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Biodiversity G. Perry GY3155, 2790155 2011 Undergraduate study in Economics, Management, Finance and the Social Sciences This is an extract from a subject guide for an undergraduate course offered as part of the University of London International Programmes in Economics, Management, Finance and the Social Sciences. Materials for these programmes are developed by academics at the London School of Economics and Political Science (LSE). For more information, see: www.londoninternational.ac.uk This guide was prepared for the University of London International Programmes by: Dr G. Perry, BSc, MSc, PhD, Senior Lecturer, The School of Geography, Geology and Environmental Science, University of Auckland, New Zealand. This is one of a series of subject guides published by the University. We regret that due to pressure of work the author is unable to enter into any correspondence relating to, or arising from, the guide. If you have any comments on this subject guide, favourable or unfavourable, please use the form at the back of this guide. University of London International Programmes Publications Office Stewart House 32 Russell Square London WC1B 5DN United Kingdom Website: www.londoninternational.ac.uk Published by: University of London © University of London 2008 Reprinted with minor revisions 2011 The University of London asserts copyright over all material in this subject guide except where otherwise indicated. All rights reserved. No part of this work may be reproduced in any form, or by any means, without permission in writing from the publisher. We make every effort to contact copyright holders. If you think we have inadvertently used your copyright material, please let us know. Contents Contents Chapter 1: Introduction ......................................................................................... 1 Introduction .................................................................................................................. 1 Aims of the course.......................................................................................................... 2 Learning outcomes......................................................................................................... 2 The structure of the subject guide................................................................................... 3 Syllabus.......................................................................................................................... 3 Reading advice............................................................................................................... 4 Online study resources.................................................................................................... 6 Activities........................................................................................................................ 8 Examination................................................................................................................... 8 Chapter 2: The ecosystem concept and scale........................................................ 11 Aims of the chapter...................................................................................................... 11 Learning outcomes....................................................................................................... 11 Essential reading.......................................................................................................... 11 Introduction................................................................................................................. 12 The ecosystem concept................................................................................................. 12 Components of ecosystems........................................................................................... 12 Flows of energy............................................................................................................ 13 Ecosystems and the ‘balance of nature’ ........................................................................ 15 Looking at the ecosystem: the importance of scale........................................................ 16 Summary...................................................................................................................... 17 A reminder of your learning outcomes........................................................................... 17 Sample examination questions...................................................................................... 17 Chapter 3: Species and speciation........................................................................ 19 Aims of the chapter...................................................................................................... 19 Learning outcomes....................................................................................................... 19 Essential reading.......................................................................................................... 19 Further reading............................................................................................................. 19 Introduction................................................................................................................. 20 The taxonomic hierarchy............................................................................................... 20 Defining species: the species concept............................................................................ 21 Geographic variation and speciation............................................................................. 22 Extinction..................................................................................................................... 24 Species richness: how many species are there?.............................................................. 24 The Procrustean fallacy?............................................................................................... 25 Summary...................................................................................................................... 25 A reminder of your learning outcomes........................................................................... 25 Sample examination questions...................................................................................... 25 Chapter 4: Historical biogeography: patterns of global diversity......................... 27 Aims of the chapter...................................................................................................... 27 Learning outcomes....................................................................................................... 27 Essential reading.......................................................................................................... 27 Further reading............................................................................................................. 28 Introduction ................................................................................................................ 28 i 155 Biodiversity Distribution of species.................................................................................................. 28 Disjunct and relict populations...................................................................................... 28 Explaining disjunct distributions.................................................................................... 29 Vicariance biogeography............................................................................................... 31 Current-day patterns of species richness....................................................................... 32 The latitudinal diversity gradient (LDG).......................................................................... 33 Species–energy relationships........................................................................................ 34 Summary...................................................................................................................... 35 A reminder of your learning outcomes........................................................................... 36 Sample examination questions...................................................................................... 36 Chapter 5: Island ecosystems, island biogeography and reserve design.............. 37 Aims of the chapter...................................................................................................... 37 Learning outcomes....................................................................................................... 37 Essential reading.......................................................................................................... 37 Further reading............................................................................................................. 37 Introduction................................................................................................................. 38 Islands and biogeography............................................................................................. 38 Characteristics of islands.............................................................................................. 38 Assembly of insular communities.................................................................................. 39 Evolutionary patterns.................................................................................................... 40 A case study: the Hawai’ian islands............................................................................... 41 The species–area relationship (SAR).............................................................................. 42 Building on the SAR: island biogeography..................................................................... 42 ETIB rules for reserve design: single large or several small (SLOSS)................................. 44 Summary...................................................................................................................... 45 A reminder of your learning outcomes........................................................................... 46 Sample examination questions...................................................................................... 46 Chapter 6: Population regulation: limits to growth and life-history trade-offs.... 47 Aims of the chapter...................................................................................................... 47 Learning outcomes....................................................................................................... 47 Essential reading.......................................................................................................... 47 Introduction................................................................................................................. 47 Population growth and density-dependence.................................................................. 48 The exponential model................................................................................................. 48 Limits to growth........................................................................................................... 50 The logistic model........................................................................................................ 51 Population models: summary........................................................................................ 52 Life-history traits and the ideal organism?..................................................................... 52 Life-history trade-offs.................................................................................................... 53 Summary: life-history traits............................................................................................ 55 A reminder of your learning outcomes........................................................................... 55 Sample examination questions...................................................................................... 55 Chapter 7: Interspecific interactions: competition and predation........................ 57 Aims of the chapter...................................................................................................... 57 Learning outcomes....................................................................................................... 57 Essential reading.......................................................................................................... 57 Introduction................................................................................................................. 57 Interspecific interactions: species diversity and niches.................................................... 58 Competition and resources........................................................................................... 59 Predator–prey interactions............................................................................................ 63 ii Contents Interactions: a summary................................................................................................ 67 A reminder of your learning outcomes........................................................................... 67 Sample examination questions...................................................................................... 67 Chapter 8: Succession and climax: temporal dynamics in ecological communities.......................................................................................................... 69 Aims of the chapter...................................................................................................... 69 Learning outcomes....................................................................................................... 69 Essential reading.......................................................................................................... 69 Further reading............................................................................................................. 69 Introduction ................................................................................................................ 70 Successional change..................................................................................................... 70 Succession: primary versus secondary succession........................................................... 70 Primary succession....................................................................................................... 71 Secondary succession................................................................................................... 72 Models of succession.................................................................................................... 73 Current views: do climax communities actually exist?.................................................... 76 Summary...................................................................................................................... 77 A reminder of your learning outcomes........................................................................... 77 Sample examination questions...................................................................................... 77 Chapter 9: Equilibrium and non-equilibrium models of biodiversity.................... 79 Aims of the chapter...................................................................................................... 79 Learning outcomes....................................................................................................... 79 Essential reading.......................................................................................................... 79 Further reading............................................................................................................. 79 Introduction................................................................................................................. 80 The equilibrium view..................................................................................................... 80 Equilibrium models....................................................................................................... 81 Environmental variability and non-equilibrium models................................................... 83 Comparing equilibrium and non-equilibrium models...................................................... 87 Summary...................................................................................................................... 87 A reminder of your learning outcomes........................................................................... 88 Sample examination questions...................................................................................... 88 Chapter 10: Conclusions: where are we going?.............................................................................................................. 89 Aims of the chapter...................................................................................................... 89 Learning outcomes....................................................................................................... 89 Essential reading.......................................................................................................... 89 Further reading............................................................................................................. 89 Introduction................................................................................................................. 89 Themes in biogeography and ecology............................................................................ 90 Where now for biogeography?...................................................................................... 91 Conclusions.................................................................................................................. 93 A reminder of your learning outcomes........................................................................... 93 Appendix 1: Activities............................................................................................ 95 Activity 1: Cemetery demography .................................................................. 95 Activity 2: Patterns in the spatial distribution of plants................................. 95 Activity 3: Naturalness and equilibrium........................................................ 100 Activity 4: Population dynamics models........................................................ 100 Activity 5: Courseware: internet-based ecological teaching software.......... 101 Activity 6: Financial incentives for protecting biodiversity........................... 101 iii 155 Biodiversity Appendix 2: Works cited...................................................................................... 103 Appendix 3: Sample examination paper 1.......................................................... 107 Appendix 4: Sample examination paper 2.......................................................... 109 Appendix 5: Guidance on answering Sample examination paper 1.................... 111 Section A questions.................................................................................................... 111 Section B questions.................................................................................................... 113 iv Chapter 1: Introduction Chapter 1: Introduction Introduction Few would dispute that we face a biodiversity ‘crisis’. Species are currently becoming extinct 100 to 1,000 times faster than the background rate (i.e. the ‘normal’, or average, rate of extinction outside mass extinction events – see Chapter 3) as estimated from the palæoecological record. There is a need for improved scientific understanding and informed scientific debate to manage and mitigate this global problem. This subject guide addresses issues of relevance to the biodiversity debate. It introduces patterns of biodiversity across different space and time scales and looks at some of the major processes that have created and maintained these patterns. Biogeography and ecology have a long tradition. Serious interest in how the natural world ‘works’ can be traced back to the ancient Greeks and the writings of philosophers such as Plato and Aristotle. Modern biogeography and ecology date back to the eighteenth century, and they are now wide, and growing, fields of scientific and social significance. Cox and Moore (2005, Chapter 2) and Lomolino et al. (2010, Chapters 1 and 2) give interesting histories of the discipline of biogeography. Biogeography and ecology are concerned with the distribution and abundance of organisms and their habitats in time and space and, as such, have a very broad remit. The types of questions that biogeographers and ecologists ask include: • What allows a species to live where it does, and what prevents it from colonising other areas? • Why are the animals and plants of large, isolated regions such as Australia and Madagascar so distinctive? • Why are there so many more species in the tropics than in temperate latitudes? This 300 course can be divided into three parts. First, we will look at what we mean by biogeography and ecology and at some of the types of questions ecologists ask. We will also consider the ecosystem as a fundamental unit of study and we will examine the significance of scalerelated issues for ecological research. Secondly, we will consider historical biogeography and the key issues relating to speciation, large-scale patterns of distribution in time and space, and island biogeography. Thirdly, we will consider ecological biogeography, focusing on the processes that might help to explain the fine-scale distribution and abundance of organisms, and on the topics of competition, predation and succession. The entire syllabus is structured around the question: ‘Why are there so many species?’ Knowledge of biogeographic and ecological processes underpins sustainable use and management of the environment. The debate surrounding sustainable development has a clear ecological component. Although we will not specifically consider management issues in this course, you should be able to apply the material covered in this course in a critical manner to specific environmental issues and problems. The syllabus for this course is wide ranging, but it is not intended to be allembracing. Among the topics that we will not explicitly consider are: specific field or laboratory methods/techniques, palæoecology, soils and plant-soil relations, conservation biology and in-depth ecological modelling. 1 155 Biodiversity Aims of the course This course provides you with a broad background in the principles and theoretical underpinnings of the scientific study of biodiversity. This serves to give you a solid conceptual knowledge in the disciplines of biogeography and ecology, building on the material covered in the prerequisite course 149 Biogeography. Specific aims of the course are to: • identify patterns of biodiversity at regional and global levels and at small (local) scales • enable you to consider the various concepts of biodiversity, the processes generating and maintaining biodiversity, and the issues surrounding the conservation of biodiversity for the future • consider how the processes generating biodiversity interact across spatial and temporal scales. The aim of this course is to introduce you to the scientific study of biological diversity. You will consider the various concepts of biodiversity, the processes generating and maintaining diversity, and the issues surrounding the conservation of biodiversity for the future. Biogeographers and ecologists have been concerned with explaining ecosystem structure, composition and function across a full continuum of spatio-temporal scales. This course will consider patterns and processes from the largest scales (historical biogeography) to the smallest (ecological biogeography). Regional and larger scale patterns of biodiversity, usually the result of evolutionary and geological factors, will be discussed as will the ecological processes and interactions that drive patterns at smaller (local) scales. I assume that you have a basic familiarity with rudimentary biological concepts, but beyond that no knowledge is expected, although some familiarity with basic mathematics will be helpful. Along with the bare facts outlined in this subject guide, I have presented a few specific case studies and examples. As this is one area in which you will need to try to focus your reading, the textbooks mentioned below all contain a wealth of such examples. A list of relevant reading material is presented at the start of each chapter, from both the main textbooks and also from relevant journal articles. Each chapter ends with a list of Learning outcomes, which provide an indication of what you should be able to do once you have finished studying the material in each chapter, and a list of Sample examination questions related to the material in the chapter. Some chapters have simple exercises associated with them: these are designed to help make some of the more abstract concepts more accessible (e.g. mathematical models of population growth). Learning outcomes After you have worked your way through the subject guide, completed the essential reading and learning activities, you should have: • foundation knowledge of the range of autecological, population and community processes that are responsible for the variation and maintenance of patterns of biodiversity across different spatial and temporal scales 2 Chapter 1: Introduction • an appreciation of the role of biodiversity in ecosystem functioning in general • an appreciation of the complexity of ecological systems and of some of the different ways in which this complexity has been conceptualised. The structure of the subject guide The subject guide is divided into 10 (short) chapters. The first part of the guide (Chapters 2 to 4) is concerned with large-scale patterns and processes (those traditionally addressed by biogeography), and the remainder of the guide is concerned with smaller scales (those covered by ecology). It is important to remember that this guide is intended as a ‘jumping off’ point and needs to be supplemented by additional reading (as discussed below and listed at the start of each chapter). At the end of each chapter is a list of Learning outcomes. When you have completed a chapter of the guide, and the essential reading for the material covered, you should read over this list and reflect on whether what you have learnt fulfils those outcomes. A good test of your learning would be whether you could explain the concepts we cover to an intelligent layperson who may not be as familiar with them as you are. Syllabus The questions ‘Why are there so many species?’, or, conversely, ‘Why aren’t there just a very few, very widely distributed, dominant species?’ remain at the forefront of contemporary ecology; satisfactorily resolving them is of conceptual and practical importance. This course considers these questions from a range of different perspectives. It considers the various concepts of biodiversity, the processes generating and maintaining biodiversity, and the issues surrounding the conservation of biodiversity for the future. At regional and global levels, patterns of biodiversity are usually the result of evolutionary and geological factors while at smaller (local) scales they are the result of ecological processes and interactions. Therefore, consideration will be given to the processes generating and maintaining biodiversity at a wide range of spatiotemporal scales (from single years to millions of years and from individual organisms to the entire globe). This course provides the necessary background to understand some of the most important problems in contemporary biogeography and ecology and to understand other important principles and theories in biogeography and ecology. Specifically, the course covers: • The ecosystem concept and scale • Species and speciation • Historical biogeography: patterns of global diversity • Island ecosystems, island biogeography and reserve design • Population regulation: limits to growth and life history trade-offs • Interspecific interactions: competition and predation • Succession and climax: temporal dynamics in ecological communities • Equilibrium and non-equilibrium models of biodiversity • Conclusions: where are we going? 3 155 Biodiversity Reading advice Essential reading There are many textbooks and journals devoted to the topics covered in this course. Those which we will make use of are: Begon, M., C.R. Townsend and J.L. Harper Ecology: From Individuals to Ecosystems. (Oxford: Blackwell, 2006) fourth edition [ISBN 9781405111171]. Cox, C.B. and P.D. Moore Biogeography: An Ecological and Evolutionary Approach. (Oxford: Blackwell, 2005) seventh edition [ISBN 9781405118989]. Lomolino, M.V., B.R. Riddle and J.H. Brown Biogeography. (Sunderland, Mass.: Sinauer Associates, 2010) fourth edition [ISBN 9780878934942 (hbk)] . Detailed reading references in this subject guide refer to the editions of the set textbooks listed above. New editions of one or more of these textbooks may have been published by the time you study this course. You can use a more recent edition of any of the books; use the detailed chapter and section headings and the index to identify relevant readings. Also check the VLE regularly for updated guidance on readings. Journals There are many journals dedicated to the biogeographic and ecological sciences. Some of the most important ones include: • American Naturalist • Annual Review of Ecology and Systematics • Conservation Biology • Ecological Applications • Ecological Monographs • Ecology • Journal of Animal Ecology • Journal of Biogeography • Journal of Ecology • Oikos • Progress in Physical Geography • Trends in Ecology and Evolution (TREE). The Annual Review of Ecology and Systematics, Progress in Physical Geography and TREE provide good ‘state-of-the-art’ reviews, while the other journals are mainly concerned with publishing the results of primary research. To help you read extensively, all University of London International Programmes students have free access to the University of London Online Library where you will find the full text or an abstract of some of the journal articles listed in this guide. You will need a username and password to access this resource. Details can be found in your Student Handbook or online at: www.external.shl.london.ac.uk. Each chapter contains references to material in these textbooks and draws to a greater or lesser extent on the material covered in them. You should try to read some of the case-study material presented in the chapters and use the material covered in the textbooks as a point of departure to the primary literature (i.e. journals). Much of the material that we will cover is 4 Chapter 1: Introduction contained in other biogeography and ecology textbooks, so if you are unable to access some of the books listed above, then other sources will most likely suffice. Referencing of material within the chapters themselves is limited; instead a reading list (both material from the core texts and other relevant journal material) is provided at the end of the subject guide in Appendix 2. If you are to understand the material covered in this subject guide, you need to make every effort to read the essential reading material listed at the start of each chapter. It is not sufficient just to read the subject guide – indeed its purpose is really to help you read and synthesise the wealth of information that is available on all of the topics we will discuss. Further reading The complete list of all the references referred to in the main body of the text at the end of the subject guide (Appendix 2) has not been divided into chapters. This is because some of the material is referenced in more than one chapter, and I do not want you to think that biogeography and ecology can be easily divided into discrete self-contained parcels. When you think about the material introduced here, it is important that you consider links between the different topics. Although it is not essential that you read all of this extra material, reading primary research (in journals) will provide you with a good introduction to the types of research ecologists and biogeographers are engaged in. Some of the older papers are likely to be difficult to obtain – they are included here for completeness. They will also allow you to see how some of the concepts introduced in the subject guide might be applied and developed. In terms of the material considered in this introductory chapter, Lomolino et al. provide a good introduction and overview of the scope and history of biogeography in Chapters 1 and 2 of their book. Finally, there are two popular science books that contain much useful and related information, which are well worth reading if you can get a copy of them: Quammen, D. Song of the Dodo: Island Biogeography in an Age of Extinctions. (London: Hutchinson, 1996) [ISBN 9780712673334]. Wilson, E.O. The Diversity of Life. (London: Penguin Press Science, 2001) [ISBN 9780140291612]. Please note that as long as you read the Essential reading you are then free to read around the subject area in any text, paper or online resource. You will need to support your learning by reading as widely as possible and by thinking about how these principles apply in the real world. To help you read extensively, you have free access to the virtual learning environment (VLE) and University of London Online Library (see below). Unless otherwise stated, all websites in this subject guide were accessed in 2008. We cannot guarantee, however, that they will stay current and you may need to perform an internet search to find the relevant pages. Other useful texts for this course include: Books Morin, P.J. Community Ecology. (Oxford: Blackwell, 1999) [ISBN 9780865423503]. Stearns, S. and R. Hoekstra Evolution: an Introduction. (Oxford: Oxford University Press, 2000) second edition 2005 [ISBN 9780199255634]. 5 155 Biodiversity Journals Connell, J.H. ‘Diversity in rainforests and coral reefs’, Science 199 (1978), 1302–10. Gaston, K.J. ‘Global patterns in biodiversity’, Nature 405 (2000), 220–27. Levin, S.A. ‘The problem of pattern and scale in ecology’, Ecology 73 (1992),1943–67. Lomolino, M.V. ‘Ecology’s most general, yet protean pattern: the species–area relationship’, Journal of Biogeography 27 (2000), 17–26. Lomolino, M.V. ‘The species–area relationship: new challenges for an old pattern’, Progress in Physical Geography 25 (2001), 1–21. May, R.M. ‘How many species are there on earth?’, Science 241 (1988), 1441–49. McCook, L.J. ‘Understanding ecological community succession: causal models and theories, a review’, Vegetatio 110 (1994), 115–47. Morrone, J.J. and J.V. Crisci ‘Historical biogeography: introduction to methods’, Annual Review of Ecology and Systematics 26 (1995), 373–401. Myers, N., R.A. Mittermeier, C.G. Mittermeier, G.A.B. da Fonseca and J. Kent ‘Biodiversity hotspots for conservation priorities’, Nature 403 (2000), 853–58. Perry, G.L.W. ‘Landscapes, space and equilibrium: some recent shifts’, Progress in Physical Geography 26 (2002), 339–59. Pickett, S.T.A., S.L. Collins and J.J. Armesto ‘Models, mechanisms, and pathways of succession’, Botanical Review 53 (1987), 335–71. Pimm, S.L. and P. Raven ‘Extinction by numbers’, Nature 403 (2000), 843–45. Purvis, A. and A. Hector ‘Getting the measure of biodiversity’, Nature 405 (2000), 212–19. Rees, M., R. Condit, M. Crawley, S. Pacala, and D. Tilman ‘Long-term studies of vegetation dynamics’ Science 293 (2001), 650–55. Reice, S.R. ‘Non-equilibrium determinants of biological community structure’, American Scientist 82 (1994), 424–35. Schluter, D. ‘Ecology and the origin of species’, Trends in Ecology and Evolution 16 (2001), 372–80. Tilman, D. ‘Causes, consequences and ethics of biodiversity’, Nature 405 (2000), 208–11. Weiner, J. ‘On the practice of ecology’, Journal of Ecology 83 (1995), 153–58. This article provides an interesting discussion of the role of theory in ecology. Wiens, J.A. ‘Spatial scaling in ecology’, Functional Ecology 3 (1989), 385–97. Whittaker, R.H. ‘Scale, succession and complexity in island biogeography: are we asking the right questions?’, Global Ecology and Biogeography 9 (2000), 75–85. Wiley, E.O. ‘Vicariance biogeography’, Annual Review of Ecology and Systematics 19 (1988), 513–42. Wu, J. and O.L. Loucks. ‘From balance of nature to hierarchical patch dynamics: a paradigm shift in ecology’, Quarterly Review of Biology 70 (1995), 439–66. Online study resources In addition to the subject guide and the Essential reading, it is crucial that you take advantage of the study resources that are available online for this course, including the VLE and the Online Library. You can access the VLE, the Online Library and your University of London email account via the Student Portal at: http://my.londoninternational.ac.uk You should receive your login details in your study pack. If you have not, or you have forgotten your login details, please email uolia.support@ london.ac.uk quoting your student number. 6 Chapter 1: Introduction The VLE The VLE, which complements this subject guide, has been designed to enhance your learning experience, providing additional support and a sense of community. It forms an important part of your study experience with the University of London and you should access it regularly. The VLE provides a range of resources for EMFSS courses: • Self-testing activities: Doing these allows you to test your own understanding of subject material. • Electronic study materials: The printed materials that you receive from the University of London are available to download, including updated reading lists and references. • Past examination papers and Examiners’ commentaries: These provide advice on how each examination question might best be answered. • A student discussion forum: This is an open space for you to discuss interests and experiences, seek support from your peers, work collaboratively to solve problems and discuss subject material. • Videos: There are recorded academic introductions to the subject, interviews and debates and, for some courses, audio-visual tutorials and conclusions. • Recorded lectures: For some courses, where appropriate, the sessions from previous years’ Study Weekends have been recorded and made available. • Study skills: Expert advice on preparing for examinations and developing your digital literacy skills. • Feedback forms. Some of these resources are available for certain courses only, but we are expanding our provision all the time and you should check the VLE regularly for updates. Making use of the Online Library The Online Library contains a huge array of journal articles and other resources to help you read widely and extensively. To access the majority of resources via the Online Library you will either need to use your University of London Student Portal login details, or you will be required to register and use an Athens login: http://tinyurl.com/ollathens The easiest way to locate relevant content and journal articles in the Online Library is to use the Summon search engine. If you are having trouble finding an article listed in a reading list, try removing any punctuation from the title, such as single quotation marks, question marks and colons. For further advice, please see the online help pages: www.external.shl.lon.ac.uk/summon/about.php 7 155 Biodiversity Activities Six ‘activities’ are provided in Appendix 1 that relate to the material covered in the subject guide. Some of these are ‘active’ in the sense that they require you to collect some data and perform some simple analyses on them. Others are more ‘passive’ in that they involve reading a journal article and thinking about some of the issues it raises. Finally, two of them make use of internet-based resources to allow you to explore the dynamics of some of the concepts and models we will encounter. The various activities are included at the end of the subject guide because some of the descriptions are quite long and because many of the activities relate to more than one chapter. Examination Important: the information and advice given in the following section are based on the examination structure used at the time this guide was written. Please note that subject guides may be used for several years. Because of this we strongly advise you to check both the current Regulations for relevant information about the examination, and the VLE where you should be advised of any forthcoming changes. You should also carefully check the rubric/instructions on the paper you actually sit and follow those instructions. In the examination for this course, you are required to answer three questions from a choice of seven. Remember, it is important to check the VLE for: • up-to-date information on examination and assessment arrangements for this course • where available, past examination papers and Examiners’ commentaries for the course which give advice on how each question might best be answered. Examination technique Students are often worried about examinations and how best to approach them – some simple strategies can maximise the result you get from the effort you put into your examination revision. Perhaps the most important part of the examination process – and the one that most often ends in examination failure – is the initial reading of the question paper. It cannot be emphasised enough that each question should be read carefully, once when you decide which questions to answer, and a second time when you start to write the answer itself. It is recommended that you draw up an essay plan – this does not need to take more than five minutes – which should outline the major issues you intend to discuss in your answer, together with a note of the main points to be addressed under each issue. It is vital to consider carefully the way that the question is phrased. A common reason for poor (or fail) marks is an overly descriptive answer when the question itself starts with the words ‘discuss’ or ‘examine’. Specific, and well-chosen, examples always help to clarify the points you are making and provide evidence of depth of knowledge and reading. Where the question takes the form: ‘With reference to specific examples, discuss...’ you must include examples. 8 Chapter 1: Introduction Good diagrams are also an essential part of a well-constructed answer. These do not need to be works of art – rough sketches are acceptable – and can illustrate concepts or processes in a diagrammatic manner that will both add to and clarify the written answer. Similarly, using formulas in the answer indicates, when suitably used, that you are able to understand the quantitative nature of much contemporary ecological and biogeographic research. In order to get good marks you must be able not only to describe ecological patterns, but also to show that you understand the underlying mechanisms responsible for those patterns. Sample examination questions At the end of each chapter, I have given two or three sample questions related to the material, and at the end of the subject guide (Appendix 3) two full sample examination papers are presented. To prepare for your examinations you should try to answer some of these questions under similar conditions to those of an examination. In Appendix 4 you will find further guidance on answering the examination questions. 9 155 Biodiversity Notes 10 Chapter 2: The ecosystem concept and scale Chapter 2: The ecosystem concept and scale Aims of the chapter The principal aim of this chapter is to provide the intellectual context for subsequent chapters. It will do this by: 1. introducing you to the study of biogeography and explaining the similarities and differences between historical and ecological biogeography 2. outlining the concept of the ecosystem and exploring how ecosystems are structured and function 3. exploring how scale and scaling are important if we are to describe and explain biogeographic patterns and processes. Learning outcomes Once you have read this chapter and the essential readings listed below, you should be able to: • describe the ecosystem concept and the key components of ecosystems • illustrate some of the different conceptualisations of the ecosystem and the community, in particular the idea of the ‘balance of nature’ • explain the idea of scale-dependence, and the importance of scale when looking at spatial and temporal pattern(s) in ecological systems. Essential reading Cox, C.B. and P.D. Moore Biogeography: An Ecological and Evolutionary Approach. (Oxford: Blackwell, 2005) seventh edition [ISBN 9781405118989] Chapter 5 covers the community and ecosystem concepts. Begon, M., C.R. Townsend and J.L. Harper Ecology: From Individuals to Ecosystems. (Oxford: Blackwell, 2006) fourth edition [ISBN 9781405111171] Chapters 18 and 19 discuss the flux of energy and materials through communities and ecosystems. There is useful information scattered throughout this volume. The authors introduce ecosystem structure and function in more detail, and Chapter 1 provides a useful overview of organism–environment interactions. Issues of scale are not particularly well covered in any of the textbooks that we are using in this course, and are (sadly) absent from most ecology/biogeography textbooks, so it is necessary to refer you to journal articles. Of relevance are: Levin, S.A. ‘The problem of pattern and scale in ecology’, Ecology 73 (1992), pp.1943–67. Perry, G.L.W. ‘Landscapes, space and equilibrium: shifting viewpoints’, Progress in Physical Geography 26 (2002), pp.339–59. Wiens, J.A. ‘Spatial scaling in ecology’, Functional Ecology 3 (1989), pp.385–97. Of these, the article by Levin in Ecology (1992) is the most important and provides a large number of other references of potential interest. 11 155 Biodiversity Introduction Biogeography is the science that attempts to document and understand spatial patterns of biodiversity. It is the study of the spatio-temporal distributions of organisms, both past and present, and of related patterns of variation throughout the earth. Hence, it is the subset of geography that deals with living things. Biogeography is closely allied to the discipline of ecology. Ecology is the scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the transformation and flux of energy and matter in ecosystems. Although biogeography and ecology are often treated as separate disciplines, they are in reality very similar and new sub-disciplines, such as macroecology and landscape ecology, draw on them equally (and, in fact, I will use the terms almost interchangeably). There are two broad types of biogeography: historical biogeography and ecological biogeography. Historical biogeography is concerned with the study of the distribution of organisms in the context of the history of the taxa and the long-term history (i.e. over geological time scales) of the areas in which they live. Ecological biogeography, on the other hand, is concerned with the study of the current distributions of organisms in the context of physical and biotic factors (i.e. ecological constraints). Of course, both branches of biogeography inform each other. The ecosystem concept Central to biogeographic and ecological enquiry is the concept of the ecosystem. The term ‘ecosystem’ was first used in 1935 by the British ecologist Tansley; the word is derived from ‘oikos’, the Greek for ‘home’. Ecosystems are hierarchically organised, and are embedded within a broader spatio-temporal setting; that is, they are set in landscapes and regions that exert an important influence on them. Ecosystems can be described by their structure, composition and function in both space and time. All ecosystems have both spatial (e.g. area, depth and height) and temporal dimensions. Nevertheless, their precise delineation is often arbitrary, because they are influenced by nutrient and energy exchanges and by species interactions, both internally and between adjacent and non-contiguous ecosystems. Furthermore, ecosystems have a past (as well as a present and a future), and this history influences the present and thus the future. First, in some ways, ecosystems can be described as having a ‘memory’ (e.g. soils retain the effects of past agricultural practices) and, secondly, an ecosystem’s history can also affect its developmental trajectories. Components of ecosystems Ecosystems can be considered in terms of their composition (what elements make them up?), their structure (how are the elements that make them up arranged and linked?) and their function (how do matter and energy flow between the elements in the ecosystem and to other ecosystems?). Ecosystems have abiotic and biotic components. The abiotic component consists of the physical environment (e.g. microclimate, radiant energy, gases, soil mineralogy, elevation, etc.), and the biotic component is concerned with the biological parts of the ecosystem and their interactions with each other and the abiotic environment (e.g. plant, animal and microbial species, plant productivity, etc.). However, ecosystems can also be considered in 12 Chapter 2: The ecosystem concept and scale terms of material and energy fluxes. There are two main energy sources for ecosystems. The first, and most important, is the sun; capture of radiant energy via photosynthesis is the key to plant growth. A second, and less significant, energy source is via inorganic chemical reactions. Autotrophs (‘producers’) are organisms that produce their own energy; they capture energy from the environment and use it to fix carbon (CO2 → CH2O). Plants are photoautotrophs; that is, they use radiant energy to fix carbon (C). The other type of autotrophs is chemoautotrophs: free-living micro-organisms that capture energy from inorganic chemical reactions to fix carbon (note that most micro-organisms are not chemoautotrophs). Heterotrophs (‘consumers’) are organisms that require a supply of organic matter from the external environment. In essence, they take dead organic matter (e.g. detritus) and decompose it into simpler components. Some heterotrophs make direct use of living tissues (e.g. phytophagous insects). The flow of nutrients through ecosystems and the biosphere (i.e. biogeochemical cycling) is driven by primary production. Primary production is, in turn, a result of photosynthesis, which requires water, light and carbon dioxide (CO2): 12H2O + 6CO2 + Light → C6H12O6 (Glucose) + 6O2 + 6H2O Based on the number of photons that a plant must absorb in its photosynthetic system until one molecule of glucose is synthesised, the process of photosynthesis is quite efficient, and has theoretically been estimated to be approximately 35 per cent. The total amount of energy fixed in photosynthesis is called gross primary production (GPP). Some of this energy is subsequently lost through respiration – the burning of chemical energy to power the work, growth and maintenance undertaken by all animals and plants: C6H12O6 + 6H2O + 6O2 → 6CO2 + 12H2O + energy The energy fixed in photosynthesis minus that lost through respiration is called net primary production (NPP). Flows of energy All ecosystems can be described and compared in terms of energy flow and nutrient cycles. In the vast majority of ecosystems the sun provides the source and green plants the point of entry for the energy required to drive ecosystem processes. The issues we are interested in are: (i) how energy moves through ecosystems, and (ii) how efficient these fluxes are. To understand energy fluxes we need to remember the first two laws of thermodynamics: • first law: energy may be transformed from one state to another, but it is neither created nor destroyed • second law: energy transformations are accompanied by degradation of energy from a non-random to random form (i.e. heat loss). The energy budget of any organism must operate ‘in credit’. Organisms cannot give out more energy than they have taken in. Energy is available to organisms from only three sources: • metabolic energy from consumption; in the case of autotrophs (plants) this means the ‘consumption’ of sunlight • direct energy from sunlight (as heat) • heat energy from ground re-radiation. At the same time, all organisms lose energy through metabolic processes (e.g. transpiration in plants) and black-body radiation. 13 155 Biodiversity Ecosystem efficiency A good place to start our discussion is to consider how efficient ecosystems are at capturing energy; in other words, we need to measure the efficiencies of plants. In theory, the efficiency of plants in terms of fixing energy is approximately 35 per cent. We can estimate this by considering the amount of energy (photons from the sun) that is required to produce one glucose molecule via photosynthesis: 1968 photons produce one molecule of glucose, which yields an increase free energy of 686 (686/1968 = 0.35). The first thing to consider is whether the process of photosynthesis is as efficient in the ‘real world’. To explore this, Transeau (1926) conducted an experiment in which he attempted to estimate the efficiency of an Ohio cornfield. On one acre of bare ground he grew 10,000 corn plants and calculated their production over a period of 100 days (in terms of kilograms of sugar); simultaneously in a greenhouse experiment he calculated the amount of respiration (again in terms of kilograms of sugar). Gross production was estimated by summing the production in the field and that lost by respiration; and was found to be 8,723 kg sugar, which is equivalent to 3.3 × 107 kCal. Over the same period inputs from solar energy were 2.04 × 109 kCal. This is equivalent to a conversion efficiency of just 1.6 per cent (3.3 × 1007 / 2.04 × 1009)! We will now discuss the implications of this very low efficiency for ecosystem structure and function, and consider some of the reasons that might explain it. Trophic pyramids We can conceptualise ecosystems as trophic pyramids (Figure 2.1), with the different trophic levels representing the productivity or biomass associated with all the organisms at that level. At the base of this pyramid are the primary producers (i.e. autotrophs), those species that make their own food. The successive steps (upward) in the pyramid are heterotrophic levels (primary consumers and above). Tertiary consumers Secondary consumers Primary consumers Primary producers Figure 2.1: Schematic representation of the trophic pyramid; area at the different levels is proportional to biomass and/or production. As energy is transferred between the different levels of the trophic pyramid, energy is lost to heat. The exact amount of energy that is ‘lost’ to heat depends on how efficient each organism in the trophic pyramid is at converting energy to tissue. This efficiency of flow is called the trophic level transfer efficiency. There are three components to the trophic level transfer efficiency (see Begon et al. 2006, pp.519–20 for more details): 14 Chapter 2: The ecosystem concept and scale • Consumption efficiency: the percentage of the total production at one level that is ingested by the next trophic compartment. • Assimilation efficiency: the percentage of food energy that is taken up by the consumers is assimilated and is then available for work. • Production efficiency: the percentage of assimilated energy that is converted to biomass; the remainder is lost as respiratory heat. Lindeman (1942) attempted to quantify trophic efficiency between levels in a lake system by looking at the amount of energy passed from one trophic level to the next (i.e. plants to herbivores, etc.). He found that the trophic efficiency between different levels was consistently around 10 per cent – this rule-of-thumb has become known as the ‘10 per cent rule’. Progressively less energy is available to the higher levels of the trophic pyramid; this ecological efficiency is around 5–20 per cent. The significance of such a low efficiency is that it limits the number of levels in any trophic pyramid to just four or five – the amount of energy moving up through the trophic pyramid is simply insufficient to support any more levels. Pauly and Christensen (1995) reviewed 48 studies of energy flows in aquatic ecosystems and found a mean efficiency of 10.3 per cent (SE = 0.49), with a range of 2–24 per cent. Trophic efficiencies are so low due to: • the loss of energy via respiration • heat loss (black-body radiation, etc.) • the cost of search (energy is lost while individuals search for the next food item they will consume) • inefficient digestion (a considerable amount of energy is lost, as heat, during the process of digestion). Begon et al. (2006, Chapters 17–18) discuss these fluxes of energy and material in a range of different ecosystems. Ecosystems and the ‘balance of nature’ Early ecologists considered nature to be in ‘balance’; in fact, this is a very old idea dating back to Aristotle and other Greek philosophers. Under this view, systems are regarded as stable and tend to move back towards equilibrium conditions after perturbations. Events such as disturbance (e.g. fire, avalanche, wind-throw, floods, etc.) that move ecosystems away from stability were considered ‘unnatural’. This view was epitomised by Clements’ ‘super-organism’ or ‘community-unit’ concept. Clements (1916) saw ecosystems as ‘super-organisms’. He considered ecosystems to be co-evolved complexes which, given enough time, would reappear in the landscape, so long as the climate was the same. Gleason (1926) proposed the ‘individualistic’ concept. He considered that environmental factors vary in space and time, and that each plant species has its own tolerance of these factors. Gleason argued that because plants tend to disperse their seeds randomly, communities are somewhat random collections of species that are able to tolerate each other and the environment, and thus coexist. Gleason’s view is closer to the modern conceptualisation of the ecological community or ecosystem. Now ecologists realise that ecosystems are rarely in a state of stable equilibrium; stochastic events mean that stable endpoints are rarely (if ever) achieved, and these events are now seen as being a ‘natural’ part of most ecosystems. Indeed, as will be discussed later, such events may be very important in driving local patterns of biodiversity. Perry (2002) reviews recent ideas regarding the balance of nature and equilibrium in ecology and biogeography; related ideas are discussed in Chapters 8 and 9. 15 155 Biodiversity Looking at the ecosystem: the importance of scale As organisms interact with each other and the abiotic environment in both space and time, spatial and temporal patterns form in their abundance and distribution(s). However, the patterns that we see when we are studying an ecosystem (or indeed any phenomena) are dependent on the scale at which we view it. This ‘scale-dependence’ is fundamental when studying dynamic entities such as ecological systems, so we will explore it a little further. The issues touched on here are covered in detail, with good examples, by Levin (1992) – you should try to read this journal article. Cox and Moore (2005: pp.2–5) briefly consider the issue of scale. We can define scale as the length scale of measurement. Thus, scale, as we have defined it, refers to the spatio-temporal domain we are interested in. It is not the same as ‘level of organisation’. A statement based on the level of organisation depends on the criteria used to define the system (e.g. population-level studies are concerned with interactions among conspecific [i.e. same species] individuals). Scale in our context has two components: grain and extent. Grain refers to the minimum resolution of the data in space or time (e.g. the size of the sample unit we use), and extent refers to the domain over which measurements are recorded (i.e. the overall area encompassed by the study in space and time). As an example, consider Table 2.1, which relates to a hypothetical study that looked at snails grazing algae on a rocky beach, using sampling quadrats of 0.25 m2 over a total area of 100 m2 every month for a year. Grain Extent Space 0.25m 100m2 Time 1 Month 2 1 Year Table 2.1: Grain and extent in an example ecological study. Any measurements we collect are taken over particular units of space and time. For example, we use spatial units such as kilometres, metres and centimetres, and in different dimensions (e.g. length, area, volume); similarly we use temporal units such as centuries, years, months, days, etc. However, if we change the scale over which something is measured we almost always observe different results or patterns. This is called scaledependency, and it means that our conclusions are limited to our scales of measurement. ‘Scale-dependent patterns’ are patterns that change in some way, with changes in either the grain or extent of measurement. ‘Scaledependent processes’ are those where the ratio of one rate to another changes with the grain or extent of measurement. The main implication of scale-dependence is that our perception of heterogeneity (pattern) depends upon scale. Thus, when studying ecological processes we need to choose an appropriate scale at which to conduct our study. Scale should be defined relative to the organism or ecological phenomena of interest. The crux of the scale problem is that nature is fine-grained and of large extent. But, in ecological studies, grain and extent are negatively correlated because the logistics of measurement necessitate a trade-off between the two. As grain decreases, and as we measure more variables at finer grains, information content tends to increase. In reality, most ecological studies are conducted over limited scales and consist of either short-term studies done at single locations1 (e.g. manipulative experiments) or longer-term studies conducted over entire regions but with poor site-level information (e.g. palæoecological studies). This means that we are forced to tradeoff between space and time and, more importantly, to make implicit 16 Recently attempts to address these problems have been made. Notable examples include the long-term ecological research network (LTER) set up in a range of different ecosystem types in the USA and elsewhere. 1 Chapter 2: The ecosystem concept and scale assumptions about which scales are not important. The key problem is that we do not understand very well how variables change with scale and, as a result, it is difficult to translate information across scales. Nearly all of the most pressing problems in biogeography and ecology entail substantial scale-ups in space and time; we often have small-scale studies but need large-scale solutions. The standard solutions (e.g. multiplication factors) for translating data and models across scales are often inadequate because most ecological phenomena are non-linear and non-additive. A good example of how scale-dependence is important is the idea of the ‘balance of nature’ that was introduced above. As we change the scale at which we view a system, we change the patterns we see and, as a result, equilibrium is likely to be scale-dependent. For example, in a forest individual trees will continually grow, reproduce and die (and so at finescales the system may appear to be disequilibrial), while the composition of the forest, as a whole, may remain constant (and so at coarser-scales the system may show a dynamic equilibrium). Summary The ecosystem is one of the fundamental units of investigation in biogeography and ecology. We can consider ecosystems in terms of their composition, structure and function. Ecosystems can be divided into abiotic and biotic components, and can also be considered in terms of energy and material fluxes (from producers to consumers). If we are to understand the patterns (in space and time) that we observe in ecosystems, then we must consider scale. The spatial and temporal patterns we see in the environment are determined by the scale(s) at which we view the system. Scale can be decomposed into grain and extent. The concept of scale-dependency is of fundamental importance to the description and the understanding of biogeographic and ecological patterns. Activity Activity 2 (p.93) is a study of the spatial distribution of plants. Much of it relates to this chapter, although to complete the activity successfully you will also need to read Chapters 7 to 10. You will probably find it useful to make a preliminary study of the activity before starting work on the next chapter of this guide. A reminder of your learning outcomes Once you have read this chapter and the essential readings listed earlier, you should be able to: • describe the ecosystem concept and the key components of ecosystems • illustrate some of the different conceptualisations of the ecosystem and the community, in particular the idea of the ‘balance of nature’ • explain the idea of scale-dependence, and the importance of scale when looking at spatial and temporal pattern(s) in ecological systems. Sample examination questions 1. What is scale-dependence and why do ecologists need to be aware of it when designing ecological experiments? 2. Compare and contrast Clements’ and Gleason’s concepts of the community. 3. In terms of energy flow, how efficient are ecosystems? 17 155 Biodiversity Notes 18 Chapter 3: Species and speciation Chapter 3: Species and speciation Aims of the chapter Understanding biogeographic and ecological patterns and processes requires that we understand the fundamental processes of speciation – how new species arise – and extinction – the disappearance of existing species. This chapter aims to do this by: 1. exploring what is meant by ‘species’ and introducing two of the major ways that species have been and are defined 2. outlining the mechanisms driving speciation – the ‘birth of new species’ 3. outlining what is meant by ‘mass extinction’ and what drives such dramatic events. Learning outcomes Once you have read this chapter and the essential readings listed below, you should be able to: • explain the biological species concept and summarise the main problems involved with its application • describe the different processes of speciation (allopatric, sympatric and parapatric) and state when they might occur • describe what is meant by a ‘mass extinction’; and understand some of the causes of previous (and current) extinction events. Essential reading Begon, M., C.R. Townsend and J.L. Harper Ecology: From Individuals to Ecosystems. (Oxford: Blackwell, 2006) fourth edition [ISBN 9781405111171] Chapter 1 considers the evolutionary context of ecology. Cox, C.B. and P.D. Moore Biogeography: An Ecological and Evolutionary Approach. (Oxford: Blackwell, 2005) seventh edition [ISBN 9781405118989] Chapter 2 provides an overview of issues related to biodiversity and its assessment including the questions ‘How many species are there?’. Chapter 6 describes the principles of natural selection and speciation. Lomolino, M.V., B.R. Riddle and J.H. Brown Biogeography. (Sunderland, Mass.: Sinauer Associates, 2010) fourth edition [ISBN 9780878934942] Chapter 8 offers a good review of issues related to the species concept and the processes of speciation and extinction. Further reading Stearns, S. and R. Hoekstra Evolution: an Introduction. (Oxford: Oxford University Press, 2000) [ISBN 9780199255634] Chapter 11 provides an excellent overview of the issues of species definition and speciation (in an evolutionary context). Interesting journal articles related to the issues discussed in this chapter are: May, R.M. ‘How many species are there on earth?’, Science 241 (1988), pp.1441–49. Schluter, D. ‘Ecology and the origin of species’, Trends in Ecology and Evolution 16 (2001), pp.372–80. 19 155 Biodiversity Introduction Biodiversity is often taken to mean the ‘total variety of life on earth’. Thus, it includes all genes, individuals, species and ecosystems, and the ecological processes of which they are a part. The ‘species’ is a fundamental unit with which to explore and understand the concept of biodiversity. In this chapter we will look at the changing concept of the species by considering the morphological and biological concepts of the species, and then the three main processes of speciation (sympatric, allopatric and parapatric). The taxonomic hierarchy All organisms are classified using the Linnaean classification system – that is, they all have a place in the taxonomic hierarchy shown in Figure 3.1. Estimates of biodiversity based on counts of number of species rely on descriptions of species according to the Rules of Nomenclature (the Linnaean system). Based on these rules, species are classified and placed in the taxonomic hierarchy. Each level (‘taxon’, plural ‘taxa’) of the taxonomic hierarchy contains species defined by particular features (usually morphological). Taxonomic classifications are the basis of species identification, and the species within any particular taxon have a shared evolutionary history (i.e. a common ancestor). As we move down the hierarchy, the classifications become more and more specific (members of the same genus have more in common than with members of the same family but different genera, for example). Species are defined by an italicised Latin binomial.1 The generic (genus) name always begins with a capital (e.g. Vombatus) and the species name always starts in lower case (e.g. ursinus); thus, we have Vombatus ursinus (the common wombat). KINGDOM (Animalia; Metazoa) PHYLUM (Chordata) CLASS (Mammalia) ORDER (Primates) FAMILY (Hominidae) GENUS (Homo) SPECIES (Homo sapiens) Figure 3.1: The taxonomic hierarchy (using Homo sapiens as an example). 20 Species also often have a common name. However, this common name may refer to more than one species (for instance, in New Zealand the name mingi-mingi refers to three to four distinct species), whereas a species can only have one ‘proper’ name, so it is less confusing to use the proper name. 1 Chapter 3: Species and speciation Defining species: the species concept Despite being seen as one of the fundamental units of enquiry in evolutionary biology and in biogeography/ecology, defining the ‘species’ has proved difficult. Many different definitions of the ‘species’ have appeared in the literature. Here we will focus on two of the most important: the morphological (or classical) species concept and the biological species concept. Lomolino et al. (2010, Chapter 7) discuss the problems of defining species and higher taxa in some detail. Morphological definition of the species Morphology refers to the form and structure of an organism or of any of its parts. The morphological species concept is based on the view that the members of a species are individuals that look similar to one another. This morphological definition was the basis for Linnaeus’s original taxonomic hierarchy and it is an intuitive method for delineating different species. However, some biologists have criticised the use of morphology for discriminating between species as somewhat arbitrary. There are many examples of morphologically similar individuals of two populations that do not reproduce. Further complications are provided by phenomena such as mimicry complexes and, for example, in situations where organisms of the same species can look very different, depending upon where they are reared or their life cycle stage. Biological definition of the species ‘A group of interbreeding natural populations that are reproductively isolated from other groups.’ (Mayr, 1969, p.26) ‘a reproductive community of sexual and cross-fertilising individuals which share in a common gene pool.’ (Dobzhansky, 1950, p.405) The biological species concept (BSC) is currently the most widely accepted definition of the ‘species’. Under the BSC, species are defined by the reproductive isolation of populations, not by the fertility of individuals. In the BSC the species is seen as a reproductive unit with genetic material able to flow freely between individuals. The species is also an ecological unit; regardless of the types of individuals in a species, it interacts as a unit with other species, with which it shares the environment. A species can also be conceived as a genetic unit whose internal genetic cohesion is based on a common evolutionary history, with individuals being vessels that hold a fraction of the species’ total genetic variation. Reproductive isolation is the key to the BSC. Table 3.1 outlines some of the major mechanisms through which such reproductive isolation might be achieved. These can be divided into pre-zygotic (before fertilisation) and post-zygotic (after fertilisation) barriers. Pre-zygotic (before fertilisation) Post-zygotic (after fertilisation) Phenological – e.g. flowering season Hybrid weakness – no subsequent reproduction Ecological – e.g. different habitats etc. Hybrid sterility – low F12 viability, Morphological – e.g. pollinator access Introgression – hybrid can only cross back to pure parent Chemical – e.g. pollen-stigma incompatibility Table 3.1: Some major mechanisms of reproductive isolation The parental generation is denoted as the P1 generation. The offspring of the P1 generation are the F1 generation (first filial). The self-fertilising F1 generation produces the F2 generation (second filial). 2 21 155 Biodiversity However, the BSC is not without its own difficulties. First, reproductive isolation is rarely proven, even for extant species. Little is known about dispersal and the reproductive cycle for many species (e.g. aquatic insects). Asexual reproduction also poses a problem for the BSC, as for some such species most individuals in a population may be clones. Finally, there are intermediate situations where hybridisation occurs/has occurred and so it is difficult to apply the BSC. Many species, although reproducing sexually, have only weak barriers to interspecific breeding. For example, interspecific breeding that produces fertile hybrids is relatively common in plants and fungi. The question here is: with how much interspecific gene flow can the BSC retain its meaning? For example, wolves (Canis spp.) and coyotes (Canis latrans) are considered separate species but they can and do hybridise, so where do they stand with regard to the BSC? Although this may seem a somewhat esoteric concern, hybridisation is of direct relevance to conservation biology. Some species are threatened by hybridisation in the sense that ‘pure-bred’ individuals are very rare. Furthermore, while pure-bred offspring may be protected under conservation legislation, hybrids usually are not. Geographic variation and speciation Genetically fixed variation among segregated populations is the beginning of speciation (the process in which new species arise – the ‘birth of species’). Such variation represents adaptation to local conditions. However, not all species will exhibit such variation and we might expect it to be limited when: • a species range is very small • dispersal ability is good • the genotype is (very) stable. Variation can be expressed in the form of ‘ecotypes’, which are populations or individuals of a species resulting from natural selection by the special conditions of a locality. These are generally subdivided by physical or other conditions (e.g. edaphic, climatic [termed a cline], geographic race, variety, etc.). Taxonomic recognition of variation is in the form of subspecies, varieties and forms. It is this spatial variation that is central to Darwinian modes of speciation. There are three basic methods by which new species can be formed (see Lomolino et al. 2010, Chapter, 7, pp.217–36): • allopatric speciation – a physical barrier separates populations and prevents gene flow (i.e. geographic separation results in reproductive isolation) • sympatric speciation – populations are overlapping but attain reproductive isolation • parapatric speciation – local adaptation at the ends of a species’ range, populations are contiguous. Stearns and Hoekstra (2000, Chapter 11) provide a good schematic overview of these speciation processes. Schluter (2001) also provides an interesting overview of the mechanisms through which new species arise. In allopatric speciation, populations become geographically isolated, are then exposed to divergent selection, and so speciation occurs. Both the initial splitting mechanism and barrier may be caused by ecological or geological events. A classic example is ‘Darwin’s finches’ (Geospiza spp.) on the Galapagos Islands. In this case a single mainland ancestor has allopatrically speciated to form a complex of 13 different species over the last three million years or so. 22 Chapter 3: Species and speciation Sympatric speciation occurs where there is reduced (but non-zero) gene flow between populations that are not divided by a physical barrier. The process of sympatric speciation is controversial and may occur only under restrictive conditions. Some possible examples include host shifts in phytophagus insects, the divergence of flowering time in plants, or sudden sympatric speciation by polyploidisation (the retention of extra sets of chromosomes). The third type of speciation, parapatric speciation, is similar to allopatric speciation but differs in that the two populations do not become completely separated but remain connected over a narrow contact zone. This may occur, for example, if a species’ population is distributed across a large geographic range that encompasses a large environmental gradient. Speciation can proceed either through anagenesis or cladogenesis (Figure 3.2). In anagenesis, a single ancestral species changes enough to be considered a new species, while in cladogenesis, new species (one or many) arise from a small population of the ancestral species. The ‘branching evolution’ of cladogenesis gives rise to the diversity of species and is thought to account for the formation of most new species. Anagenesis Cladogenesis Figure 3.2: Schematic views of the processes of anagenesis and cladogenesis. The speciation processes described to this point all occur over long time periods. However, there is some evidence in the fossil record of periods of quite rapid change. Eldredge and Gould (1977) developed ‘punctuated equilibrium’ as a model of evolutionary change to explain the observation that many fossil lineages show long periods of stasis followed by brief periods of rapid change. This punctuated model challenged neo-Darwinian views, which predict slow and steady change. Eldredge and Gould inferred that most morphological change occurred at the time of speciation and that most evolutionary change is cladogenetic rather than anagenetic. If cladogenesis and anagenesis occur independently of each other, then we would expect slow and gradual phyletic change. However, if cladogenesis triggers periods of anagenesis, then we might see punctuated equilibrium. A key point to remember here is ‘How quick is quick?’ – remember we are dealing with geological rather than ecological timeframes when we talk of processes such as evolution and speciation. 23 155 Biodiversity Extinction Just as species are born, they die. The death of a species is called extinction. Strictly speaking, extinction is the failure of a taxonomic group to produce any direct descendants, resulting in its total disappearance. The loss of a species from a significant part of its range is called extirpation. Extinction is a natural process; 99 per cent of all species that have ever lived are extinct. The fossil record allows us to estimate the lifespan of the ‘average’ species – this varies between taxa but for mammals, for example, is in the order of one million years. Extinction rates over geological time have averaged at about one species per million species per year. This continuous low-level rate extinction is termed background extinction. However, past rates of extinctions have varied dramatically and include mass extinction events where a large proportion of species and higher taxa suffer total extinction over a short period of geological time. Five or six such mass extinction events have occurred, each leading to fundamental changes in the biota (e.g. dinosaur extinction at the Cretaceous [around 65 million years ago] and the rise of flowering plants at the end of the Jurassic [around 130 MYA]). Mass extinction events are believed to have occurred due to: • Climate change: sudden profound changes nearly always lead to a large extinction event; for example, Pleistocene glaciation (over the last 2.5 million years) resulted in loss of grazing animals from North America and Eurasia. • Geologic events: events such as changes in sea-level, atmospheric pollution due to volcanic and fire activity, etc. have all driven mass extinctions. For example, the Permian-Triassic event (approximately 250 million years ago) was possibly caused by increased volcanism accompanied by oceanic Methane hydrate gasification. • Meteorites: both single, large meteors and meteor swarms have also triggered mass extinctions – most famously, the extinction of the dinosaurs at the end of the Cretaceous was probably caused by a meteorite hitting the earth near Yucatan, Mexico. It is the indirect effects of meteor impact such as increased dust in the atmosphere and accompanying climate change that cause most extinctions. Current extinction rates are believed to be 100 to 1000× higher than the longterm background rate, and are possibly as high or higher than at any other time in Earth’s history. This has led some to argue that we are entering a sixth mass extinction event. Humans (Homo sapiens; Hominidae) are the driving force behind the present mass extinction. The main drivers of extinction are habitat loss and fragmentation, the effects of exotic (invasive) species, overexploitation and pollution. This sixth mass extinction is compounded by the fact that only a minority of species are formally described; it is likely species are being lost faster than they are being discovered and described. Species richness: how many species are there? So, how many species are there? Estimates of global species diversity vary from 2 million to 100 million species. The current ‘best’ conservative estimate is ≈12.5 million, of which only about 1.8 million have been formally named (described). The accuracy of this estimation varies greatly between taxonomic groups and types of ecosystem. Cox and Moore (2005: pp.46–52) discuss this question and its significance, and May (1988) provides an interesting discussion of the problems involved with estimating the number of species on the planet. 24 Chapter 3: Species and speciation The Procrustean fallacy? As was discussed above, taxonomic classifications are the basis of species identification. However, genetic data are casting doubt on many previously accepted classifications; we also need to ask whether these new classifications are actually ecologically useful. We need to be careful of the ‘Procrustean fallacy’.3 We have to be careful not to invent concepts or theories and then try to ‘cut and stretch’ nature to fit them. Nature is too diverse to be described by a single, rigid concept, and this is likely the case with the idea of the ‘species’. 3 Procrustes, whose name means ‘he who stretches’, was one of Theseus’s challenges on his way to becoming a hero. Summary The biological species concept defines a species as a group of reproductively isolated, interbreeding populations; species are dynamic, not static entities. However, this biological species concept is problematic and, in some cases, is difficult to apply. The key processes of speciation are allopatric, sympatric and parapatric – the most important of these is allopatric speciation which occurs when a physical barrier separates populations and prevents gene flow (i.e. it results from geographic separation). A reminder of your learning outcomes Once you have read this chapter and the essential readings listed earlier, you should be able to: • explain the biological species concept and summarise the main problems involved with its application • describe the different processes of speciation (allopatric, sympatric and parapatric) and state when they might occur • describe what is meant by a ‘mass extinction’; and understand some of the causes of previous (and current) extinction events. Sample examination questions 1. Critically review the biological species concept (BSC). 2. Explain how hybridisation and asexual reproduction provide a challenge to the biological species concept. 3. Using examples compare and contrast allopatric and sympatric speciation. 25 155 Biodiversity Notes 26