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Size-structured populations: Ecology and evolution
TREE vol. 4, no. 12, December
7989
bryological rules of operation cannot safely be ignored by any evolutionist. The molecular evolutionist may still argue that all such phenomena will always ultimately be reducible to the far more definitive and concrete terms of hereditary molecules. This the embryologist would not dispute: the mechanisms he studies are not incompatible with, or independent of, a molecular basis. But as cell- and tissue-based embryological mechanisms are shown to account directly for more and more features of mormolecular explanations phology, may be expected to become increasingly indirect, complex, impracticable and unnecessary. Molecular biologists may only see in this book a confirmation of their suspicions regarding the failure of embryologists to trace through the phenomena systemically in rigorous causal terms, but they may also recognize that mechanisms at the embryological level could require additional, unfamiliar types of explanation. One suspects that the embryologists’ failure results from the difficulty in arriving at acceptable, not primarily molecular, explanatory alternatives. But the data pointing to the power of embryological mechanisms are already considerable. In the end, abstract statements about hierarchies, reductionism or levels of explanation are not likely to convince anybody; this will only come from specific and fully substantiated examples of how embryological rules can mediate evolutionary change. Above all, this book shows how progress is to be made. It demonstrates the importance of establishing agreed overall objectives and the difficulties in achieving an integration among the wealth of available data and issues. It also brings into focus the most problematical themes in the field. Difficult though it is to re-think an issue like recapitulation, Thomson’s brief and total dismissal of the concept surely excludes arguments that have to be faced: his reasons would have provided an interesting and important discussion. Thomson proposes that integrated, phyletic changes in morphology are uniquely the product of properties of the embryonic domain; small changes occurring at early embryonic stages can potentially achieve global and integrated changes of adult Bauplan that are characteristic of, and foundations for, higher-order taxa. The openness of even the earliest stages of development to change clearly undermines the totally Haeckelian interpretation of recapitulation, but is not the relative evolutionary stability of a Bauplan and its underlying early em-
bryological causes - without which taxonomy would be impossible indicative of a form of recapitulation? How otherwise does one explain why early stages of development are more similar than later within large taxonomic groups? Is it not premature to invoke solely embryological effects to explain rapid, apparently global morphological jumps as supposedly required by the ‘correlated progression’ seen in macroevolution? Not only does this assume that genetic choices and selection forces are not themselves sufficient to generate such rapid changes; until we understand genetic and embryological mechanisms better we surely cannot yet exclude gradualistic mechanisms as the basis for even the most fundamental morphological changes. Historians of science argue that the ‘Theoretical Biology Club’ of the
1930s represented an important stepping stone to the age of molecular biology. This book, the heir to a ‘Heretical Studies Group’ at Yale, may prove to be significant as a model for how some of today’s most profound biological puzzles are likely to be resolved. It illustrates the need for open-minded ways of thinking to match the complexity of evolutionary biology itself. Progress is likely to occur, not so much in the lab, but in the pub and the library, and it will be measured out by books like this one. T. J. Horder Dept of Human Anatomy, University of Oxford, South Parks Road, Oxford OX1 3QX, UK
Reference
1 Waddington, C.H. (1940) Organisers and Genes, Cambridge University Press
Dynamicsof Size Size-Structured Populations: Ecology and Evolution edited by Bo Ebenman and Lennart Persson, Springer-Verlag, 7988. DM 148.00 (xiv+284 pages) ISBN 3 540 50188 6 Many standard theories in ecology have dealt with biological populations as consisting of homogeneous individuals or as merely age-structured. Biological populations in the real world, however, consist of individuals of different sizes, and thus should be dealt with in this context. Fifteen contributions in SizeStructured Populations (the proceedings of a symposium held in Sweden in 1987) shed light on the significance of size structure in the study of the ecology and evolution of populations. Although size structure itself is familiar to ecologists mostly in the appearance of rightward skewed frequency distributions of body size in relation to competition between individuals in a population, there have been only a few theories or models that incorporate size structure to approach ecological and evolutionary problems of populations. As seen in this book, several workers have just begun to figure out such theories and models. This is a book on the right subject at just the right time. It shows that when size structure is taken into account, a much more realistic and mechanistic insight is gained into the ecology and evolution of populations and communities. For example, most
standard models of exploitative trophic-level interactions excluding size structure fail to account for a well-known phenomenon in freshwater lakes, namely, that the biomass of producers and consumers increases along a gradient of increased nutrient loading; only the model incorporating size structure can predict it (Mittelbach et al.). Many such models and examples are given in this book. Most of the examples are from animals (e.g. rotifers, crustaceans, zooplankton, dipterans; scorpions, fish and amphibians), and the authors are mostly zoologists. Aside from Caswell’s purely theoretical contribution on matrix population models, which uses several plant species as examples, only one contribution (Norberg) deals with plant ecology, and the last few paragraphs of the contribution by Ebenman mention some possible applications of his model to plant ecology. These authors too are zoologists! Some small drawbacks derive from the animal-oriented nature of the book. The dynamics of size structure themselves and their effects on population dynamics are scarcely mentioned. Plant ecologists have studied the pattern and dynamics of size distributions in even-aged plant populations in relation to the interference between individuals. Even individual plants of the same age have differences in size and thus in performance, and even individual plants of the same size have different performance as affected by neighbouring 391
TREE vol. 4, no. 12, December
environmental individuals and heterogeneity. This point is almost ignored in the book, in which ‘size structure’ means ‘stage structure’ that is, the size difference is attributed to the difference in stage, e.g. juvenile versus adult, and mean values for each stage constitute the whole size structure of the population. However, even a juvenile or adult cohort has size structure and thus sizedependent performance that affects
1989
the whole population dynamics. Of course, it is true that the ‘stagestructured’ approach is much more informative than the conventional mean-based models or than the agestructured models, and I believe that this book is a good first step toward the study of ‘size-structured’ populations, not only for animal ecologists but also for plant ecologists. Some parts of this animal-oriented book were difficult for me, as a plant
ecologist, to follow. But I enjoyed reading it, because of its consistent approach based on the general proposition that ‘how you look, and what “life stage” you have attained, is usually more important than your chronological age’ (May).
tors is long and includes persecution, human disturbance, habitat degradation, pollutants, pesticides and acid waters. Persecution has to do with tolerance and attitudes. Nowadays this is a minor problem in North America and Europe, but not in the Third World: here, ospreys on their wintering grounds may be attracted to foraging sites where they run the risk of being regarded as competitors to humans (e.g. in aquacultures). Pesticides are coming under increasing control in developed countries, but their use is very high in many tropical regions, e.g. in tsetse control programmes where waters important for native species of fisheating birds as well as for migrants from temperate areas are affected. Habitat degradation affects breeding sites in temperate areas as well as the tropical wintering grounds. In modern forestry, trees are normally cut down before they have grown large enough to harbour the heavy nest of a large raptor. Even if ospreys reproduce successfully in artificial nests, the management of populations that to a large extent rely on nest platforms involves a long-term responsibility for maintenance. Acid rain threatens ospreys in several ways. Scandinavian experiences of how ospreys are affected are different from those reported for North America by Poole. A decline in reproductive success among pairs breeding in South-west Sweden during the 1970s was related to acidification of lakes and primarily to the decline of fish stocks rather than to exposure to metals’. Most of the conclusions about factors affecting breeding density and success of ospreys are based on circumstantial evidence. Experimental studies are few. Nevertheless, I believe that the osprey is an excellent subject for field studies where problems of general and applied character could be combined.
A review of this book from a narrow and purely scientific perspective could result in a long list of detailed criticisms of conclusions based on weak evidence. However, such an effort would miss its goal: I feel confident that Poole, most of the time, has made correct conclusions on a combination of sometimes fragmentary data and a good general knowledge. This reflects a general dilemma in research on the factors affecting populations of large birds and mammals. There are normally practical problems involved: good field experiments can seldom be done without manipulations of populations or habitats over very large areas. This must not be an excuse for doing bad research; managers, administrators or researchers must base their decisions only on the best research available. The fact that decisions must sometimes be based on imperfect research (even if the scientist has done her/his best) emphasizes the need for experienced wildlife biologists who combine a qualified scientific training with a deep knowledge and good intuition. Alan Poole has been successful in his effort to write a fluent text. This book has a large target group, from non-professionals interested in wildlife or aquatic ecology to administrators working on a national or international basis. Equally important is its value as a contribution to international natureconservation: the text emphasizes that the osprey is a ‘world citizen’ and that its successful management is dependent on international cooperation.
Toshihiko Hara Dept of Biology, Faculty of Science, Tokyo Metropolitan University, Tokyo 158, Japan
Birds of Prey Ospreys: A Natural and Unnatural History by Alan F. Poole, Cambridge University Press, 1989. f77.50/$27.95 hbk (xviii+246 pages) ISBN 0 52 7 30623 X The osprey (Pandion haliaetus) is a conspicuous raptor that occurs in five of the world’s six continents. Thus, it deserves its position in the logotype of The International Council for Bird Preservation (ICBP). Although not an endangered species, it is exposed to several threats and is therefore a good candidate for a study of approaches to conservation and management at the international level. The osprey has been the subject of a large number of studies, mostly in the developed world and especially in North America. The results have been presented in everything from local or regional reports to well-known refereed journals with international distribution. Alan Poole has been successful in compiling these disparate sources, and he has produced a text that is more than just the sum of the contributing parts. Furthermore, from his acknowledgements it appears that most living scientists involved in osprey research have been consulted. I appreciated, for instance, the thorough presentation of the very important but still unpublished thesis by Dr Yves Prevost, especially as it is one of the few detailed studies of ospreys at their tropical wintering sites. Poole is optimistic about the future of the osprey. I agree up to a point: when its primary needs - such as a suitable nest site and sufficient food (fish) that is not too heavily contaminated with organochlorines - are met, the osprey can apparently breed almost anywhere, even in urban areas. Recent population trends in North America and Europe are stable or upwards. But I also disagree, in that almost no threat related to human activities is under long-term control. The list of problematic fac392
Mats O.G. Eriksson The National Environmental Protection Board, PO Box 1302, S-171 25 Solna, Sweden Reference 1 Eriksson, Oscarson, 293-300
M.O.G., Henrikson, L. and H.G. (1983) !I& FZgelvdrld42,
7989
bryological rules of operation cannot safely be ignored by any evolutionist. The molecular evolutionist may still argue that all such phenomena will always ultimately be reducible to the far more definitive and concrete terms of hereditary molecules. This the embryologist would not dispute: the mechanisms he studies are not incompatible with, or independent of, a molecular basis. But as cell- and tissue-based embryological mechanisms are shown to account directly for more and more features of mormolecular explanations phology, may be expected to become increasingly indirect, complex, impracticable and unnecessary. Molecular biologists may only see in this book a confirmation of their suspicions regarding the failure of embryologists to trace through the phenomena systemically in rigorous causal terms, but they may also recognize that mechanisms at the embryological level could require additional, unfamiliar types of explanation. One suspects that the embryologists’ failure results from the difficulty in arriving at acceptable, not primarily molecular, explanatory alternatives. But the data pointing to the power of embryological mechanisms are already considerable. In the end, abstract statements about hierarchies, reductionism or levels of explanation are not likely to convince anybody; this will only come from specific and fully substantiated examples of how embryological rules can mediate evolutionary change. Above all, this book shows how progress is to be made. It demonstrates the importance of establishing agreed overall objectives and the difficulties in achieving an integration among the wealth of available data and issues. It also brings into focus the most problematical themes in the field. Difficult though it is to re-think an issue like recapitulation, Thomson’s brief and total dismissal of the concept surely excludes arguments that have to be faced: his reasons would have provided an interesting and important discussion. Thomson proposes that integrated, phyletic changes in morphology are uniquely the product of properties of the embryonic domain; small changes occurring at early embryonic stages can potentially achieve global and integrated changes of adult Bauplan that are characteristic of, and foundations for, higher-order taxa. The openness of even the earliest stages of development to change clearly undermines the totally Haeckelian interpretation of recapitulation, but is not the relative evolutionary stability of a Bauplan and its underlying early em-
bryological causes - without which taxonomy would be impossible indicative of a form of recapitulation? How otherwise does one explain why early stages of development are more similar than later within large taxonomic groups? Is it not premature to invoke solely embryological effects to explain rapid, apparently global morphological jumps as supposedly required by the ‘correlated progression’ seen in macroevolution? Not only does this assume that genetic choices and selection forces are not themselves sufficient to generate such rapid changes; until we understand genetic and embryological mechanisms better we surely cannot yet exclude gradualistic mechanisms as the basis for even the most fundamental morphological changes. Historians of science argue that the ‘Theoretical Biology Club’ of the
1930s represented an important stepping stone to the age of molecular biology. This book, the heir to a ‘Heretical Studies Group’ at Yale, may prove to be significant as a model for how some of today’s most profound biological puzzles are likely to be resolved. It illustrates the need for open-minded ways of thinking to match the complexity of evolutionary biology itself. Progress is likely to occur, not so much in the lab, but in the pub and the library, and it will be measured out by books like this one. T. J. Horder Dept of Human Anatomy, University of Oxford, South Parks Road, Oxford OX1 3QX, UK
Reference
1 Waddington, C.H. (1940) Organisers and Genes, Cambridge University Press
Dynamicsof Size Size-Structured Populations: Ecology and Evolution edited by Bo Ebenman and Lennart Persson, Springer-Verlag, 7988. DM 148.00 (xiv+284 pages) ISBN 3 540 50188 6 Many standard theories in ecology have dealt with biological populations as consisting of homogeneous individuals or as merely age-structured. Biological populations in the real world, however, consist of individuals of different sizes, and thus should be dealt with in this context. Fifteen contributions in SizeStructured Populations (the proceedings of a symposium held in Sweden in 1987) shed light on the significance of size structure in the study of the ecology and evolution of populations. Although size structure itself is familiar to ecologists mostly in the appearance of rightward skewed frequency distributions of body size in relation to competition between individuals in a population, there have been only a few theories or models that incorporate size structure to approach ecological and evolutionary problems of populations. As seen in this book, several workers have just begun to figure out such theories and models. This is a book on the right subject at just the right time. It shows that when size structure is taken into account, a much more realistic and mechanistic insight is gained into the ecology and evolution of populations and communities. For example, most
standard models of exploitative trophic-level interactions excluding size structure fail to account for a well-known phenomenon in freshwater lakes, namely, that the biomass of producers and consumers increases along a gradient of increased nutrient loading; only the model incorporating size structure can predict it (Mittelbach et al.). Many such models and examples are given in this book. Most of the examples are from animals (e.g. rotifers, crustaceans, zooplankton, dipterans; scorpions, fish and amphibians), and the authors are mostly zoologists. Aside from Caswell’s purely theoretical contribution on matrix population models, which uses several plant species as examples, only one contribution (Norberg) deals with plant ecology, and the last few paragraphs of the contribution by Ebenman mention some possible applications of his model to plant ecology. These authors too are zoologists! Some small drawbacks derive from the animal-oriented nature of the book. The dynamics of size structure themselves and their effects on population dynamics are scarcely mentioned. Plant ecologists have studied the pattern and dynamics of size distributions in even-aged plant populations in relation to the interference between individuals. Even individual plants of the same age have differences in size and thus in performance, and even individual plants of the same size have different performance as affected by neighbouring 391
TREE vol. 4, no. 12, December
environmental individuals and heterogeneity. This point is almost ignored in the book, in which ‘size structure’ means ‘stage structure’ that is, the size difference is attributed to the difference in stage, e.g. juvenile versus adult, and mean values for each stage constitute the whole size structure of the population. However, even a juvenile or adult cohort has size structure and thus sizedependent performance that affects
1989
the whole population dynamics. Of course, it is true that the ‘stagestructured’ approach is much more informative than the conventional mean-based models or than the agestructured models, and I believe that this book is a good first step toward the study of ‘size-structured’ populations, not only for animal ecologists but also for plant ecologists. Some parts of this animal-oriented book were difficult for me, as a plant
ecologist, to follow. But I enjoyed reading it, because of its consistent approach based on the general proposition that ‘how you look, and what “life stage” you have attained, is usually more important than your chronological age’ (May).
tors is long and includes persecution, human disturbance, habitat degradation, pollutants, pesticides and acid waters. Persecution has to do with tolerance and attitudes. Nowadays this is a minor problem in North America and Europe, but not in the Third World: here, ospreys on their wintering grounds may be attracted to foraging sites where they run the risk of being regarded as competitors to humans (e.g. in aquacultures). Pesticides are coming under increasing control in developed countries, but their use is very high in many tropical regions, e.g. in tsetse control programmes where waters important for native species of fisheating birds as well as for migrants from temperate areas are affected. Habitat degradation affects breeding sites in temperate areas as well as the tropical wintering grounds. In modern forestry, trees are normally cut down before they have grown large enough to harbour the heavy nest of a large raptor. Even if ospreys reproduce successfully in artificial nests, the management of populations that to a large extent rely on nest platforms involves a long-term responsibility for maintenance. Acid rain threatens ospreys in several ways. Scandinavian experiences of how ospreys are affected are different from those reported for North America by Poole. A decline in reproductive success among pairs breeding in South-west Sweden during the 1970s was related to acidification of lakes and primarily to the decline of fish stocks rather than to exposure to metals’. Most of the conclusions about factors affecting breeding density and success of ospreys are based on circumstantial evidence. Experimental studies are few. Nevertheless, I believe that the osprey is an excellent subject for field studies where problems of general and applied character could be combined.
A review of this book from a narrow and purely scientific perspective could result in a long list of detailed criticisms of conclusions based on weak evidence. However, such an effort would miss its goal: I feel confident that Poole, most of the time, has made correct conclusions on a combination of sometimes fragmentary data and a good general knowledge. This reflects a general dilemma in research on the factors affecting populations of large birds and mammals. There are normally practical problems involved: good field experiments can seldom be done without manipulations of populations or habitats over very large areas. This must not be an excuse for doing bad research; managers, administrators or researchers must base their decisions only on the best research available. The fact that decisions must sometimes be based on imperfect research (even if the scientist has done her/his best) emphasizes the need for experienced wildlife biologists who combine a qualified scientific training with a deep knowledge and good intuition. Alan Poole has been successful in his effort to write a fluent text. This book has a large target group, from non-professionals interested in wildlife or aquatic ecology to administrators working on a national or international basis. Equally important is its value as a contribution to international natureconservation: the text emphasizes that the osprey is a ‘world citizen’ and that its successful management is dependent on international cooperation.
Toshihiko Hara Dept of Biology, Faculty of Science, Tokyo Metropolitan University, Tokyo 158, Japan
Birds of Prey Ospreys: A Natural and Unnatural History by Alan F. Poole, Cambridge University Press, 1989. f77.50/$27.95 hbk (xviii+246 pages) ISBN 0 52 7 30623 X The osprey (Pandion haliaetus) is a conspicuous raptor that occurs in five of the world’s six continents. Thus, it deserves its position in the logotype of The International Council for Bird Preservation (ICBP). Although not an endangered species, it is exposed to several threats and is therefore a good candidate for a study of approaches to conservation and management at the international level. The osprey has been the subject of a large number of studies, mostly in the developed world and especially in North America. The results have been presented in everything from local or regional reports to well-known refereed journals with international distribution. Alan Poole has been successful in compiling these disparate sources, and he has produced a text that is more than just the sum of the contributing parts. Furthermore, from his acknowledgements it appears that most living scientists involved in osprey research have been consulted. I appreciated, for instance, the thorough presentation of the very important but still unpublished thesis by Dr Yves Prevost, especially as it is one of the few detailed studies of ospreys at their tropical wintering sites. Poole is optimistic about the future of the osprey. I agree up to a point: when its primary needs - such as a suitable nest site and sufficient food (fish) that is not too heavily contaminated with organochlorines - are met, the osprey can apparently breed almost anywhere, even in urban areas. Recent population trends in North America and Europe are stable or upwards. But I also disagree, in that almost no threat related to human activities is under long-term control. The list of problematic fac392
Mats O.G. Eriksson The National Environmental Protection Board, PO Box 1302, S-171 25 Solna, Sweden Reference 1 Eriksson, Oscarson, 293-300
M.O.G., Henrikson, L. and H.G. (1983) !I& FZgelvdrld42,