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Species diversity in ecological communities
BOOK
REVIEWS
Symmetry versus crypsis In their recent TREE article, Polak and Triversl say that the study of symmetry and its fluctuations in biology was largely restricted to morphology and systematics until 1953. However, in 1889 A.R. Wallace* remarked that coloration patterns of wild animals are more symmetrical than those of their domesticated descendants; he thought that symmetry would help specific recognition. In one respect Wallace’s observation seems paradoxical. Domestic animals have less need to be cryptic than their wild counterparts, but, at least for
Community ecology Species Diversity in Ecological Communities edited by Robert E. Ricklefs and Dolph Schluter University of Chicago Press, 1993. $105.00 hbk, $35.00 pbk (414 pages) ISBN 0 226 71822 O/O 226 71823 9
S
everal years ago, Bob Ricklefs published a paper in Science1 in which he drew attention to how geography and history could affect the species diversity of local communities. This message may seem obvious in retrospect, but it contrasted sharply with the prevailing views of the time. Even though such influential ecologists as MacArthur and Hutchinson had discussed historical and geG graphical effects2j3, community ecologists generally believed (or pretended) that these effects were commonly overridden by processes such as competition, predation or disturbance that operated at a local scale over short periods. If this were so, then we should expect local species diversity to be correlated with local environmental conditions and to reach saturation (i.e. an equilibrium speciesrichness) quickly. Communities in similar habitats on different continents should also attain similar diversities. We could test these predictions, and understand diversity, by studying the local, immediate dynamics of communities and by employing equilibrium models that ignored spatial variation. Apparent progress was great, and explanations of community patterns proliferatedb. This local approach has provided the basis for much of our thinking about diversity (and diversity management) for the past several decades. If Ricklefs’ Science paper hinted at weaknesses in this approach, then there can be no doubt remaining after one reads the contributions in Species Diversity in Ecological Communities, The underlying themes, in addition to history and geography, are represented by a suite of terms that have become ecological buzzwords: scale, metapopu-
346
humans, the presence of symmetry is a major failing of camouflage. Symmetrical patterning gives away animals that are otherwise superbly concealed. The few cryptic animals that are asymmetrically patterned may be the exceptions that prove this rule, one example is the wryneck (Lynx torquilla), an unusual woodpecker. Do the demands of natural or sexual selectioni.* compromise crypsis, or are most animals much less good than humans3 at finding such patterns? The task of finding a symmetrical region embedded in a random background has (to my knowledge) defeated computer vision, and might be beyond brains that are simpler than ours. It would be worthwhile to investigate the ability of animals to find symmetrical but
lation, biodiversity, trophospecies, nonequilibrium, source-sink effects, and the like. An introductory section (five chapters) deals with several aspects of local community dynamics, especially the relationship of diversity to system productivity. This leads into a consideration of ‘mesoscale’ patterns and processes (five chapters), in which theories of metapopulations and spatially structured multispecies interactions figure importantly. The bulk of the book is devoted to broaderscale relationships in space (regional and intercontinental patterns in a variety of systems; ten chapters) and in time (phylogenetic and palaeontological constraints; eight chapters). Collectively, these contributions lead to the conclusion that we should probably abandon attempts to develop a simple, unifying theory to explain community patterns. In addition to the local ecological interactions traditionally emphasized, community diversity is likely to be a product of local movement and openness to external effects, dispersal among habitats and metapopulation effects, range expansions and regional diversification of taxa, speciation effects, biogeographic interchanges, and unique historical effects operating over a range of time scales. The importance of these factors varies among regions or continents, and among taxa that operate on different scales in space or time. Reality, as Holt observes in his chapter, is messy. How, then, should studies of diversity be approached? Ricklefs and Schluter emphasize the importance of comparisons, especially those conducted over broad gradients of ecology, geography and history. Because they are necessarily confined in space and time, field experiments, which have become the modus operandi in community ecology, may in fact not tell us much about the regional or historical processes that govern local diversity. If equilibrium is an unusual state of nature (as many of the contributions suggest), then models based on equilibrium assumptions may be of limited value as well. Of course, there are problems in assuring comparability in compara-
otherwise random patterns, rather as Barlow and Reeves3 have done for humans.
D. Osorio School of Biological Sciences, Sussex University, Falmer, Brighton, UK BNl 9QC References 1 Polak, M. and Trivers, R. (1994) Trends Ecol. Evol. 9, 122-124 2 Wallace, A.R. (1889) Danvinism,Ch. 8, Macmillan 3 Barlow, H.B. and Reeves, B.C. (1979) Vision Res. 19.783-793
tive studies, especially because of differences in space-time scaling among systems. Underwood and Petraitis provide an excellent assessment of these difficulties, which some of the other contributors to the book might well have heeded. Although threads of classical (i.e. niche/ competition-based) thinking appear here and there in this book, the emphasis is clearly upon a new and potentially more rewarding view of the processes determining community patterns. As Blonde1 and Vigne conclude (p. 146): ‘Integrating historical and spatial components of diversities with population biology and community ecology leads to more realistic ideas of the real world, which is no longer the orderly, predictable, and deterministic world of the sixties, but a world where factors such as competition, predation, parasitism, species-specific life histories, behavior, population structure and geographical variation of both phenotypes and genotypes share their roles in the shaping of biodiversity with disturbances, patchiness, historical events, and the impact of humanity’. While the contributions to this book do not provide a definitive synthesis of these factors, they do provide a richly detailed and stimulating documentation of the diversity of processes affecting biodiversity. The book is an ideal foundation for graduate student discussions, and is essential reading for ecologists or resource managers concerned with communities or with conservation issues. More than any other recent book, it marks the changing way we are thinking about ecological communities. John A. Wiens Dept of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523, USA
References Ricklefs, R.E. (1987) Science 235, 167-171 MacArthur, R.H. (1973) Geographical Ecologv, Harper & Row Hutchinson, GE. (1959) Am. Nat. 93, 145-159 Wiens, J.A. (1989) The Ecology ofBird Communities, Cambridge University Press TREE vol.
9, no.
9 September
1994
REVIEWS
Symmetry versus crypsis In their recent TREE article, Polak and Triversl say that the study of symmetry and its fluctuations in biology was largely restricted to morphology and systematics until 1953. However, in 1889 A.R. Wallace* remarked that coloration patterns of wild animals are more symmetrical than those of their domesticated descendants; he thought that symmetry would help specific recognition. In one respect Wallace’s observation seems paradoxical. Domestic animals have less need to be cryptic than their wild counterparts, but, at least for
Community ecology Species Diversity in Ecological Communities edited by Robert E. Ricklefs and Dolph Schluter University of Chicago Press, 1993. $105.00 hbk, $35.00 pbk (414 pages) ISBN 0 226 71822 O/O 226 71823 9
S
everal years ago, Bob Ricklefs published a paper in Science1 in which he drew attention to how geography and history could affect the species diversity of local communities. This message may seem obvious in retrospect, but it contrasted sharply with the prevailing views of the time. Even though such influential ecologists as MacArthur and Hutchinson had discussed historical and geG graphical effects2j3, community ecologists generally believed (or pretended) that these effects were commonly overridden by processes such as competition, predation or disturbance that operated at a local scale over short periods. If this were so, then we should expect local species diversity to be correlated with local environmental conditions and to reach saturation (i.e. an equilibrium speciesrichness) quickly. Communities in similar habitats on different continents should also attain similar diversities. We could test these predictions, and understand diversity, by studying the local, immediate dynamics of communities and by employing equilibrium models that ignored spatial variation. Apparent progress was great, and explanations of community patterns proliferatedb. This local approach has provided the basis for much of our thinking about diversity (and diversity management) for the past several decades. If Ricklefs’ Science paper hinted at weaknesses in this approach, then there can be no doubt remaining after one reads the contributions in Species Diversity in Ecological Communities, The underlying themes, in addition to history and geography, are represented by a suite of terms that have become ecological buzzwords: scale, metapopu-
346
humans, the presence of symmetry is a major failing of camouflage. Symmetrical patterning gives away animals that are otherwise superbly concealed. The few cryptic animals that are asymmetrically patterned may be the exceptions that prove this rule, one example is the wryneck (Lynx torquilla), an unusual woodpecker. Do the demands of natural or sexual selectioni.* compromise crypsis, or are most animals much less good than humans3 at finding such patterns? The task of finding a symmetrical region embedded in a random background has (to my knowledge) defeated computer vision, and might be beyond brains that are simpler than ours. It would be worthwhile to investigate the ability of animals to find symmetrical but
lation, biodiversity, trophospecies, nonequilibrium, source-sink effects, and the like. An introductory section (five chapters) deals with several aspects of local community dynamics, especially the relationship of diversity to system productivity. This leads into a consideration of ‘mesoscale’ patterns and processes (five chapters), in which theories of metapopulations and spatially structured multispecies interactions figure importantly. The bulk of the book is devoted to broaderscale relationships in space (regional and intercontinental patterns in a variety of systems; ten chapters) and in time (phylogenetic and palaeontological constraints; eight chapters). Collectively, these contributions lead to the conclusion that we should probably abandon attempts to develop a simple, unifying theory to explain community patterns. In addition to the local ecological interactions traditionally emphasized, community diversity is likely to be a product of local movement and openness to external effects, dispersal among habitats and metapopulation effects, range expansions and regional diversification of taxa, speciation effects, biogeographic interchanges, and unique historical effects operating over a range of time scales. The importance of these factors varies among regions or continents, and among taxa that operate on different scales in space or time. Reality, as Holt observes in his chapter, is messy. How, then, should studies of diversity be approached? Ricklefs and Schluter emphasize the importance of comparisons, especially those conducted over broad gradients of ecology, geography and history. Because they are necessarily confined in space and time, field experiments, which have become the modus operandi in community ecology, may in fact not tell us much about the regional or historical processes that govern local diversity. If equilibrium is an unusual state of nature (as many of the contributions suggest), then models based on equilibrium assumptions may be of limited value as well. Of course, there are problems in assuring comparability in compara-
otherwise random patterns, rather as Barlow and Reeves3 have done for humans.
D. Osorio School of Biological Sciences, Sussex University, Falmer, Brighton, UK BNl 9QC References 1 Polak, M. and Trivers, R. (1994) Trends Ecol. Evol. 9, 122-124 2 Wallace, A.R. (1889) Danvinism,Ch. 8, Macmillan 3 Barlow, H.B. and Reeves, B.C. (1979) Vision Res. 19.783-793
tive studies, especially because of differences in space-time scaling among systems. Underwood and Petraitis provide an excellent assessment of these difficulties, which some of the other contributors to the book might well have heeded. Although threads of classical (i.e. niche/ competition-based) thinking appear here and there in this book, the emphasis is clearly upon a new and potentially more rewarding view of the processes determining community patterns. As Blonde1 and Vigne conclude (p. 146): ‘Integrating historical and spatial components of diversities with population biology and community ecology leads to more realistic ideas of the real world, which is no longer the orderly, predictable, and deterministic world of the sixties, but a world where factors such as competition, predation, parasitism, species-specific life histories, behavior, population structure and geographical variation of both phenotypes and genotypes share their roles in the shaping of biodiversity with disturbances, patchiness, historical events, and the impact of humanity’. While the contributions to this book do not provide a definitive synthesis of these factors, they do provide a richly detailed and stimulating documentation of the diversity of processes affecting biodiversity. The book is an ideal foundation for graduate student discussions, and is essential reading for ecologists or resource managers concerned with communities or with conservation issues. More than any other recent book, it marks the changing way we are thinking about ecological communities. John A. Wiens Dept of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523, USA
References Ricklefs, R.E. (1987) Science 235, 167-171 MacArthur, R.H. (1973) Geographical Ecologv, Harper & Row Hutchinson, GE. (1959) Am. Nat. 93, 145-159 Wiens, J.A. (1989) The Ecology ofBird Communities, Cambridge University Press TREE vol.
9, no.
9 September
1994