- Email: [email protected]
Vascular epiphytes
TREE vol. 6, no. 3, March
1997
egorizing the chapters in another way, four are general reviews, five are reviews of particular case studies, and three present original data alone. Other than mentioning patches (which some chapters do only in the introductory remarks), there is very among many of the little in common chapters. The book does not give a comprehensive view of what it means to live in a patchy environ-
ment. But I do not think it was meant to. Instead, the book is a collection of well-written and interesting essays that, regardless of the fact that they are unrelated, provide good reading. With the exception of Baker’s chapter on the t-gene complex in house mice, I think all of the chapters are accessible to readers without specialized backgrounds, provided they do not have mathephobia. I have chose three chapters to summarize in more detail. My choices reflect my personal interests, and another reviewer might have equally chosen any of the other chapters. The chapter by McNamara and Houston presents a dynamic programming model in which foraging animals must select among patches based on the availability of food and the risk of predation within each patch. The
method of dynamic programming allows the hypothetical foragerto make decisions based on its internal food reserves (how hungry it is). This differs from static models in which the forager must choose a fixed foraging strategy at the beginning of the foraging period. McNamara and Houston give an example in which an optimal dynamic forager can do substantially better than an optimal static forager. This is important, because in many previous models the static strategies perform almost as well as the dynamic strategies. Shorrocks gives a very crisp review of the mechanisms other than classical resource partitioning that may facilitate the coexistence of competitors. He discusses the ideas of (1) fugitive refuges, in which an inferior competitor can persist by being a good disperser into patches free from other competitors; and (2) probability refuges, in which the aggregative patterns of competing species among patches may promote coexistence. He also considers priority refuges, in which species can persist simply by becoming established in a patch and then outcompeting any new species. However, as Shorrocks points out, models of priority refuges either do
not demonstrate facilitated coexistence, or can be shown to be special cases of the other two general mechanisms for coexistence. Finally, Wright addresses the conservation question of whether it is better to have one large reserve or two small reserves to prevent the extinction of species. Wright first presents a model that shows an advantage for one large reserve. He then presents data on the present distribution patterns of species among islands that have been separated from the mainland since the rise in sea level during the Pleistocene. Although data were only available for seven relict biota, there is a distinct trend that indicates that one large island supports more species than two islands whose combined area equals that of the large island. I very much enjoyed this book, and I think that people not dissuaded by the book’s diversity will be gratified. Diversity is its strength, and it makes the book particularly suitable for group discussions.
Benzing provides an informative and thoughtful introduction to the systematic occurrence, phylogeny, physiology, ecology and distribution of those plants that during at least a portion of their life cycle reside entirely upon another plant. In some ways, epiphytes form a rather peculiar assemblage. No single feature - growth form, seed type, pollen vector, water-carbonbalance regimen, nutrient source, or mechanism of resource procurement -is shared by all of the approximately 25 000 epiphytic species. In fact, their one defining characteristic-a ‘seeming intolerance of terrestrial soil’ - is itself something of a puzzle, since many epiphytes can be cultivated in regular potting mixture. In tropical rain forests one rarely finds epiphytic species growing or even germinating on the ground. Clearly, if strangler figs, those most ‘earthly’ of epiphytes (they are hemiepiphytes, which begin life as epiphytes but often grow to be large, free-standing trees), are not accidental components of the ‘terrestrial’ flora, then the epiphytic habit must be seen as more than a series of physiological tolerances that allow the rigors of the canopy environment to be withstood.
Epiphytes form an important component of the vegetation in many environments; they constitute up to 50% of the total vascular flora in some wet tropical areas and can have a green biomass equal to ‘or even exceeding the foliar mass of the supporting trees. Yet they are one of the least-studied groups of plants and many questions remain unanswered (or even unaddressed). Why do so many epiphytes root exclusively on bark and yet never form a physiological connection with their support? Why are foliar osmotic potentials generally less negative than those of tree leaves? Why are hemiepiphytes found in only a few evolutionary lineages? While Vascular Epiphytes does not provide any simple answers to these questions, it does bring together a great deal of information (including much unpublished work) and ideas relating to these and other issues, and thus should provide a strong stimulus for future research. Since epiphytism has been viewed primarily as a physiological problem, less attention has been paid from either the populationor ecosystemlevel perspective. One of the strengths of Vascular Epiphytes is that it covers not only carbon, water
Anthony R. Ives Deptof Zoology, Universityof Wisconsin-Madison, Madison,WI 53706,USA
Livingin the Canopy Vascular Epiphytes by David H. Benzing, Cambridge University Press, 7990. f40.00/$59.95 hbk (xvii + 354 pages) LSBN 0 52 7 26630 0 Our father leaned out the window. ‘When you’re tired of being up there, you’ll change your mind!’ he shouted. ‘I’ll never change my mind,’ exclaimed my brother from the branch. ‘You’ll see as soon as you come down!’ ‘I’ll never come down again!’ and he kept his word. Thus, in ltalo Calvino’s novel The
Baron in the Trees, young Cosimo makes his exit through the upstairs window of the family mansion, never to set foot on the ground again. The story depicts a life that is at the same time restricted by its treetop environs and enriched through the deployment of novel behaviors that allow an arboreal existence. While of an entirely different literary tradition, David Benzing’s Vascular Epiphytes is also an expos6 of the conditions, constraints and consequences of the epiphytic habit. In this long overdue work (the last treatment of such scope was by Schimper in 1888’),
103
TREE vol. 6, no. 3, March
and nutrient relations, but also topics such as reproductive strategies, plant-animal interactions and nutrient cycling. This breadth of coverage allows the interplay between physiological constraints and life history characteristics to be considered. For example, epiphytes often have low population densities (perhaps due to limitations in appropriate microsites within the canopy) and thus may require more specialized or loyal pollinators since the floral displays of scattered and relatively small epiphytes would be inadequate to compete with the more massive flowering of canopy lianas and trees. Despite the recognition that epiphytes cannot be easily summarized, Vascular Epiphytes is much more than a compendium of specific epiphytic strategies and adaptations. In general, the book strikes well the
balance between documenting the diversity of epiphytic life forms, and marshalling data and the author’s extensive experience in the search for patterns and synthesis. As with many ambitious ventures, strong points can also be seen as avenues of weakness, Thus, while the attempt to bring together so many aspects of epiphyte biology (and concomitantly to be of interest to a wide range of biologists) has been quite successful, it necessarily means that the more technical chapters are somewhat burdened with basic information that is not specific to epiphytes. Similarly, an occasionally excessive use of terminology, abbreviations and categorizations may reflect the author’s desire to treat his subject in a precise and rigorous fashion. These complaints, however, do little to diminish the pleasure one
1991
obtains in reading such a comprehensive and thought-provoking discussion of this fascinating group of plants. Vascular Epiphytes should be of interest not only to obligate botanists, but also to zoologists, horticulturalists and ecosystem biologists - as well as anyone who is our troubled considering forsaking times and treetops.
joining
Cosimo
up
in the
N. Michele Holbrook Dept of Biological Sciences, Stanford University, Stanford, CA 94305-5020, USA
Reference 1 Schimper, Epiphytische
A.F.W.
(1888)
Die
Vegetation Amerikas (Bat. Mitt. Tropen II), Fischer
-
Historical Ecology Phylogeny, Ecology, and Behavior: A Research Program in Comparative Biology by Daniel R. Brooks and Deborah A. McLennan, University of Chicago 7990. $45.OO/f35.95 hbk, Press, $27.OO/f76.75 pbk (xii + 434 paged ISBN 0 226 07571 0 Ecologists, Brooks and McLennan believe, have concentrated excessively on the characters of individual modern species, and on interactions between modern species, all stripped of their evolutionary past. They figure that ecology would benefit from a phylogenetic perspective. Phylogenetic effects, or phylogenetic constraints, they argue, are powerful, because most of the characters and interactions we see in modern species did not evolve in these species: they evolved in ancestors, perhaps under different conditions. In particular, Brooks and McLennan describe how the methods of cladism can be used to study four ecological questions: what biogeographic ranges are occupied by sister groups of species, whether a character has evolved in a given species or was inherited from an ancestor, whether the phylogenies of two groups of possibly coevolving species (such as parasites and their hosts) form mirror-images, and whether ecological interactions between species are newly evolved in a given community or are inherited from an ancestral community. All of these can be interesting inferences, and one valu104
able feature of the book is the clear introduction it gives to the methods used to make them. It is vanity on the authors’ part to announce these questions, and the approach they take to them, as a new scientific discipline (which they call ‘historical ecology’), but vanity is a venial sin: so let it pass. The most suggestive results in the book are for the phylogenies of parasites and hosts. Brooks and McLennan give results for a number of groups, and find that some do seem to speciate together whereas others do not. There are thus degrees of ‘cospeciation’ and of ‘host switching’. As they hint, the differences might be explicable by some such factor as the dispersal powers of different kinds of parasites, but they do not follow it up; they are interested in the phenomenology, not its explanation. They also discuss Ehrlich and Raven’s famous theory of radiation in insects and plants, but conclude that the necessary phylogenies are too poorly known to allow a test. The authors recommend their new discipline to the rest of us because ‘historical ecology expands the spatial and temporal perspective of evolutionary biology, opening doors on more, and more varied research programs’. I fear that argument would be compelling only to an ecologist with an empty head as well as empty hands. Those who have to make positive choices among actual research opportunities will judge it more by the coherence of its philos-
ophy, the reliability of its methods and the stimulation of its ideas. I welcome the book, but in all three respects there is room for improvement. The main philosophical difficulty lies in Brooks and McLennan’s distinction between constraint and change. In the case of adaptation, for example, they apply the term only to the character when it first evolves. If it is later retained that is because of constraint, and the character now ceases to be an adaptation. The usage has precedents, but it has awkward consequences, such as not allowing us to call the human eye an adaptation. We are said to possess eyes because of constraint, not adaptation to the environment. A similar problem arises for ecological interactions. Many biologists will also object that the methods are unreliable. Brooks and McLennan reconstruct biogeographic ranges on the assumption that ranges do not change (i.e. no dispersal), and phylogenies on the assumption of parsimony. If you do not like these assumptions, your confidence not only in the results but also in the entire discipline will be proportionally reduced. (By the way, it is interesting to juxtapose the assumptions of vicariance and parsimony, which minimize the effects of history, with the justification of the discipline, which is that historical effects are powerful.) The main defect of historical ecology as Brooks and McLennan present it is that it does not contain anv ideas.
1997
egorizing the chapters in another way, four are general reviews, five are reviews of particular case studies, and three present original data alone. Other than mentioning patches (which some chapters do only in the introductory remarks), there is very among many of the little in common chapters. The book does not give a comprehensive view of what it means to live in a patchy environ-
ment. But I do not think it was meant to. Instead, the book is a collection of well-written and interesting essays that, regardless of the fact that they are unrelated, provide good reading. With the exception of Baker’s chapter on the t-gene complex in house mice, I think all of the chapters are accessible to readers without specialized backgrounds, provided they do not have mathephobia. I have chose three chapters to summarize in more detail. My choices reflect my personal interests, and another reviewer might have equally chosen any of the other chapters. The chapter by McNamara and Houston presents a dynamic programming model in which foraging animals must select among patches based on the availability of food and the risk of predation within each patch. The
method of dynamic programming allows the hypothetical foragerto make decisions based on its internal food reserves (how hungry it is). This differs from static models in which the forager must choose a fixed foraging strategy at the beginning of the foraging period. McNamara and Houston give an example in which an optimal dynamic forager can do substantially better than an optimal static forager. This is important, because in many previous models the static strategies perform almost as well as the dynamic strategies. Shorrocks gives a very crisp review of the mechanisms other than classical resource partitioning that may facilitate the coexistence of competitors. He discusses the ideas of (1) fugitive refuges, in which an inferior competitor can persist by being a good disperser into patches free from other competitors; and (2) probability refuges, in which the aggregative patterns of competing species among patches may promote coexistence. He also considers priority refuges, in which species can persist simply by becoming established in a patch and then outcompeting any new species. However, as Shorrocks points out, models of priority refuges either do
not demonstrate facilitated coexistence, or can be shown to be special cases of the other two general mechanisms for coexistence. Finally, Wright addresses the conservation question of whether it is better to have one large reserve or two small reserves to prevent the extinction of species. Wright first presents a model that shows an advantage for one large reserve. He then presents data on the present distribution patterns of species among islands that have been separated from the mainland since the rise in sea level during the Pleistocene. Although data were only available for seven relict biota, there is a distinct trend that indicates that one large island supports more species than two islands whose combined area equals that of the large island. I very much enjoyed this book, and I think that people not dissuaded by the book’s diversity will be gratified. Diversity is its strength, and it makes the book particularly suitable for group discussions.
Benzing provides an informative and thoughtful introduction to the systematic occurrence, phylogeny, physiology, ecology and distribution of those plants that during at least a portion of their life cycle reside entirely upon another plant. In some ways, epiphytes form a rather peculiar assemblage. No single feature - growth form, seed type, pollen vector, water-carbonbalance regimen, nutrient source, or mechanism of resource procurement -is shared by all of the approximately 25 000 epiphytic species. In fact, their one defining characteristic-a ‘seeming intolerance of terrestrial soil’ - is itself something of a puzzle, since many epiphytes can be cultivated in regular potting mixture. In tropical rain forests one rarely finds epiphytic species growing or even germinating on the ground. Clearly, if strangler figs, those most ‘earthly’ of epiphytes (they are hemiepiphytes, which begin life as epiphytes but often grow to be large, free-standing trees), are not accidental components of the ‘terrestrial’ flora, then the epiphytic habit must be seen as more than a series of physiological tolerances that allow the rigors of the canopy environment to be withstood.
Epiphytes form an important component of the vegetation in many environments; they constitute up to 50% of the total vascular flora in some wet tropical areas and can have a green biomass equal to ‘or even exceeding the foliar mass of the supporting trees. Yet they are one of the least-studied groups of plants and many questions remain unanswered (or even unaddressed). Why do so many epiphytes root exclusively on bark and yet never form a physiological connection with their support? Why are foliar osmotic potentials generally less negative than those of tree leaves? Why are hemiepiphytes found in only a few evolutionary lineages? While Vascular Epiphytes does not provide any simple answers to these questions, it does bring together a great deal of information (including much unpublished work) and ideas relating to these and other issues, and thus should provide a strong stimulus for future research. Since epiphytism has been viewed primarily as a physiological problem, less attention has been paid from either the populationor ecosystemlevel perspective. One of the strengths of Vascular Epiphytes is that it covers not only carbon, water
Anthony R. Ives Deptof Zoology, Universityof Wisconsin-Madison, Madison,WI 53706,USA
Livingin the Canopy Vascular Epiphytes by David H. Benzing, Cambridge University Press, 7990. f40.00/$59.95 hbk (xvii + 354 pages) LSBN 0 52 7 26630 0 Our father leaned out the window. ‘When you’re tired of being up there, you’ll change your mind!’ he shouted. ‘I’ll never change my mind,’ exclaimed my brother from the branch. ‘You’ll see as soon as you come down!’ ‘I’ll never come down again!’ and he kept his word. Thus, in ltalo Calvino’s novel The
Baron in the Trees, young Cosimo makes his exit through the upstairs window of the family mansion, never to set foot on the ground again. The story depicts a life that is at the same time restricted by its treetop environs and enriched through the deployment of novel behaviors that allow an arboreal existence. While of an entirely different literary tradition, David Benzing’s Vascular Epiphytes is also an expos6 of the conditions, constraints and consequences of the epiphytic habit. In this long overdue work (the last treatment of such scope was by Schimper in 1888’),
103
TREE vol. 6, no. 3, March
and nutrient relations, but also topics such as reproductive strategies, plant-animal interactions and nutrient cycling. This breadth of coverage allows the interplay between physiological constraints and life history characteristics to be considered. For example, epiphytes often have low population densities (perhaps due to limitations in appropriate microsites within the canopy) and thus may require more specialized or loyal pollinators since the floral displays of scattered and relatively small epiphytes would be inadequate to compete with the more massive flowering of canopy lianas and trees. Despite the recognition that epiphytes cannot be easily summarized, Vascular Epiphytes is much more than a compendium of specific epiphytic strategies and adaptations. In general, the book strikes well the
balance between documenting the diversity of epiphytic life forms, and marshalling data and the author’s extensive experience in the search for patterns and synthesis. As with many ambitious ventures, strong points can also be seen as avenues of weakness, Thus, while the attempt to bring together so many aspects of epiphyte biology (and concomitantly to be of interest to a wide range of biologists) has been quite successful, it necessarily means that the more technical chapters are somewhat burdened with basic information that is not specific to epiphytes. Similarly, an occasionally excessive use of terminology, abbreviations and categorizations may reflect the author’s desire to treat his subject in a precise and rigorous fashion. These complaints, however, do little to diminish the pleasure one
1991
obtains in reading such a comprehensive and thought-provoking discussion of this fascinating group of plants. Vascular Epiphytes should be of interest not only to obligate botanists, but also to zoologists, horticulturalists and ecosystem biologists - as well as anyone who is our troubled considering forsaking times and treetops.
joining
Cosimo
up
in the
N. Michele Holbrook Dept of Biological Sciences, Stanford University, Stanford, CA 94305-5020, USA
Reference 1 Schimper, Epiphytische
A.F.W.
(1888)
Die
Vegetation Amerikas (Bat. Mitt. Tropen II), Fischer
-
Historical Ecology Phylogeny, Ecology, and Behavior: A Research Program in Comparative Biology by Daniel R. Brooks and Deborah A. McLennan, University of Chicago 7990. $45.OO/f35.95 hbk, Press, $27.OO/f76.75 pbk (xii + 434 paged ISBN 0 226 07571 0 Ecologists, Brooks and McLennan believe, have concentrated excessively on the characters of individual modern species, and on interactions between modern species, all stripped of their evolutionary past. They figure that ecology would benefit from a phylogenetic perspective. Phylogenetic effects, or phylogenetic constraints, they argue, are powerful, because most of the characters and interactions we see in modern species did not evolve in these species: they evolved in ancestors, perhaps under different conditions. In particular, Brooks and McLennan describe how the methods of cladism can be used to study four ecological questions: what biogeographic ranges are occupied by sister groups of species, whether a character has evolved in a given species or was inherited from an ancestor, whether the phylogenies of two groups of possibly coevolving species (such as parasites and their hosts) form mirror-images, and whether ecological interactions between species are newly evolved in a given community or are inherited from an ancestral community. All of these can be interesting inferences, and one valu104
able feature of the book is the clear introduction it gives to the methods used to make them. It is vanity on the authors’ part to announce these questions, and the approach they take to them, as a new scientific discipline (which they call ‘historical ecology’), but vanity is a venial sin: so let it pass. The most suggestive results in the book are for the phylogenies of parasites and hosts. Brooks and McLennan give results for a number of groups, and find that some do seem to speciate together whereas others do not. There are thus degrees of ‘cospeciation’ and of ‘host switching’. As they hint, the differences might be explicable by some such factor as the dispersal powers of different kinds of parasites, but they do not follow it up; they are interested in the phenomenology, not its explanation. They also discuss Ehrlich and Raven’s famous theory of radiation in insects and plants, but conclude that the necessary phylogenies are too poorly known to allow a test. The authors recommend their new discipline to the rest of us because ‘historical ecology expands the spatial and temporal perspective of evolutionary biology, opening doors on more, and more varied research programs’. I fear that argument would be compelling only to an ecologist with an empty head as well as empty hands. Those who have to make positive choices among actual research opportunities will judge it more by the coherence of its philos-
ophy, the reliability of its methods and the stimulation of its ideas. I welcome the book, but in all three respects there is room for improvement. The main philosophical difficulty lies in Brooks and McLennan’s distinction between constraint and change. In the case of adaptation, for example, they apply the term only to the character when it first evolves. If it is later retained that is because of constraint, and the character now ceases to be an adaptation. The usage has precedents, but it has awkward consequences, such as not allowing us to call the human eye an adaptation. We are said to possess eyes because of constraint, not adaptation to the environment. A similar problem arises for ecological interactions. Many biologists will also object that the methods are unreliable. Brooks and McLennan reconstruct biogeographic ranges on the assumption that ranges do not change (i.e. no dispersal), and phylogenies on the assumption of parsimony. If you do not like these assumptions, your confidence not only in the results but also in the entire discipline will be proportionally reduced. (By the way, it is interesting to juxtapose the assumptions of vicariance and parsimony, which minimize the effects of history, with the justification of the discipline, which is that historical effects are powerful.) The main defect of historical ecology as Brooks and McLennan present it is that it does not contain anv ideas.