Department of Genetics, Eötvös University, Budapest, Hungary
Zoltán Botta-Dukát
MTA Centre for Ecological Research, Vácrátót, Hungary
Gabriella Magyar
Formerly of Department of Plant Systematics, Ecology and Theoretical Biology, Eötvös University, Budapest, Hungary
Tamás Czárán
Research Group of Theoretical Biology and Evolutionary Ecology, MTA and Eötvös University, Budapest, Hungary
Géza Meszéna
Department of Biological Physics, Eötvös University, Budapest, Hungary
Great Clarendon Street, Oxford, OX2 6DP, United Kingdom
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To the memory of Pál Juhász Nagy, our professor and friend; without his inspiration and intellectual influence we would never have thought of writing a book on the principles of ecology.
Preface
‘Anyone familiar with the history of science knows … that its only real rules are honesty and validity of logic, and that even these are open to public scrutiny and correction.’
More than ever before, ecology as a scientific discipline is in a challenging state. While several theoreticians have invested considerable effort in attempts to build ecology upon first principles, others doubt even the feasibility of a general ecological theory, essentially on the basis of ‘too much complexity’ type arguments. In line with this scepticism, mathematical descriptions of ecological phenomena are far too often considered rudimentary tools to aid the understanding of single isolated (or at best a few specific) empirical situations. For many the hope that universal ecological principles might guide empirical research seems to have faded. The resulting mass of unconnected theoretical and empirical research is overwhelming, and contributes to the prevailing misperception of ecology as jumble of special cases lacking universal insights. This has led to justified concerns regarding the future success of both research and education in ecology, especially in view of escalating environmental problems.
Our ambition in writing this book was to meet this challenge head on. We integrate empirical knowledge from several fields of ecology within a unified and coherent conceptual framework. The output of our effort is an advanced undergraduate/graduate-level textbook that provides the outlines for a theory of ecology built on seven logically related principles illustrated with empirical studies. Our primary motivation is our conviction that a general theoretical framework to structure and organize empirical knowledge is crucial to reduce conceptual uncertainty and aid comprehension, and thus to provide a fundamental basis for education in ecology.
The book consists of four parts. Part I presents a reinterpretation of Darwin’s theory of natural selection, emphasizing its roots in population regulation. We
(Robert MacArthur 1972)
introduce the key concepts used in the book while also discussing the problems and complications that a sufficiently general and operative theory of ecology must consider, along with the mathematical tools for treating them. Part II starts with two chapters on exponential, i.e. unregulated, growth: the first one deals with unstructured, the second one with structured populations. The third chapter of Part II focuses on concepts of ecological tolerance, geographical distribution, and adaptation, all based on the environmental dependence of the rate of exponential population growth. The third—and bulkiest—part is devoted to different aspects of population regulation, the central concept of the book. Starting from a detailed discussion of the most elementary types of regulation, we analyze general conditions for coexistence of reproductive units, to conclude with a formal analysis of the roles species play in their communities, and introduce the reader to a new, operational niche theory. The two chapters of the last, fourth part consider the ecological consequences of finite population size, and the connections between diversity patterns and population regulation. The universal potential of all organisms for exponential population growth and the resultant inevitable checks on growth through population regulation are overarching topics of the book and put in a different light time and again in various chapters. Though some issues of ecology (e.g., nutrient cycling) are not discussed, we regard population regulation and natural selection to be important in all of the areas of ecology.
Each chapter either starts with one of the seven principles introduced in the first chapter or with a brief overview, followed by the main text organized into a modular structure. In the main text, the treatment is sufficiently detailed to cover all the theoretical
essentials, but it does not dive into the depths of theory that might divert attention from the main message. It includes numerous empirical examples, with focal organisms ranging from microbes to plants and animals. The main text is complemented by three kinds of modules: Notes, Warnings, and Theory Boxes (the latter abbreviated as ‘TBox’). Notes are short, mainly historical, reflections related to the topic being discussed. Warnings draw attention to misinterpretations or misconceptions that we have frequently encountered in publications, discussions, or during teaching. Abundant cross-references aid in finding and connecting relevant interrelated information within the book. Chapter cross-references are in a special format (e.g., Ch3Expo) to help the reader recall the content of the chapter referred to without actually turning pages. A detailed and structured index is also provided to aid finding definitions of mathematical and ecological concepts. Online material accompanying the book (referred to as ‘OLM’) contains detailed presentations of some interesting models or empirical studies that are beyond the scope of the book. The web page (at http://tbe.elte.hu/) also provides a forum for discussions with readers—colleagues and students alike.
The main text introduces concepts together with simple verbal formulations of the corresponding body of theory and its conclusions, along with illustrations taken from empirical studies on various species and communities on different spatial and temporal scales. The mathematical formulation and formal discussion of the theory are located in TBoxes, which make up about 20% of the total volume of the book. The comprehension of abstract concepts and processes is supported by flow charts, and wherever possible with illustrations of the dynamical properties of the systems under study (steady states, other attractors, and spatial and temporal qualities) and their functional connections, in the form of graphs and plots. We have provided numerical or experimental realizations of the abstract constructs so that theoretical predictions or conclusions can be readily connected to empirical situations. TBoxes are not indispensable for understanding the main text, yet we strongly recommend that readers dig into the TBox material: this will add substantially to the depth of understanding. The main goal of the TBoxes is not to provide mathematically rigorous derivations: we have instead put the main emphasis on comprehensibility. We have done our best to make the notations consistent throughout the book. For mathematical background we refer wherever possible to specific locations in Otto and Day (2007), or to Case (2000) as a secondary source.
For the accurate treatment of a few issues we could not avoid referring to a wider range of mathematical literature. However, these issues are of lesser importance relative to the main thread of thought.
We believe that the material of this book can be a part of many different types of course, ranging from field ecology to biomathematics. Ideally, these courses should be taught by a combination of theoretical and field ecologists. Parts of the material can also be used in BSc level courses on ecology and evolutionary biology. We have assumed our readers will come from quite different backgrounds; some will prefer to focus on the TBoxes, others on the main text. The modular structure of the book allows and supports both strategies: one can first selectively browse the topics covered in the book—which are diverse both in their subjects and methods—to follow this up with a more systematic reading.
We owe a debt of gratitude to many excellent colleagues and friends who, in one way or another, have all taken part in our adventure of writing this book. First of all, we wish to thank Robert May and Hans Metz for their primary roles in initiating, supporting, and carrying out this project. Hans Metz and Jim Mallet contributed a great deal to the final structure and style of the book through their invaluable comments at an early stage of the project. Hans commented on essential issues in nearly all chapters of the manuscript regarding both the actual content and the style, and his comments have helped make the material more consistent and readable. Axel Rossberg and György Barabás were careful reviewers of several chapters. Judit Padisák, Nick Barton, Jim Mallet, David Reznick, Imelda Somodi, and Ferenc Jordán commented on specific issues. We thank Dee Shields for her grammatical and stylistic advice and corrections. All these contributions helped eliminate errors and inconsistencies, and we are grateful for them.
Some of our colleagues were so kind as to supply us with their original data sets—in more than a few cases collected over decades—for producing figures or tables. They are (in order of appearance): Petr Pysek (Ch3Expo), József Kiss (Ch5Toler), János Török (Ch6Regul), Brian Husband (Ch6Regul), Román Carrasco (Ch7Excl), Cyrille Violle (Ch7Excl), Anita Narwani (Ch9Coex), Jeremy Thomas (Ch10Niche), Stina Drakare (Ch12Divers), and Sándor Bartha (Ch12Divers).
We wish to thank our students for their help in various matters: Milan Janosov, Lívia Hanusovszky, Zsuzsa Milkovics, and Anikó Zölei were our readers and commenters; Benjámin Márkus (Figs 2.7 and 9.9), Domícián
Kovács (simulations of OLM 9.8), and Lénárd L. Szánthó (simulations of TBox 9.5) have contributed to the content of the book. Our colleague György Barabás has produced the simulations in TBox 10.5.
Some of our institutes have provided substantial support in the form of reducing unrelated workload during the book-writing project, which we gratefully acknowledge. Liz Pásztor would like to thank Tibor Vellai, the head of the Genetics Department of Eötvös University, for his backing during the six years of work on the book. Tamás Czárán has spent the last half year of manuscript preparation at the Niels Bohr Institute of the University of Copenhagen in an inspiring and pleasant environment and in the company of interested and competent colleagues. Thanks for this to the staff of NBI and the University, and in particular to Kim Sneppen and Bjarke Bak Christensen. Gabriella Magyar would like to thank her head of department, Margit Kapás, who supported her book writing despite it not being related to her present job. We would also like to acknowledge the financial support received from the Hungarian Scientific Research Fund (OTKA) in the form of grants K81628, K100806, and K83595.
We are all particularly grateful for the moral, emotional, and intellectual support we have received from our families and friends throughout all the years of writing: their love, care, and tolerance were indispensable ingredients of this lengthy project.
Last, but not least, we wish to thank the staff of Oxford University Press for their professional work on the manuscript: special thanks go to Ian Sherman, Helen Eaton, and Lucy Nash. Thanks to the patience and endurance of Sathya Sridharan and the production
team at Newgen even the latest of our numerous corrections have found their way into the text.
This book is the result of teamwork. Individual chapters cannot be assigned to any one of us, since we all contributed to each one. The first versions of the majority of TBoxes were written by Géza Meszéna, but even they were repeatedly revised and re-written, just like the other parts of the book. We have definitely developed and learned a great deal from our common adventure of discussions, argumentation, writing, and mutual criticism along the way. We very much hope that our readers will also benefit from the sincere effort we made to consistently solve all the problems that emerged in the process.
Ecology is an extremely diverse discipline, so no ecology book with a general scope can be perfect. We are ecologists dealing with animals, plants, bacteria and theory, but we are not dedicated specialists of the taxa mentioned in the book or experts on all the empirical systems studied. Although we have done our best to find good examples to illustrate our messages, it is likely that other case studies—unknown to us—could perhaps have been more enlightening. We welcome readers who would like to share with us any better examples they have, or to help us correct possible weaknesses or errors for future editions of this book.
Budapest, April 2016
Reference
MacArthur, R. H.: 1972, ‘Coexistence of species’, p. 259 in J. A. Behnke (ed.), Challenging Biological Problems, Oxford University Press, New York.
5.1
6.2
6.3
6.4
TBox
6.4.1
6.4.2
TBox
6.4.3
TBox 6.5
6.4.4
TBox 6.6
7.1
7.2
7.3
7.4
TBox 10.4 Temporal niche segregation and storage effect
TBox 10.5 Coexistence in cyclic environments
10.2.6 Niche segregation and evolution
10.6 Evolutionary consequences of competition
11.1 Viability of small populations
TBox 11.1 Branching process: connecting individual and population-level stochasticity
TBox 11.2 Stochastic individual contributions to population growth and demographic stochasticity
11.3 Extinction time and diffusion
11.2 Loss of genetic diversity and its consequences
TBox 11.4 Genetic drift: the Fisher–Wright model and coalescence
11.3 Diversity patterns under neutral dynamics
Note 11.1 Species–abundance distributions
12.1 Global structures and diversity patterns
12.1.2
12.2 The effect of regional diversity on the diversity and function of local communities
12.3 Spatial patterns of community composition and regulation
List of Online Materials
The following online materials (OLMs) can be found at the book’s companion website: http://tbe.elte.hu/
OLM1.1 Deductive and inductive approaches to ecology
OLM1.2 The water-clock
OLM2.1 The state of the lion, and forecasting its behaviour
OLM2.2 The stochastic process of dying
OLM2.3 Likelihood function and model selection
OLM4.1 Age or size classification for plants
OLM4.2 How do we calculate eigenvalues and eigenvectors?
OLM4.3 Two age classes and a non-generic exception
OLM4.4 Sensitivity and elasticity
OLM5.1 Estimation of the multivariate response function
OLM5.2 Correlative distribution models
OLM6.1 Local density dependence
OLM6.2 Potential large-scale consequences of temperature-dependent Allee effect
OLM6.3 An example for revealing the mechanism of regulation
OLM6.4 Regulation of structured populations: an example
OLM7.1 The equilibrium resource density in case of fluctuating growth rates
OLM7.2 Impact of alien species through shared parasites
OLM8.1 Growth-defence trade-off and its role in biological invasions
OLM8.2 Fitness in age-structured populations
OLM8.3 Relationship of protoplasm/cell wall ratio and photosynthetic capacity
OLM8.4 Calculation of the optimal clutch size in the great tit population of Wytham Woods
OLM8.5 Density-dependent optimization: partial selection pressures for higher reproductive effort
OLM8.6 Division of labour in clonal plants
OLM9.1 K or r0?
OLM9.2 Coexistence of two E. coli strains on a mixture of two resources
OLM9.3 Tilman model: full analysis
OLM9.4 Regulation of plant populations by self-thinning
OLM9.5 The dynamical role of spatial constraints in competitive interactions
OLM9.6 The complex dynamics of Tribolium castaneum
OLM9.7 The chemostat model of the Chlorella–Rotifera system
OLM9.8 Simulated dynamics of simple food webs, based on Wollrab et al. 2012
OLM9.9 Application of the energy pathway method to modules of complex food webs
OLM 10.1 Discrete and continuous niche: the formal problem
OLM 10.2 The requirement/impact niche concept of Leibold: similarities and differences
OLM 10.3 The Maculinea story
OLM 10.4 Historical reflections on the development of the niche concept
OLM 10.5 Niche segregations
OLM 10.6 An example for the calculation of niche overlap
OLM 10.7 Coexistence in fluctuating environments: relative nonlinearity and storage effect
OLM11.1 Predicting the variance increase due to drift
OLM11.2 Calculation of effective population size in fluctuating populations
OLM11.3 Genetic drift and inbreeding
OLM11.4 Spatial differentiation of populations and migration
OLM11.5 Species–abundance distribution under neutral dynamics
OLM11.6 Bestiary of species–abundance distributions
OLM12.1 Comparison of species numbers in samples
OLM12.2 Relationship between species- and community-level variation of biomass
OLM12.3 Hysteresis in high productivity lakes
OLM12.4 Kelp forests and urchin barrens as alternative stable states
List of Figure and Table Credits
Figure 3.3 Reprinted by permission of John Wiley & Sons. 36
Figure 3.4 By permission of Oxford University Press. 41
Figure 3.5 Adapted with permission from Knapp, C.W., Dolfing, J., Ehlert, P.A.I. and Graham, D.W. Evidence of Increasing Antibiotic Resistance Gene Abundances in Archived Soils since 1940. Environmental Science & Technology, 44(2): 580–87. Copyright (2010), American Chemical Society. 44
Figure 3.6a Reprinted from Harper, J. L. Population biology of plants, Academic Press, p.7, 523, Copyright (1977), with permission from Elsevier. 45
Figure 3.6b Selas, V., Hogstad, A., Kobro, S. and Rafoss, T. Can sunspot activity and ultraviolet-B radiation explain cyclic outbreaks of forest moth pest species? Proceedings of the Royal Society B-Biological Sciences, 271(1551): 1897–901. Figure 1b, 2004, by permission of the Royal Society. 45
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Figure 4.12 Reprinted by permission of John Wiley & Sons. 59
Figure 4.13b Republished with permission of Ecological Society of America, from Holmes, E. E., L. W. Fritz, et al. Age-structured modeling reveals long-term declines in the natality of western Steller sea lions. Ecological Applications 17(8): 2214–2232. 2007, permission conveyed through Copyright Clearance Center, Inc. 61
Figure 4.13c Reprinted from Boyd, I. L. Assessing the effectiveness of conservation measures: Resolving the “wicked” problem of the Steller sea lion. Biological Conservation 143(7): 1664–1674. Copyright (2010), with permission from Elsevier. 61
Figure 4.14a,b,c Republished with permission of Ecological Society of America, from Holmes, E. E., L. W. Fritz, et al. Agestructured modeling reveals long-term declines in the natality of western Steller sea lions. Ecological Applications 17(8): 2214–2232. 2007, permission conveyed through Copyright Clearance Center, Inc. 61
Figure 4.19 Republished with permission of Ecological Society of America, from de Kroon, H., J. van Groenendael, et al. Elasticities: a review of methods and model limitations. Ecology 81(3): 607–618. 2000, permission conveyed through Copyright Clearance Center, Inc. 65
Figure 4.21a,b Reprinted by permission of John Wiley & Sons. 67
Figure 4.22 By permission of the Royal Society. 68
Figure 5.2a,b,c Republished with permission of Ecological Society of America, from Hooper, H.L., Connon, R., Callaghan, A., et al. The ecological niche of Daphnia magna characterized using population growth rate. Ecology, 89, 1015–1022. 2008, permission conveyed through Copyright Clearance Center, Inc. 74
Figure 5.5, Azolla Reprinted by permission of John Wiley & Sons. 76
Table 5.2 Reprinted by permission of John Wiley & Sons. 80
Figure 5.9 Reprinted by permission of John Wiley & Sons. 80
Figure 5.10b Reprinted by permission of John Wiley & Sons. 81
Figure 5.12 Reprinted by permission of John Wiley & Sons. 82
Figure 5.13a,b By permission of the British Ecological Society. 83
Figure 5.16a,b By permission of the Royal Society. 87
Figure 5.18 By permission of Oxford University Press. 88
Figure 5.21a,b Figure 1, 4b from Dong, Y. et al. 2008. Biol. Bull. 215: 173–181. Reprinted with permission from the Marine Biological Laboratory, Woods Hole, MA. 91
Figure 6.2 Republished with permission of Ecological Society of America, from Brook, B. W. and C. J. A. Bradshaw. Strength of evidence for density dependence in abundance time series of 1198 species. Ecology 87(6): 1445–1451. 2006, permission conveyed through Copyright Clearance Center, Inc. 97
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Figure 6.6 Thomas, J.A., Simcox, D.J. and Hovestadt, T. (2011). Evidence based conservation of butterflies. Journal of Insect Conservation, 15(1–2): 241–58. With kind permission from Springer Science and Business Media. Original caption: Fig. 6. Relationship between values of the observed intrinsic growth rate (λ) and the observed carrying capacity (or equilibrium level) (K) of Maculinea arion populations following colonisation of 13 unoccupied conservation sites in the UK. 100
Figure 6.10a,b Reprinted by permission of John Wiley & Sons. 103
Figure 6.11a,b Tobin, P. C., C. Robinet, et al. (2009). The role of Allee effects in gypsy moth, Lymantria dispar (L.), invasions. Population Ecology 51(3): 373–384. With kind permission from Springer Science and Business Media. Original caption: Fig. 5 The relationship
between population density (moths/trap) in year t and population replacement rates in t+1 (a) and the estimate of the Allee threshold (based upon data from the shaded area in a), for West Virginia, Virginia, and North Carolina, 1996 to 2004 (mean, black line; 95% confidence intervals, grey lines; reprinted from Tobin et al. 2007b). Source: Tobin, P.C., Whitmire, S.L., Johnson, D.M., Bjørnstad, O.N. and Liebhold, A.M. (2007). Invasion speed is affected by geographical variation in the strength of Allee effects. Ecology Letters, 10(1): 36–43, by permission of John Wiley & Sons. 105
Figure 6.12a Reprinted by permission of John Wiley & Sons. 106
Figure 6.12b By permission of the Royal Society. 106
Figure 6.13b Reprinted from de Villemereuil, P.B. and López-Sepulcre, A. Consumer functional responses under intra- and interspecific interference competition. Ecological Modelling, 222(3): 419–26. Copyright (2011), with permission from Elsevier. 107
Figure 6.15a Republished with permission of Ecological Society of America, from Tilman, D. Secondary Succession and the Pattern of Plant Dominance Along Experimental Nitrogen Gradients. Ecological Monographs, 57(3): 189–214. 1987, permission conveyed through Copyright Clearance Center, Inc. 110
Figure 6.15b Republished with permission of Ecological Society of America, from Ritchie, M.E. and Tilman, D. Responses of Legumes to Herbivores and Nutrients During Succession on a Nitrogen-Poor Soil. Ecology, 76(8): 2648–55. 1995, permission conveyed through Copyright Clearance Center, Inc. 110
Figure 6.16 Reprinted from Creel, S. and Christianson, D. Relationships between direct predation and risk effects. Trends in Ecology & Evolution, 23(4): 194–201. Copyright (2008), with permission from Elsevier. 114
Figure 6.20 Reprinted from Hodgkin, S.E. Scrub encroachment and its effects on soil fertility on Newborough Warren, Anglesey, Wales. Biological Conservation, 29(2): 99–119. Copyright (1984), with permission from Elsevier. 119
Figure 7.2a Reprinted from Dykhuizen, D.E. and Dean, A.M. Enzyme activity and fitness: Evolution in solution. Trends in Ecology & Evolution, 5(8): 257–62. Copyright (1990), with permission from Elsevier. 123
Figure 7.2c From Cook, L.M., Mani, G.S. and Varley, M.E. (1986). Postindustrial Melanism in the Peppered Moth. Science, 231(4738): 611–13. Reprinted with permission from AAAS. 123
Figure 7.4a,b Reprinted by permission of John Wiley & Sons. 126
Figure 7.7a,b Republished with permission of Ecological Society of America, from Passarge, J., Hol, S., Escher, M. and Huisman, J. Competition for nutrients and light: stable coexistence, alternative stable states, or competitive exclusion? Ecological Monographs, 76(1): 57–72. 2006, permission conveyed through Copyright Clearance Center, Inc. 127
Figure 7.10 Adapted by permission from Macmillan Publishers Ltd: Nature.
Lenormand, T., Bourguet, D., Guillemaud, T. and Raymond, M. Tracking the evolution of insecticide resistance in the mosquito Culex pipiens. Nature, 400(6747): 861–64. Copyright (1999). 131
Figure 7.11 By permission of Cambridge University Press. 132
Figure 7.12a Reprinted by permission of John Wiley & Sons. 132
Figure 7.12b Reprinted by permission of John Wiley & Sons. 132
Figure 7.17a,b Reprinted by permission of John Wiley & Sons. 137
Figure 7.18 Reprinted from Fox, J.W. The intermediate disturbance hypothesis is broadly defined, substantive issues are key: a reply to Sheil and Burslem. Trends in Ecology & Evolution, 28(10): 572–73. Copyright (2013), with permission from Elsevier. 137
Figure 7.23a,b Republished with permission of Ecological Society of America, from Venable, D.L. Bet hedging in a guild of desert annuals. Ecology, 88(5): 1086–90. 2007, permission conveyed through Copyright Clearance Center, Inc. 141
Figure 7.24 Reprinted from Fox, J.W. The intermediate disturbance hypothesis is broadly defined, substantive issues are key: a reply to Sheil and Burslem. Trends in Ecology & Evolution, 28(10): 572–73. Copyright (2013), with permission from Elsevier. 142
Figure 8.1a,b From Boag, P.T. and Grant, P.R. (1981). Intense Natural Selection in a Population of Darwin’s Finches (Geospizinae) in the
Galapagos. Science, 214(4516): 82–85. Reprinted with permission from AAAS. 145
Figure 8.2a,b By permission of Oxford University Press. 146
Figure 8.4 From Grant, P.R. and Grant, B.R. (2006). Evolution of character displacement in Darwin’s finches. Science, 313(5784): 224–26. Reprinted with permission from AAAS. 146
Figure 8.8a,b,c Pan, X.-Y., Jia, X., Chen, J.-K. and Li, B. (2012). For or against: the importance of variation in growth rate for testing the EICA hypothesis. Biological Invasions, 14(1): 1–8. With kind permission from Springer Science and Business Media. Original caption: Fig. 6. Relationships between stem TMD (tissue mass density) versus stem growth rate, TMD versus feeding rate, and TMD versus pupation rate of Agasicles hygrophila on Alternanthera philoxeroides. 148
Figure 8.12 Reprinted by permission of John Wiley & Sons. 150
Figure 8.15 Kisdi, E., Meszena, G. and Pasztor, L. (1998). Individual optimization: Mechanisms shaping the optimal reaction norm. Evolutionary Ecology, 12(2): 211–21. With kind permission from Springer Science and Business Media. Original caption: Figure 1. Direct and cost-modifying effects of quality. 153
Figure 8.19a,b Bell, G. 1997. Selection: the mechanism of evolution. Springer. p. 287–288, block 93. With kind permission from Springer Science and Business Media. 157
Figure 8.22 Reprinted by permission of John Wiley & Sons. 159
Figure 8.24a,c Republished with permission of Ecological Society of America, from Both, C., Tinbergen, J.M. and Visser, M.E. Adaptive density dependence of avian clutch size. Ecology, 81(12): 3391–403. 2000, permission conveyed through Copyright Clearance Center, Inc. 161
Figure 8.25a,b Reprinted by permission of John Wiley & Sons. 164
Figure 8.27a,b,c Copyright (2008) National Academy of Sciences, U.S.A. 165
Figure 8.29a Republished with permission of Ecological Society of America, from Tilman, D. and Wedin, D. Plant traits and resource reduction for five grasses growing on a nitrogen gradient. Ecology, 72(2): 685–700. 1991, permission conveyed through Copyright Clearance Center, Inc. 168
Figure 8.30 Republished with permission of Ecological Society of America, from Tilman, D. Competition and Biodiversity in Spatially Structured Habitats. Ecology 75(1): 2–16. 1994, permission conveyed through Copyright Clearance Center, Inc. 169
Figure 9.4a,b Copyright (2001) National Academy of Sciences, U.S.A. 174
Figure 9.5, 9.6 Reprinted from Ayala, F. J., Gilpin, M. E., & Ehrenfeld, J. G. Competition between species: theoretical models and experimental tests. Theoretical Population Biology, 4(3), 331–56. Copyright (1973), with permission from Elsevier. 175
Figure 9.7a,b Reprinted by permission of John Wiley & Sons. 177
Figure 9.8a,b Adapted by permission from Macmillan Publishers Ltd: Nature. Fitzpatrick, M. J., Feder, E., Rowe, L., & Sokolowski, M. B. Maintaining a behaviour polymorphism by frequency-dependent selection on a single gene. Nature, 447(7141): 210–12. Copyright (2007). 178
Figure 9.12 Republished with permission of Ecological Society of America, from Tilman, D. Resource competition between plankton algae: an experimental and theoretical approach. Ecology, 58(2), 338–48. 1977, permission conveyed through Copyright Clearance Center, Inc. 183
Figure 9.15 Reprinted from Weiner, J. Asymmetric competition in plant populations. Trends in Ecology & Evolution, 5(11): 360–64. Copyright (1990), with permission from Elsevier. 187
Figure 9.16, 9.17a,b Reprinted by permission of John Wiley & Sons. 188
Figure 9.18a,b, 9.19 Martinsen, G.D., Cushman, J.H. and Whitham, T.G. (1990). Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community. Oecologia, 83(1): 132–38. With kind permission from Springer Science and Business Media. Original caption for 9.18a,b: Fig. 4. The relationship between magnitude of pocket gopher disturbance and A abundance of perennial grasses (y = 0.976−0.843x; r2=0.70; P<0.005), B abundance of perennial dicots (y = 0.0132+0.821x; r2=0.71 ; P<0.005). For 9.19: Fig. 2. The relationship between magnitude of pocket gopher disturbance and species diversity (H’) for all 21 subplots (y=0.1+1.76x−1.34x2; r2=0.67; P<0.005). 190, 191
Figure 9.20a,b,c Reprinted by permission of John Wiley & Sons. 192
Figure 9.20d From Fussmann, G.F., Ellner, S.P., Shertzer, K.W. and Hairston, N.G. (2000). Crossing the Hopf bifurcation in a live predator-prey system. Science, 290(5495): 1358–60. Reprinted with permission from AAAS. 192
Figure 9.23a,b Reprinted by permission from Macmillan Publishers Ltd: Nature. Yoshida, T., Jones, L.E., Ellner, S.P., Fussmann, G.F. and Hairston, N.G. Rapid evolution drives ecological dynamics in a predator-prey system. Nature, 424(6946): 303–06. Copyright (2003). 195
Figure 9.27 Reprinted by permission of John Wiley & Sons. 198
Figure 9.28 Reprinted by permission of John Wiley & Sons. 198
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Figure 10.7a,b By permission of the Royal Society. 207
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Figure 10.9b Reprinted by permission of John Wiley & Sons. 212
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Figure 10.13a,b Szilágyi, A. and Meszéna, G. (2009). Two-patch model of spatial niche segregation. Evolutionary Ecology, 23(2): 187–205. With kind permission from Springer Science and Business Media. Original caption for panel a: Figure 4. Robustness of coexistence of two species against an extra mortality. For panel b: Figure 5. Robustness of coexistence of the two species of Fig. 2 against an extra mortality ▵1 of the first species is plotted as a function of migration rate. 216
Figure 10.15a,b,c Copyright (2006) National Academy of Sciences, U.S.A. 219
Figure 10.20 Reprinted by permission of John Wiley & Sons. 225
Figure 10.21a,b By permission of the Royal Society. 225
Figure 10.21c By permission of the Royal Society. 225
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Figure 10.24 Reprinted figure with permission from Meszéna, G., Gyllenberg, M., Jacobs, F.J. and Metz, J.A.J. Physical Review Letters, 95(7): 078105. Copyright (2005) by the American Physical Society. 227
Figure 11.7a,b Reprinted by permission of John Wiley & Sons. 238
Figure 11.8a,b,c Reprinted by permission of Sinauer Associates. 241
Figure 11.11a,b,c By permission of Oxford University Press. 244
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Figure 12.5a,b,c,d Republished with permission of Ecological Society of America, from Staver, A.C., Archibald, S. and Levin, S. Tree cover in sub-Saharan Africa: Rainfall and fire constrain forest and savanna as alternative stable states. Ecology, 92(5): 1063–72. 2011, permission conveyed through Copyright Clearance Center, Inc. 254
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Figure 12.11c Reprinted by permission of John Wiley & Sons. 258
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Figure 12.26b Reprinted by permission of John Wiley & Sons. 267
Figure 12.29a,b From Chase, J.M. (2010). Stochastic Community Assembly Causes Higher Biodiversity in More Productive Environments. Science, 328(5984): 1388–91. Reprinted with permission from AAAS. 268
Figure 12.30a Reprinted from Rietkerk, M. and Van de Koppel, J. Regular pattern formation in real ecosystems. Trends in Ecology & Evolution, 23(3): 169–75. Copyright (2008), with permission from Elsevier. 269
Figure 12.31a,b,c, 12.32a,b From van de Koppel, J., Gascoigne, J.C., Theraulaz, G., Rietkerk, M., Mooij, W.M. and Herman, P.M.J. (2008).
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Figure 12.33a,b,c, 12.34 Reprinted by permission of John Wiley & Sons. 272
Figure 12.36 Reprinted by permission of John Wiley & Sons. 273
Figure 12.37 From Staver, A.C., Archibald, S. and Levin, S.A. (2011). The Global Extent and Determinants of Savanna and Forest as Alternative Biome States. Science, 334(6053): 230–32. Reprinted with permission from AAAS. 274
PART I Introduction
CHAPTER 1
Introduction: Darwinian ecology
Overview
Tempted by the impossible, we build ecology from first principles. We start out by explaining the seven principles that constitute the basis of our Darwinian ecological theory. These principles are rules of generic validity, supported by a coherent mathematical theory and providing a firm logical structure to ecology. Since the central concept of theory-based ecology is population regulation and its essential method is the separation of scales, we devote a complete subchapter to the explanation and illustration of these issues through simple physical analogues of selfregulating ecological systems, our main subjects in this book. To introduce the reader to the logical and formal treatment used in further chapters, we recall and explain a few fundamental abstract concepts like speed, derivative, and state description in a theory box, and introduce the central variable of ecology: the rate of population growth. An understanding of the concepts of population and fitness, which we define with a semantic range different from the usual, is also essential. It was Darwin who first revealed the ecological principles of, and gave a process-based, mechanistic explanation to, the origin of species, i.e., the evolution of diversity. That is why we call our ecology Darwinian, and start the book with revealing the logical blueprint of his theory.
1.1 Darwin’s explanation for the emergence and maintenance of diversity
For the general public—and even for some professional biologists—the characteristic image of Charles Darwin is that of the old-fashioned scientist meticulously collecting and classifying huge amounts of data with the intention to build the theory of evolution fact by fact. He is much less often considered as a modern scientist who based his theory on general principles and hypotheses, thinking in terms of the mechanisms of evolution acting in nature. He is thought to be much more of an excellent detective—the pre-image of Sherlock Holmes and Hercule Poirot—systematically collecting evidence to illustrate the hypothesis of evolution than the founding father of a coherent scientific theory consistent with the entirety of modern biology. A closer look at the third and fourth chapters of ‘The Origin of Species’ (Darwin 1876) immediately reveals, however, that Darwin himself refers to a set of logically
interconnected principles when he explains the mechanisms of species divergence, and his (numerous) examples are direct illustrations of these principles. It is the rule of ‘Geometrical Ratio of Increase’ (ibid., p.50), the doctrine of Malthus on the inevitable limits to population increase (ibid., p.50), the fact of individual variability (ibid., p.48.), the principle of natural selection (ibid., p.49), the conclusion that ‘the struggle will almost invariably be most severe between the individuals of the same species’ (ibid., pp.58–59), and the principle of ‘divergence of character’ (ibid., p.46, p.87) which form the inner structure of Darwin’s deductive theory. The modus operandi of Darwin was in fact the hypothetico-deductive approach (Penny 2009), which was built on coherent principles.
Understanding the emergence and maintenance of species diversity, and thus the explanation of biodiversity, is a crucial problem common to ecology and evolutionary theory. For Darwin himself, the coexistence of varieties and the emergence of new species was the inevitable product of natural selection for divergence.
In Darwin’s view natural selection on inherited variants stems from the empirical fact—as well as logical necessity—that, owing to the inherent potential for exponential population growth of all living creatures (rule of geometric ratio of increase), the population increase of any variety must be checked sooner or later (doctrine of Malthus). Even if there is only a tiny advantage of a certain new variant over existing ones, its relative frequency will increase (principle of natural selection). As a result of this process each slight variation, if useful, is preserved (ibid., p.49) whereas less fit variants vanish sooner or later. With these considerations in mind, a nontrivial question arises: in what ways does the struggle for existence give rise to an evolutionary process which diverges into many branches of varieties?
Darwin claims that the universal potential of all living creatures for exponential population growth saturates Nature: ‘Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants’ (ibid., p.85). The contest must be toughest among the most similar variants which share their checks to increase: ‘But the struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers’ (ibid., pp.58–59). We nowadays consider space (shared ‘districts’), resources (‘same food’), and natural enemies (‘common dangers’) as potential regulating factors of natural populations. Severe struggle between these similar varieties leads to the extinction of the less fit variants as ‘It may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being prevented) could be kept up for half-adozen generations if they were allowed to struggle together’ (ibid., p.59). Thus, because each area is fully stocked, ‘as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare’ (ibid., p.85)— Darwin completed his argument.
Darwin explained how variants become different in order to survive in each others’ presence, i.e., to coexist, by the principle of divergence: ‘ the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers’ (ibid., p.87). (Here the ‘place in the polity of nature’ corresponds to the modern notion of niche.) Only those varieties are able to spread in the presence of the other variants that are adapted to new niches. Darwin also gave a very enlightening ‘imaginary illustration’
to this thesis: ‘ Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; […] there are two varieties of the wolf inhabiting the Catskill Mountains in the United States, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd’s flocks’ (ibid., pp.70–71). Darwin’s population dynamic approach is fully transparent in this example: the checks to increase, i.e., the regulating factors of the two wolf types, are different (different prey) and this provides an opportunity for both of them ‘to increase in numbers’, i.e., to coexist. This principle of divergence assumes implicitly that wolves cannot produce variants that are as perfect in attacking sheep as they are in pursuing deer (the traits are traded off).
Besides his dynamic approach Darwin, the experienced experimentalist, was aware of the stochastic nature of neutral processes and considered their potential effects: ‘Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.’ (ibid., p.63). However, he definitely excluded that ‘mere chance’ may explain species divergence, i.e., speciation: ‘Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large a degree of difference as that between the species of the same genus.’ (ibid., p. 86). Apart from its obvious relevance in generating stochastic variation of inherited traits, neutral variations do not have a role in Darwin’s theory of diversification.
1.2 The Darwinian principles of ecology
In the spirit of Darwin’s hypothetico-deductive approach the ecology to be presented in this book relies on principles and their theoretically deduced consequences (Online Material (OLM) 1.1 at http://tbe.elte. hu/). The general principles behind formalized ecological theories correspond to principles regularly recurring in Darwin’s texts (the rule of geometrical increase, the doctrine of Malthus, and the inevitable connection of similarity and the strength of competition that lead to the principle of natural selection, and the principle of character divergence) besides two basic facts (the heritability of some individual differences and the
Table 1.1 Darwinian principles in actual ecological terms.
Principle & Chapter No.
principle 1 ch3expo, ch4struct, ch5Toler
principle 2 ch6regul
principle 3
principle 4 ch11finit
principle 5 ch7excl
principle 6 ch9coex
principle 7 ch8Trade-off
Formulation of the principles
Principle of exponential growth. The number of reproductive units is expected to grow exponentially in the absence of regulating feedbacks.
Principle of growth regulation. negative feedback of density on population growth rate is inevitable, and it is necessary to provide zero, i.e., regulated, population growth of reproductive units in the long run.
Principle of inherited individual differences. replication is imprecise, thus inherited variations emerge repeatedly.
Principle of finiteness. stochastic behaviour arises from the inevitable finiteness of population size. Due to the finite number of individuals the number of varieties is also constrained, and each can go extinct merely by chance.
Principle of the survival of the fittest/competitive exclusion. in a population of reproductive units regulated by a single common factor all varieties but the one whose growth rate is highest at the extremal value of that factor will be excluded.
Principle of robust coexistence. The larger the difference between the competing varieties in their way of growth regulation, the more robust their coexistence is.
Principle of constraints and trade-offs. Variations of individual traits are usually cross-constrained, and the components of fitness are generically traded off.
fluctuations of neutral features). Darwin’s principles apply to the coupled dynamics of multiple interacting variants, connecting the principles of population dynamics to the rules of coexistence and evolution. We have merged these Darwinian principles and the two non-selective mechanisms of generating and shaping diversity into a coherent set of seven principles on which, we believe, a synthetic theory of Darwinian ecology can be built.
Even though all its elements are well known, such a coherent and comprehensive system of principles currently does not exist either in evolutionary genetics or in ecology. The first list of principles (variation, heritability, and selection) of the theoretically most advanced evolutionary discipline, evolutionary genetics, as put forward by Lewontin (1974; 2010), implicitly include the principle of regulation, which is connected to the metaphor of ‘the struggle for existence’. On the other hand, the majority of ecological theories separate the principles of the dynamics of single populations (Turchin 2001; Berryman 2003) from those of coexistence (Hardin 1960; MacArthur 1962; Levin 1970; Tilman
Sources of alternative or similar formulations
first principle of population dynamics, law of inertia (ginzburg 1983, Turchin 2001, Berryman 2003)
second principle of population dynamics (Berryman 2003), self-limitation (Turchin 2001)
principle of variation, principle of inheritance principle of mutation (lewontin 1970, 2010), principle of speciation (Vellend 2010)
genetic drift (fisher 1930, wright 1931), evolutionary rate at the molecular level (Kimura 1984), neutral theory of biodiversity (hubbell 2001), ecological drift (Vellend 2010)
coexistence band width (armstrong 1976, abrams 2001), unified principle of competitive exclusion and limiting similarity (Meszéna et al. 2006)
principle of allocation (levins 1962, 1968), principle of allocation in life-history context (sibly and calow 1986)
1980; Holt et al. 1994; Rönicke et al. 2010), and traditionally ignore the influence of within-population genetic variation (but see Hairston et al. 2005). Adaptive dynamics (Metz and Diekmann 1986) and Darwinian dynamics (Vincent and Brown 2005) formalize the Darwinian link between the principles of population dynamics and those of natural selection.
We have listed the principles and the sources of a number of their different formulations in Table 1.1. The original principles worded by Darwin are rephrased to tailor them to recent scientific terminology without altering their meaning and coherence. The first column of Table 1.1 gives the actualized form of the principles; the second one gives the sources for alternative formulations in the literature. Figure 1.1 shows the logical structure of the theory’s armature.
Principle 1 Principle of exponential growth
The principle of exponential population growth expresses that self-reproducing entities can increase
in number exponentially, i.e., their population size can be an exponential function of time (Eq. 1.6). This principle is often stated in the narrow sense, for static (unchanged) environments, even though it applies equally well to changing environments, provided that the size of the population does not affect its own rate of increase, i.e., in the absence of regulating feedback. We shall show that the inherent potential for exponential growth plays a crucial role both in population regulation and in coexistence. Exponential population growth is treated in detail in the second part of the book (Ch3Expo, Ch4Struct, and Ch5Toler).
Principle 2 Principle of growth regulation
It is easy to see that population growth is necessarily limited, because every organism needs habitable space and resources for its maintenance and reproduction; neither of these is infinite, so the exponentially accelerated growth of any population will be halted sooner or later for lack of space or resources. This may seem a triviality, yet the importance or negligibility of population regulation has been the subject of recurrent debates in ecology during the last century (Cooper 2003). All this in spite of the fact that evolutionary genetics has considered regulation as a self-evident fact from the beginning, by assuming static equilibrium populations as their most general postulate (Haldane 1956; Bell 1997; Maynard Smith 1998). The turn of the century seems to have brought a breakthrough in this field: the state of the art is a nearly complete consensus on the general importance of population regulation, and the characterization and detailed understanding of the mechanisms of regulation are essential for studying population dynamical patterns (Turchin 2001) and understanding community composition (Tilman 1982; Pimm 1991).
The various formulations of this principle (Table 1.1) are all based on the recognition that limited population growth requires a negative feedback loop between population size and population growth: increasing population size decreases the growth rate, while decreasing size increases it. To see this, one has to overcome the difficulties of thinking in terms of the abstract concepts related to change (TBox 1.1). However, it is easy to see the existence of ‘checks on growth’, as for its understanding it is sufficient to consider static systems and structures. We have devoted the next subchapter (Ch1.3) to explaining the concept of regulation and its mathematical
representation, using a simple self-regulated physical system for illustration. The chapters on population regulation (Ch6Regul), competitive exclusion (Ch7Excl) and coexistence (Ch9Coex) discuss the potential outcomes of interactions between individuals by connecting Lotka–Volterra type and processbased models of population regulation.
Principle 3 Principle of inherited individual differences
The occurrence of heritable individual differences was an empirical fact for Darwin, but—in the light of our present knowledge on the mechanism of genetic inheritance—it has become one of the principles: perfect copying of genetic material is impossible due to the inherent stochastic nature of the reactions involved in DNA replication. Richard Lewontin, a leading advocate of integrated population biology, had formulated this principle as two separate principles: those of variation and inheritance (Lewontin 1970; 2010). The primary sources of heritable individual differences are mutations producing new alleles or new genes. Heritable individual variation is also the ultimate source of variation at the population level, leading to the occurrence of new species through speciation. The fact that the same principle applies to any reproductive unit; mutant clones, alleles and new species in terms of their invasion and coexistence with other variants within the population (community) constitutes the basis for a unified treatment of population biological processes (Ch1.3.3). We shall give examples of this unified treatment in each chapter.
Besides the eternal inner source of variation, the physical conditions on Earth would be spatially heterogeneous and temporally variable even without life, which does structure its own environment in its turn, even in a test tube. The last chapter (Ch12Divers) attempts to sketch a possible scenario as to how the discussed ecological mechanisms and external environmental drives are shaping and maintaining the diversity of communities and ecosystems on various spatiotemporal scales.
Principle 4 Principle of finiteness and stochasticity
The physiological and behavioural responses of the individuals to their environments depend on their genomes, states and the environment itself. No matter how precisely we characterize the individual differences
in physiological responses to the environment, there usually remains an unexplained portion of variability between responses of individuals in the same state. Moreover, the environments of the individuals change stochastically as well. Individual behaviour is unavoidably probabilistic and populations are of finite sizes; therefore, the number of individuals and alleles fluctuates stochastically. In population genetics, the effect of finite population sizes on allele frequencies is called ‘genetic drift’, which has many different, but in essence equivalent, dynamical and statistical representations (Crow and Kimura 1970; Rice 2004; Hartl and Clark 2007). Hubbell (2001) suggested applying a similar ‘neutral’ approach to the distribution of species population sizes within ecological communities. Stochastic treatment may be necessary to consider when applying any one of the basic principles, as the size of natural populations is always finite. However, since the effects of stochasticity due to finite population sizes are separable from those of the principles themselves, and since a systematic treatment of stochastic processes is beyond the scope of this book, we will engage in stochastic illustrations only in a few, indispensable cases (like in TBox 3.2, TBox 4.1). Ch11Finit is devoted to a number of problems arising from finiteness and a brief discussion of Hubbell’s neutral theory (Hubbell 2001).
Principle 5 Principle of the survival of the fittest/competitive exclusion
The principle of competitive exclusion (Gause’s principle) is a consequence of the universal capacity of all living creatures for exponential population growth, and the inevitable limits of increase in a finite world. The most famous wording of the principle by Hardin (1960, ‘Complete competitors cannot coexist’) was meant to express the same thought as that of Hutchinson, who stated that different species occupying the same niche cannot coexist (Hutchinson 1978). The fixation of the fittest allele in a Mendelian population is a process of allele selection which is equivalent to competitive exclusion. In the context of population regulation the K-maximization principle of MacArthur (1962), the R* rule of Tilman (1980) and the P* rule of Holt et al. (1994) express the competitive exclusion principle. In the most general sense we may say that from among the varieties regulated by a single factor there is only one that can survive in the long term: the one which is able to increase in number at the worst conditions of the common regulating factor. More precisely, the winner is the variant that reaches zero growth at the pessimal (maximal or minimal) value of the regulating
factor (Metz et al. 2008). The principle of competitive exclusion is treated in Ch7Excl.
Principle 6 Principle of robust coexistence
Inherited variants of reproductive units (alleles, clones, species; Ch1.3.3) that are rare in a population can persist if and only if they enjoy growth advantage compared to more common resident variants. Their coexistence is sufficiently robust if it is possible under a wide range of environmental conditions. The principle of robust coexistence states that this can be achieved if the rare type’s population growth is regulated in a manner sufficiently different from that of the resident (Meszéna et al. 2006). A qualitative novelty of the concept— compared to conventional stability studies—is that the sufficient difference between competitors depends also on the effect of the relevant non-regulating (modifying; Ch1.3.2) variables. The principle of robust coexistence is the basis for the extension (Ch10Niche) of classical niche theory (Hutchinson 1957; MacArthur and Levins 1967; Hutchinson 1978) and has its roots in the works of Levin (1970), Vandermeer (1975), Armstrong (1976), and Abrams (2001). While the stability of dynamical equilibria, the asymptotic behaviour of populations, and complex dynamics have become integral parts of general ecological thinking, the notion of robust coexistence is used by a relatively narrow circle of theoretical ecologists so far (Abrams and Holt 2002; Abrams 2004; Meszéna et al. 2006). In the next subchapter we start explaining the concepts of regulating factors and the components of the feedback loop, which we will further elaborate on in Ch5Toler and Ch6Regul, and complement it with the concept of robust coexistence in Ch9Coex and Ch10Niche.
Principle 7 Principle of constraints and trade-offs
In spite of the tight genetic control and the highly modular organization of most living beings their different properties cannot be optimized independently. The components of fitness can often be increased only at the cost of other fitness components. For example, increasing reproductive investment in many cases decreases parental survival, or tolerance of high population density may decrease reproductive output. These are fitness-related traits, which are often traded off. In the absence of trade-offs, evolution would produce perfectly adapted, unbeatable
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les moyens dont on dispose au Sahara... L’Oudan ne paraît pas être un ancien volcan, ainsi que l’avaient fait supposer à Duveyrier des renseignements indigènes. » Cette description, due à Voinot, semble cependant indiquer un plateau basaltique, analogue à l’Adjellela.
La Tifedest se continue sur la rive gauche de l’Igharghar, jusqu’à hauteur du Mouidir, par une série de massifs isolés dont le plus important est l’Edjelé, qui se dresse à une altitude notable au milieu du reg.
Fig. 18. — L’Adr’ar’ Arigan, dyke éruptif dans les contreforts méridionaux de l’Ahaggar.
Du point d’eau de l’oued Zazir.
Sur la rive droite de l’Igharghar, au nord de la Tifedest, l’Edjéré (ou Eguéré) arrive au voisinage du tassili des Azdjer ; c’est un massif de grande étendue qui, de loin, figure vaguement un cône très aplati. Son point culminant, le Toufriq, atteint 1560 mètres. Sa structure est analogue à celle de la Coudia ; comme elle, l’Edjéré est un plateau surmonté de formations volcaniques ; les cratères ébréchés y sont nombreux, et les bombes volcaniques y abondent.
Les vallées étroites de ce massif sont, certaines années, couvertes de beaux pâturages, où se réunissent parfois les Azdjer et les Ahaggar ; les points d’eau y sont assez espacés, mais de fort débit et peu profonds ; les puits ne dépassent pas 4 mètres.
Ce massif se prolonge vers le sud par la petite chaîne de Torha qui n’est séparée de la Tifedest que par la vallée de l’Ighargar ; vers l’est, le massif de Torha se termine brusquement au-dessus de la haute plaine d’Amadr’or.
L’Anahef est un plateau très semblable à la Coudia, qu’il prolonge vers l’est ; comme elle, il est surmonté de gours, derniers témoins d’un étage disparu et d’aiguilles granitiques dont la plus remarquable semble être le Tihi n’Kalan. L’Anahef, que traversent quelques pistes allant de l’Ahaggar à R’at, paraît d’un accès peu facile ; les points d’eau, situés au pied de la montagne, sont peu nombreux ; Voinot en mentionne seulement quatre, sur le versant Atlantique.
La partie sud de l’Anahef qu’a explorée Foureau [Doc.Sc., p. 345 et 614] lorsqu’il a été reconnaître le point où est mort Flatters, ne semble pas différente de celle qu’a vue Voinot.
A son extrémité orientale, l’Anahef se relie à une série de hauteurs qui, se dirigeant vers le nord, viennent à peu de distance de Tir’ammar et du tassili des Azdjer. Elles se terminent par les deux massifs importants d’Adr’ar’ (1700) et d’Admar (1400).
Le tassili des Azdjer est formé de grès horizontaux, d’âge dévonien, et reproduit exactement les formes de terrain de l’Ahnet ou du Mouidir, dont il est la suite.
Entre ce tassili et les massifs anciens qui dépendent de l’Ahaggar, il existe, au moins depuis l’Igharghar jusqu’à l’Admar, une zone en général assez déprimée, qui offre des communications faciles entre l’est et l’ouest ; la piste qui y passe est souvent suivie par les rezzou.
Au centre du paquet montagneux que forme la Coudia et ses annexes s’étend une immense plaine dont l’origine est assez ambiguë. La haute plaine d’Amadr’or n’est pas une sebkha, mais bien un immense reg, long d’environ 120 kilomètres du nord au sud
et d’une largeur moyenne de 60 kilomètres. Cette plaine peut être considérée comme horizontale ; la différence d’altitude atteint à peine 100 mètres entre le nord et le sud ; quelques gours isolés et insignifiants de granite rose font seuls saillie sur le reg. La végétation y fait en général défaut et il n’y existe aucun point d’eau. Le cours de l’oued Amadr’or et de ses affluents n’est plus indiqué que par quelques cuvettes à peine perceptibles et qui, depuis longtemps, ont cessé de communiquer entre elles ; quelques-unes sont marquées par une très maigre végétation d’éthels et de guétaf. Au cours d’un orage qui a duré deux jours, Voinot a pu voir toute l’eau tombée se rassembler en flaques stagnantes dont chacune correspondait à l’un de ces bas-fonds ; ce n’est que plus au nord, grâce à l’Edjéré, que l’oued Amadr’or, sous le nom d’oued Tidjert, reprend un peu de vie et redevient continu.
Il existe bien du sel dans la plaine d’Amadr’or, mais il est localisé en un point unique : la sebkha d’Amadr’or se réduit à une petite dépression, située à 5 kilomètres au nord de Tissint.
Le sel, qui s’y présente en cristaux cubiques, est facile à extraire ; pour le purifier tout à fait, on souffle légèrement dessus et ce vannage rudimentaire suffit à obtenir un produit d’un beau blanc et d’excellente qualité, qui a grande réputation au Soudan : on l’exporte jusqu’à Zinder, où il a une haute valeur.
Cette petite sebkha d’Amadr’or n’est pas loin de l’extrémité méridionale de l’Edjéré ; près d’elle se dresse une gara basaltique et tout le reg qui l’avoisine est jonché de débris de laves. Le sel provient probablement du lavage des roches volcaniques.
A part le reg d’Amadr’or, toutes les parties du Massif Central saharien se ressemblent. On prendra une idée moins incomplète des aspects du pays en consultant, outre les photographies, et les croquis joints à ce volume, ceux qu’ont donné le commandant Dinaux, le capitaine Arnaud et le lieutenant Cortier[33] . Mais le soleil leur fait défaut : « Ces vues du massif de la Coudia, déchiqueté et fantastique, donnent l’impression d’un pays noir et lugubre.
« Au contraire, l’ensemble des paysages reste clair ; ce sont des pastels délicats, des jeux variés de lumière sur les blocs de granite rose, les plateaux de grès (?) pâles, les coulées grises des laves ; une richesse de tons, une délicatesse de nuances, exagérées encore par la limpidité et la profondeur de l’atmosphère.
« La Coudia est un massif informe, sans harmonie et sans ligne ; c’est un squelette décharné, mais les couleurs le transforment en décor féérique. » (Dinaux). C’est le soir et le matin surtout, que les couleurs sont merveilleuses ; dans l’après-midi, la lumière du soleil est trop écrasante, les nuances perdent toute délicatesse ; toutes les couleurs deviennent des gris.
Hydrographie. — Ce haut massif a été un centre hydrographique important. Naissant près d’Idélès, l’Igharghar, accru d’assez nombreux affluents, traversait le tassili des Azdjer près d’Amguid et allait aboutir au chott Melr’ir. Ce fleuve important et son affluent principal, l’oued Mia, descendu du Tadmaït, fertilisent encore les principales oasis du Sud constantinois. Duveyrier le premier avait pu mettre en évidence l’importance de ce bassin dont les recherches patientes de Foureau ont bien fait connaître les parties moyennes ; les officiers de Tidikelt nous en ont fait, plus récemment, connaître le bassin supérieur.
Vers l’ouest, un grand nombre de ruisseaux, descendus de la Coudia et de la Tifédest, se réunissent en deux troncs principaux, l’oued Takouiat et l’oued Tamanr’asset, que coupe le medjebed d’In Zize à Timissao. On sait que ces deux fleuves coulent encore parfois assez loin et que leurs crues se font sentir jusqu’au méridien de Timissao. Ces crues doivent être violentes, puisqu’elles suffisent à entraîner des scories basaltiques au nord du tassili Tan Adr’ar’.
Le Tamanr’asset et le Takouiat n’ont pas été suivis bien loin vers l’ouest ; on ne sait pas comment ils vont se perdre dans le tanezrouft qui relie Azzelmatti à Sounfat ; on ignore quelles relations exactes ils ont avec Taoudenni et les oueds qui descendent du nord
de l’Adr’ar’ des Ifor’as ou du Timetr’in, et dont l’oued Ilok semble être le principal collecteur.
Au sud, l’oued Zazir, l’Igharghar[34] , l’oued Tagrira, le Tin Tarabin vont se joindre presque certainement au Taffassasset, qui se rattache actuellement au bassin du Niger. Les cours supérieurs de ces rivières sont seuls connus ; il subsiste entre l’Adr’ar’ des Ifor’as et l’Aïr un blanc considérable ; mais elles ne peuvent aboutir qu’au Niger, ou à un bassin fermé inconnu, dont rien d’ailleurs ne permet de prévoir l’existence (V. carte géologique hors texte).
Les rivières qui descendent de l’est de la Coudia ou de ses contreforts ont une histoire plus obscure. Il n’y a pas de doute pour l’oued Tadent qui est sûrement un affluent du Tin Tarabin. Les origines du Taffassasset sont moins claires. L’oued Falezlez, ou Afahlehle, a été coupé par Barth et par von Bary vers le 7° longitude est ; en ce point il se dirige vers le sud-est et von Bary indique, d’après ses informateurs indigènes, qu’il aboutit à Bilma. Barth et plus tard Duveyrier ont cru au contraire que le Falezlez était la tête du Taffassasset ; une carte récente[35] du Sahara a adopté cette opinion. Il en résulte, pour le tracé du fleuve, un coude bizarre que rien jusqu’à présent ne vient justifier, sauf peut-être l’importance du Taffassasset à In Azaoua qui permet de supposer un vaste bassin. En tous cas, toutes les rivières qui, au sud de Tadent, se dirigent vers l’est et qui ont été reconnues par Barth, vont aboutir au Taffassasset. Foureau [Doc. Sc., p. 247] a donné un schéma de ce bassin hydrographique.
Les villages. — La pluie n’est pas très rare sur la Coudia, et les rivières qui en descendent présentent une structure qui permet à l’eau de se conserver assez longtemps dans certaines vallées.
Les rivières des contreforts de l’Ahaggar sont, en effet, d’ordinaire encaissées et assez indépendantes de la direction des affleurements de roches imperméables, au milieu desquels elles ont creusé leur lit ; la vallée, souvent assez large, se rétrécit toutes les fois qu’elle rencontre un seuil rocheux plus résistant, quartzite
silurienne ou filon éruptif : si ces rivières coulaient, elles présenteraient des rapides. Cette structure en chapelet est très nette presque partout.
Entre deux barrages successifs, chaque bief présente une pente notable et l’eau tend à s’accumuler contre le seuil d’aval, de sorte que la nappe aquifère est d’autant moins profonde que l’on se rapproche de ce seuil ; aussi les pâturages, situés à l’amont des barrages, sont fréquents et permettent souvent l’élevage de troupeaux assez nombreux.
De plus, l’eau, arrêtée à chaque barrage, est à un niveau un peu plus élevé que le bief suivant de la vallée ; cette particularité des oueds a été le plus souvent utilisée pour la création des petits centres de culture qui caractérisent les contreforts de l’Ahaggar.
Dans certains cas, les plus fréquents, on va chercher, par des foggaras longues de 5 à 6 kilomètres, l’eau en amont d’un barrage ; des seguias surélevées permettent une irrigation facile dans les parties plus basses ; c’est ce procédé qui est employé à Tamanr’asset, à Tit et à Tin Amensar, toutes les fois que les alluvions humides, véritables mines d’eau, sont dans une vallée trop étroite pour permettre facilement l’établissement de jardins, toutes les fois surtout que des crues violentes sont à craindre, qui enlèveraient toutes les cultures.
Parfois, dans le haut pays, il y a des ruisseaux permanents ; les foggaras deviennent alors inutiles et de simples seguias suffisent à assurer l’irrigation.
Quant aux jardins, ils sont établis dans les vallées les plus larges, dans celles où le lit de l’oued est creusé au milieu d’une plaine d’alluvion : on les cultive sur le lit majeur de l’oued quaternaire ; en général, dans ces vallées élargies, la nappe aquifère est profonde et c’est pour cette cause que l’on va chercher l’eau dans un bief supérieur.
Plus rarement la vallée est large, l’eau abondante à fleur de sol ; c’est ce qui arrive à Abalessa. Tous les oueds qui descendent du
versant occidental de la Coudia coulent d’abord dans une région déprimée, une cuvette synclinale, que limite à l’ouest la chaîne élevée de l’Adr’ar’ Aberaghettan. Cette haute sierra, formée de quartzites, a résisté à l’érosion qui n’a pu réussir à y creuser que quelques gorges resserrées, quelques brèches exiguës ; la plus importante livre un étroit passage à l’oued Endid, formé de la réunion d’une dizaine d’oueds ou de ruisseaux dont les plus notables, l’oued Outoul, l’oued Tit et l’oued Ir’eli, viennent tous converger à Abalessa.
La brèche qui donne passage à tous ces oueds arrête les eaux et, à ses abords immédiats, pendant quelques cents mètres, se trouve un véritable fourré où dominent les tamarix ; c’est, comme arbres, un des plus beaux coins de l’Ahaggar.
Lorsque, venant de Silet, on a traversé la région chauve et dénudée de l’Adr’ar’ Ouan R’elachem, au sommet du dernier col, la vue d’une telle profusion de ferzig et d’ethel est une joyeuse surprise. Les arbres sont accompagnés de nombreux arbustes et de nombreuses graminées ; il y a même quelques fleurs. Plusieurs hectares sont réellement couverts d’une véritable verdure : pareil spectacle est vraiment rare au Sahara.
L’Adr’ar’ Aberaghettan, barrant un ensemble de vallées, ne fait que reproduire en plus grand la disposition des seuils transversaux qui, dans chaque oued de l’Ahaggar, accroissent, vers l’aval du bief, l’humidité des alluvions, et dont l’effet se traduit habituellement par un accroissement des pâturages et l’apparition d’arbres plus serrés.
Rarement cette structure est autant marquée qu’à Abalessa, et peu de villages sont aussi riches.
A Abalessa, on a pu creuser, dans chaque jardin, des puits peu profonds (2 à 3 m.). Ce sont souvent des puits à bascule, type classique dans les oasis, comme aussi en Anjou ; parfois l’outre à manche et à double corde, tirée par un âne, comme dans le M’zab ou à Iférouane, vient simplifier le travail du haratin. Il y a de plus quelques foggaras et, en fait, dans la plupart des ar’érem, les deux
systèmes, puits et foggaras, coexistent : tout l’effort des cultivateurs a porté sur l’exploitation de l’eau, et la meilleure façon de l’avoir en abondance.
Parfois d’autres causes sont intervenues, qui rendent possible l’établissement de jardins ; à Silet, par exemple, la vallée est largement ouverte : une coulée de basalte, descendue de l’Adr’ar’ Ouan R’elachem recouvre les alluvions de l’oued Ir’ir’i ; la vallée de l’oued Silet est, en amont du ksar, probablement elle aussi dans le même cas [Villatte, loc. cit., p. 221] ; l’eau, protégée contre l’évaporation, est très abondante et pendant plusieurs kilomètres, en aval du front de la coulée, il suffit de creuser légèrement (0 m. 20-0 m. 30) dans l’oued, pour trouver le niveau aquifère. La vallée est couverte d’une très belle végétation ; les Salvadorapersica forment un véritable taillis qui s’étend à plusieurs kilomètres de Tibegehin.
Silet et Tibegehin sont les plus belles palmeraies de l’Ahaggar. Malheureusement, malgré leur richesse en eau et leur abondance en dattiers, les deux villages jumeaux ont du être abandonnés : on se contente de venir y cueillir les dattes, lorsqu’elles sont mûres, dans la première quinzaine du mois d’août. Le reste du temps tout est à l’abandon ; on ne coupe jamais les palmes desséchées et les hautes tiges des dattiers sont couvertes d’un manchon de djerids jaunes et desséchées qui pendent misérablement vers le sol ; ces palmes forment, il est vrai, avec leurs épines, un obstacle difficile à franchir et protègent les régimes contre le vol d’un passant.
Il ne reste à Silet que les ruines d’un ksar et des traces de seguia, longues de 300 mètres, qui partent de la coulée de basalte. Malgré les facilités de culture, Silet était mal placée. Située à la limite de l’Ahaggar, à la porte du tanezrouft, Silet ne pouvait savoir ce qui se passait dans l’ouest : les pâturages font défaut dans le tanezrouft, et nul berger ne pouvait assurer la couverture du village : les rezzou y tombaient à l’improviste ; l’insécurité trop grande a causé son abandon. On peut espérer que le calme relatif que nous imposons au Sahara permettra à ce petit centre de renaître et de se développer.
Les villages de culture de l’Ahaggar, assez nombreux, sont peu importants ; Motylinski en dénombre trente-cinq. L’expression d’oasis, qui évoque toujours l’idée d’une palmeraie, ne leur convient pas : la culture des dattiers manque dans la plupart d’entre eux ; elle est insignifiante dans les autres. La première place appartient aux céréales. Aussi vaut-il mieux conserver à ces centres de jardinage du pays Touareg, leur nom berbère de ar’érem ; l’orthographe en a été longtemps douteuse (arrem, agherim) ; on trouve même une variante qui a longtemps servi à désigner, à l’ouest de Bilma, les jardins de Fachi qui, depuis Barth, sont souvent appelés Oasis Agram, même sur des cartes récentes.
Ces villages se ressemblent tous : ils sont formés de quelques huttes rondes ou carrées, construites en terre ou en diss, mélangeant les formes soudanaises aux formes des ksour ; les plus peuplés ont à peine cent habitants. Le tableau suivant, emprunté surtout à Voinot, permettra de se rendre compte du peu d’importance de la plupart des ar’érem.
HECTARES NOMBRE DE JARDINS HOMMESFEMMESENFANTSHABITANTS
In Amdjel 120
Idélés 6 à 8 45 112 palmiers. Quelques figuiers. 3 pieds de vigne.
Tebirbirt 3-4
Tazerouk 30 38 29 16 83 ⎧ ⎨ ⎩ 140 hectares d’anciennes cultures abandonnées entre Tazerouk et Tebirbirt.
Aïtoklane 3 1/2 ? abandonné depuis 1902. Tin Tarabin 11 20 17 4 41 Tarahaouthaout 34 39 29
90 2 figuiers, 4 bœufs.
5 foggaras, la nappe d’eau à 1m,50 ou 2 m. Motylinski indique 52 habitants.
Toutes les tribus importantes possèdent quelques-uns de ces jardins ; le plus grand nombre semble appartenir aux Kel R’ela et aux Dag R’ali. On trouvera le détail dans Motylinski et surtout dans Benhazera.
Malgré leur état misérable, les ar’érem impriment cependant à l’Ahaggar un cachet particulier : la vie sédentaire est possible dans les hautes régions du Sahara.
Tout incomplet qu’il soit, ce tableau nous donne quelques renseignements intéressants ; il confirme l’état misérable des cultures ; il nous apprend que chaque jardin, cultivé par un chef de case, a une surface restreinte, variant d’un demi-hectare à un hectare ; il nous montre enfin combien la population en est anormale : les hommes sont de beaucoup les plus nombreux (46,2 p. 100) ; il y a peu de femmes (35,4 p. 100) et à peine d’enfants (17,5 p. 100).
Ces villages sont de création récente ; ils n’existaient pas, il y a un siècle, d’après les renseignements recueillis par le capitaine Dinaux [Bull.Com.Afr. fr., mars 1907, p. 65] ; ils ont été établis avec le concours, souvent involontaire, des haratins du Tidikelt et du Touat et la collaboration, toujours forcée, des esclaves achetés ou razziés au Soudan. Les cultivateurs n’ont aucune racine dans le pays ; ce sont des immigrés de date récente à peine installés dans l’Ahaggar.
