised fields, species-poor and dominated by grass. Fields ‘improved’, in the language of modern agriculture, although that’s not how a plant conservationist would see it.

      FIG 2. Downe Bank nature reserve in the Cudham Valley, Kent, in early May 2019. A short walk from Down House, and a favourite afternoon walk for Charles and Emma Darwin.

      At first glance the vegetation on the bank looks simple, comprising grassland with a small path running between ant hills. Take a closer look, however, and it reveals itself as a complex tangle of mosses, grasses and a wide variety of herbs. There are nine species of orchid growing here, although this early in the year only Common Twayblade is flowering (Fig. 3). The orchid-rich character of this bank is not new – in the 1860s, obsessed by the complexities of orchid reproduction, Charles Darwin fondly referred to this site as his ‘orchis bank’. The name fitted the site well in the 1860s when many more British orchid species were placed in the genus Orchis than is the case today, and orchis was also used in the English name of several orchid species. The Darwins lived in Downe village only a short walk away, and the orchis bank was the objective of a favourite afternoon walk for Charles and his wife Emma, and possibly one of the inspirations for Darwin’s metaphor of an ‘entangled bank’ that famously appears in the final paragraph of On the Origin of Species. Today it’s still a quiet stroll from Downe along a footpath beside the Cudham Road to reach the reserve. As a classic description of the complexity of nature that ecologists strive to understand, Darwin’s entangled bank still works today. Indeed, this book is an introduction to the ideas that help us start to untangle the entangled bank, and understand the complexities of ecology.

      FIG 3. Common Twayblade Neottia ovata flowering on Downe Bank, one of nine species of orchids that grow on Darwin’s orchis bank.

      Darwin (1859) closed one of the most famous books in the history of science with this description of the English countryside he knew:

      It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.

      Many of the banks and field margins around Downe in the mid-nineteenth century would have matched this description, and while Downe Bank is often cited as the origin of the entangled bank (simplified to a ‘tangled bank’ in later editions of The Origin), Janet Brown (2002), one of Darwin’s most important biographers, rightly deploys the caution and scepticism of a good historian, writing that the orchis bank ‘may well have been the same sweetly tangled bank’. Whatever its role in inspiring Darwin’s classic image of biodiversity, the Darwin connection gives Downe Bank added importance in British natural history, and it was the first reserve acquired by the Kent Wildlife Trust, in 1962.

      That day in May Brimstone butterflies Gonepteryx rhamni were ‘flitting about’, yellow against the bright green of the unfolding leaves on the coppice at the base of the bank, while overhead flew Ring-necked Parakeets Psittacula krameri. While much has not changed on the bank since Darwin’s time, the parakeets would have surprised him. It’s a bird he probably never saw in the wild – its native range is central Africa, India and nearby countries (places he never visited on the Beagle voyage). There were some escapes from captivity into the wild in Britain, including a population that survived for a few years in Norfolk around the time of Darwin’s death, but our current population appears to have originated from multiple escapes of captive birds during the twentieth century (Heald et al. 2020). This colourful intrusion of tropical diversity into the Kent countryside illustrates one of the many ways in which humans have changed the ecology of the planet – moving large numbers of organisms to locations far from their original home – a topic returned to in Chapter 12.

      WHAT IS ECOLOGY?

      Using Darwin’s metaphor of an entangled bank to introduce the science of ecology raises a basic question – what do we mean by ecology? The word ‘ecology’ gets used in multiple ways, several of which are compatible with using Darwin’s orchis bank as the opening example in this book. Say ‘ecology’ to many people and they will often think of nature conservation and attempts to address a whole host of environmental problems – from ‘saving the whales’ when I was growing up in the 1970s, to fears that the whole planet is potentially now in need of saving. This book doesn’t address ecology in this sense (except in the final chapter, which gives a necessarily selective brief overview of some aspects of ecology as applied to environmental problems). Instead it focuses on the science of ecology – especially the ecological concepts that attempt to explain the workings of Darwin’s entangled bank. Why is this bank (and the world at large) so diverse, and what are the relationships between the plants of many kinds, the birds singing on the bushes, and the various insects flitting about, not to mention the worms crawling through the damp earth and a host of other small and often overlooked organisms? The importance of microorganisms and fungi in ecological processes is one of the recurring themes in this book. Key ecological processes are often driven by obscure organisms that seldom feature in TV natural history documentaries or books and magazine articles written by, and for, naturalists (Fig. 4).

      The term ‘ecology’ was coined (originally as ‘oecologie’) by Ernst Haeckel and popularised in a widely read book of 1866 where he defined ecology as ‘the whole science of the relations of the organism to the environment’ (Egerton 2012). Our modern definitions are still very similar – for example, ‘the scientific study of the abundance and distribution of organisms in relation to other organisms and environmental conditions’ (Ricklefs & Relyea 2014). Haeckel certainly didn’t singlehandedly invent ecology; as so often in the history of science, it is impossible to identify a clear starting date, and many people were starting to do what we would now consider ecological studies long before 1866 (one of many examples was Gilbert White; see Chapter 8). However, giving this area of science a name and a formal definition obviously helped focus people’s attention on this way of thinking about the world, although it was the end of the nineteenth century before ecology started to become a growing area of research.

      FIG 4. Lichen-covered rock on Cadair Idris, Snowdonia. The large pale green lichen is Map Lichen Rhizocarpon geographicum (the grey patches are other species of lichens). The main body of a lichen is made up of fungus, but the ‘organism’ itself is a composite of fungus and a range of species of microbes (see Chapter 7).

      In Britain, plant ecology developed more rapidly than animal ecology, with Arthur Tansley being a particularly influential figure in the early decades of the twentieth century. It would however be incorrect to assume that the key driver for ecological research has always been the sort of environmental concern that people would now often attribute to scientists working in this area. As the science historian Peter Bowler (1992) has pointed out, many early ecological studies were ‘often initiated by scientists who hoped to modify the natural balance in order to allow sustainable exploitation’. This was particularly the case for some of the early ecological researchers in the United States of America, and it is still important – for example in applying ecological ideas to try and formulate a more sustainable agriculture. Whatever the underlying reason for studying ecology, the science is underpinned by a number of basic ideas, and the aim of this book is to introduce some of these using examples drawn from British natural history. The rest of this opening chapter discusses a couple of very basic and fundamental concepts – with many more detailed ideas considered in the subsequent chapters.

      ISLANDS OF ORDER IN A SEA OF CHAOS

      An obvious question in ecology is this – where does an organism get its energy from? Common answers are that the energy comes from photosynthesis, or that it comes from eating other organisms – be that as a herbivore eating plants (Fig. 5) or as a carnivore eating other animals. As discussed in more detail later in this book, however, other options are available, such as feeding on dead biological material (Fig. 6) or mixing photosynthesis with consuming other organisms.

      To understand why the acquisition of energy is such a fundamental process in ecology requires a brief excursion into the ideas embodied in an area of science called thermodynamics. This sounds a long way from anything to do with natural history, but as a science that’s all about the movement of energy in the form of heat it’s very relevant to many ‘why’ questions in natural history. The early history of thermodynamics makes this obvious. It was developed as an area of theory applicable to steam engines – how they are fuelled (effectively, what they eat) and how efficiently they can use these sources of power. These are also questions we can ask about the plants, birds and worms in Darwin’s entangled bank.

      Fundamental to understanding energy use by organisms (or steam engines) is the idea of the second law of thermodynamics. This ‘is of central importance in the whole of science, and hence in our rational understanding of the universe, because it provides a foundation for understanding why any change occurs’ (Atkins 2007). Peter Atkins, who wrote these words, is a distinguished physical chemist, but the second law is just as central to ecology as it is to chemistry. This should not be a surprise when you consider that metabolism (the processes in cells by which energy is stored or released) is a chemical process. This approach to thinking about thermodynamics and ecology, described below, is a brief summary of a somewhat more technical discussion in my earlier book Fundamental Processes in Ecology: an Earth Systems Approach (Wilkinson 2006).

      FIG 5. Alder Leaf Beetles Agelastica alni feeding on Alder Alnus glutinosa leaves at Askham Bog on the outskirts of York in 2019. This beetle was thought to have become extinct in Britain in the 1940s, but it reappeared in 2004, and is now becoming common at some locations in northern England.

      FIG 6. Turkeytail Trametes versicolor in Bunny Old Wood, Nottinghamshire. A very common fungus growing, and feeding, on dead wood. This is a very variably coloured species, and one of the commonest polypores in Europe (Kibby 2017).

      Look in a physics text or popular science book (such as the one by Peter Atkins from which I have just quoted) and you will see that the second law is usually defined in relation to a concept called ‘entropy’. One way to think of entropy is that it is a measure of the disorder, or our lack of information, about the state of things – this is a horribly inexact way of describing entropy, but any attempt at anything more rigorous quickly becomes rather technical (Thorne & Blandford 2017). One brief way of stating the second law of thermodynamics is that ‘the entropy of the universe is always increasing’ – so we should expect things to become more disordered over time. This certainly seems true of my study as I am working on a book, and physics suggests it is true of the universe as a whole. However, as naturalists we can sidestep ideas of entropy and instead use a more intuitive concept called ‘free energy’. This is simply the amount of energy available for doing useful work – for example, walking to the bookshelves behind me as I write, to check physics textbooks in an attempt to make sure what I am writing is at least approximately correct (give or take the odd simplification).

      Why should free energy matter to an ecologist? Think about the look of an organism such as the orchid in Figure 3 or the beetles (or the Alder leaf) in Figure 5, or look at yourself in a mirror – all these examples look ordered, and in many cases they are bilaterally symmetrical too (i.e. one side is a mirror image of the other). In a universe dominated by the second law, how do such organisms manage to stay so organised rather than falling apart? In a neat phrase, Lynn Margulis and Dorion Sagan (1995) encapsulated this idea as organisms representing ‘islands of order in an ocean of chaos’. The way organisms maintain this order – and temporarily save themselves from death and the ocean of chaos – is to use energy from their environment to maintain their ordered structure. As no organism, or machine, can be 100 per cent efficient (this follows from thermodynamics too), then all organisms must also be producing some waste. The ecological implications of these fundamental ideas from physics can be summarised as:

      energy → organism → waste

      FIG 7. The pyramid of free energy production (after Lineweaver & Egan 2008). Traditionally in ecology a pyramid – with photosynthetic organisms at the base, a smaller number of herbivores above, and an even smaller number of carnivores above them – is used to show the amount of energy available for use as one moves along food chains. This figure is effectively a more fundamental version of the same idea – applicable to any planet with life. Energy can come either from the planet’s star (in our case the sun) or from chemical sources. The light can be used as a source of energy by photosynthetic organisms (phototrophs), while chemical sources are used by chemolithotrophs. These organisms can then provide food (and energy) for heterotrophs. See Chapter 3 for a more detailed discussion of food chains and energy sources.

      So all living things are using their environment as a source of energy (and other resources) while also using their environment as a dumping ground for waste products; physics gives them no other option. Much of ecology, from food chains to nutrient cycling, involves sorting out the detailed implications of this very simple idea. As described in more detail in later chapters, the source of energy can be sunlight (for photosynthetic organisms), chemical sources of energy, or obtaining energy by eating other organisms (Fig. 7). All of these sources of energy are being utilised on Darwin’s orchis bank – and indeed in the majority of ecological systems. There are occasional apparent exceptions to this rule: for example, if you are a caving naturalist then you will be studying systems where there is no light, and so no photosynthesis – although products of photosynthesis may still have entered the cave from sunlit systems outside (Fig. 8).

      FIG 8. Herald moths Scoliopteryx libatrix hibernating in a cave in southern Cumbria. By entering the cave they provide a potential food source for cave-living organisms based on photosynthesis outside the cave – because their caterpillars feed on a number of different tree species.

      TRADE-OFFS – A KEY ECOLOGICAL CONCEPT

      Leaving Darwin’s bank, if you walk back along Cudham Road to the village of Downe you will find Darwin’s former home, Down House, which is now a museum. In Darwin’s day the gardens were a mix of flower gardens, kitchen gardens, multi-compartmented greenhouse, along with orchard, meadow and small fragments of woodland. These gardens provided not only outdoor space for the family and a source of fresh vegetables for the kitchen, but also a place where Darwin could walk and think, and a site for his experiments. The wooden superstructure of the greenhouse at Down House was replaced around 1900, but the basic layout and the brick walls are as used by Darwin, who had wide-ranging botanical interests, one of which was carnivorous plants. Today, as in Darwin’s time, part of the greenhouse is filled with carnivorous plants from around the world (Fig. 9). These plants form a charismatic introduction to ideas of trade-offs in ecology and evolutionary biology, as their leaves have to perform two rather different functions – namely photosynthesis and trapping their prey. By a trade-off we mean ‘any situation in which the quality of one thing must be decreased for another to increase’, although the plant ecologist Peter Grubb (2016), who favours this definition, does note that sometimes ‘trade-off’ has been used in ecology in a looser and less rigorous way. Before discussing this in more detail I will use a non-natural history example to introduce the key idea of trade-offs, using sports science to introduce the basic idea before applying it to carnivorous plants.

      FIG 9. The greenhouse at Down House, showing a range of carnivorous plants, including tall tropical pitcher plants and smaller sundews. Although sundews grew in the wild near Down House, Darwin brought them into the greenhouse to make his experiments easier to carry out.

      Anyone who has ever taken even a passing interest in sport will be familiar with the idea that different types of physique tend to be associated with success in different sports. If you were confronted with three high-standard sports people and told that one was a basketball player, another a jockey and the third a sumo wrestler, you would have little difficulty in guessing which one was which. The fact that different sports are associated with different physiques means that there is no one body type that can excel in all sports. A sport that requires substantial physical bulk tends to rule out also excelling as a jockey (think of the poor horse!) or as a rock climber, where too much weight is a disadvantage when fighting gravity on overhanging rock (Fig. 10). One way to make such arguments more quantitative – a favoured approach of most scientists – is to use data on the performance of top-class decathletes, where they have to compete in ten different events in order to win. Raoul Van Damme and colleagues (2002) analysed the performance of 600 ‘world class decathletes’ and found nice illustrations of the importance of trade-offs. For example, athletes who did really well at the distance running (1,500 m) tended to do less well in the shot put, and vice versa. This is unsurprising, since if asked to picture a typical distance runner most people will visualise a much thinner athlete than if asked to picture a typical shot-put specialist. The decathlon is interesting as it tends to select for athletes who are rather good in all the events – that’s how you win – but may not necessarily be top in any particular event. The key point for thinking about natural history is the authors’ conclusion that ‘in an environment in which the selection criterion is combined high performance across multiple tasks, increased performance in one function may impede performance in others’. That is effectively Peter Grubb’s preferred definition of trade-off, as given above.

      FIG 10. Nigel Smart climbing the route ‘Cave Eliminate’ on Stanage Edge in the Peak District. The main work done by a rock climber is to move his or her body weight upwards, against gravity. As muscle is a heavy tissue, what really matters is the climber’s power-to-weight ratio, and there is a trade-off between amount of muscle and body weight – the more muscle you have the more weight you need to move upwards against the force of gravity, but without at least some muscle you will make no progress at all.

      With this conclusion in mind we can return to thinking about the carnivorous plants that so interested Darwin. His fascination with such plants grew from simple natural history observation:

      During the summer of 1860, I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex. I had heard that insects were thus caught, but knew nothing further on the subject. (Darwin 1875)

      The sundew (the usual English name for this species is now Round-leaved Sundew), along with other species of insectivorous plants, quickly made its way into Darwin’s greenhouse, and he started a long series of experiments so that he came to know substantially more on the subject. My own experience suggests that 160 years later this is still a good way of doing biology, seeing something in the field, or down a microscope, and thinking ‘I wonder how that works?’ Alternatively, sometimes it can be reading about something and asking ‘how does that work?’ or ‘can that explanation really be correct?’ Darwin realised that many of these

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