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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.
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Published by: University of London
© University of London 2008
Reprinted with minor revisions 2011
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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
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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
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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.
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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?
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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].
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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
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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.
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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.
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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.
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155 Biodiversity
Notes
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