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CRAIG MORITZ, CHRISTOPHER W. DICK, AND ELDREDGE BERMINGHAM
The idea for this book arose from our conviction that there is much to be gained by increasing the level of communication and collaboration between the evolutionary biologists and ecologists engaged in the study of tropical rainforest communities. That knowledge of history and evolution should inform ecology, and vice versa, is hardly a new idea (Ricklefs and Schluter 1993a). Likewise, previous works have dealt explicitly with the effects of history on diversity in rainforests (Flenley 1979; Prance 1982a; Morley 2000). What is new is that we are reaching a sufficient understanding of ecological and evolutionary processes and, for some places, of patterns of local and regional diversity to attempt an integrative approach to the analysis of species-rich tropical rainforests.
As several contributions in this volume make clear, there is much still to be done by way of basic description of species, let alone their ranges, interactions, and phylogenetic relationships. However, the past few years have seen great advances in basic knowledge of tropical rainforests, resulting from, among other things, the establishment of networks of permanent tree inventory plots (Condit 1995), increasing knowledge of the paleoecological history of tropical regions (Morley 2000), the capacity for large-scale sequence-based analysis of phylogenetic and biogeographic history (Moritz et al. 2000), and the acquisition of fine-grained environmental data via remote sensing (Saatchi et al. 2000). These advances are moving us toward a deeper understanding of the origin and maintenance of species diversity in tropical rainforests.
Of course, the glue needed to bind these somewhat disparate fields is theory-specifically, theory that combines ecological and evolutionary approaches. The traditional view can be paraphrased as "ecological processes-productivity, demographics, and species interactions-determine local species richness, while evolutionary processes leading to speciation and extinction set species numbers at regional scales." A newer view, exemplified by Hubbell's neutral theory of biodiversity and biogeography (Hubbell 2001; Hubbell, chap. 4 in this volume), is that local community structure is determined by both ecological (productivity, [J.sub.M]) and evolutionary (speciation rate, v) dynamics, moderated by dispersal limitation, and, as a deviation from the Hubbell model, species interactions. While far from universally accepted (Nee and Stone 2003) and subject to refinement, Hubbell's theory and other attempts to combine ecological and evolutionary theory (e.g., Rosenzweig 1995) provide a promising direction for integrative studies of rainforest diversity at varying spatial and temporal scales.
A principal aim of this book is to showcase the Australian Wet Tropics, because the history of this region has been more thoroughly reconstructed through geological, climatological, and molecular genetic records than that of other rainforest regions. As evolutionary biologists, the editors of this volume have looked at rainforest communities principally through a historical lens, and we felt that the integrated evolutionary and ecological research in the Wet Tropics provides a useful case study. Therefore, this book couples an integrated view of research in the Australian Wet Tropics to chapters focusing on rainforests in Africa, Southeast Asia, and the Neotropics. All of these rainforests have distinct evolutionary and biogeographic histories (Morley 2000), which have undoubtedly had a profound influence on regional differences in alpha and beta diversity (Richards 1973; Condit et al. 2002; Dick, Abdul-Salim, and Bermingham 2003).
The book also aims to provide an improved scientific basis for conservation that we hope will serve as one more resource in the battle to retard, and in some cases reverse, the steady clearing and degradation of rainforests worldwide. While the drivers of rainforest destruction are economic and political, it is up to biologists studying rainforests to develop an understanding of rainforest community dynamics across varying spatial and temporal scales and to communicate their observations and knowledge at multiple levels in order to develop effective conservation strategies. As D. H. Janzen (1986) has written, "Engineers build bridges, writers weave words, and biologists are the representatives of the natural world." The contributors to this volume lucidly represent rainforest biomes around the world, offering suggestions about how their science might lead to improved stewardship of tropical forests.
This volume is divided into three parts. Part I presents a series of contributions spanning general evolutionary and ecological influences on the species diversity of rainforest biotas, and includes approaches referencing different temporal and geographic scales. Part II focuses on the Australian Wet Tropics, with the aim of bringing together evolutionary, paleoecological, and ecological perspectives on the only region of the world's tropics where the rainforest is adequately protected by legal and cultural stewardship. Although human effects on tropical rainforests are not the principal theme of this volume, part III closes the book with four chapters that recognize that the future of tropical rainforests will depend on human behavior and political decisions.
The concept for the book arose in connection with the symposium Rainforests: Past and Future, held in Cairns, Australia, in April 1998, cosponsored by the Cooperative Research Centre for Tropical Rainforest Ecology and Management and the Smithsonian Tropical Research Institute. The volume in your hands, however, includes chapters by authors who were recruited after the Cairns symposium, and all contributions were updated prior to their final submission in May 2003.
With so many new sources of information and theoretical understanding, this is an exciting time to be a rainforest biologist. Given the ongoing threat to this biologically rich biome, it is a critical time to be engaged in the study of rainforests. We hope that this volume will stimulate investigators, old hands and newcomers alike, to think broadly about how evolutionary and ecological factors interact to promote or maintain tropical diversity, and how this understanding can be used to better protect the integrity of rainforests around the world.
Chapter Two Overview: The History and Ecology of Tropical Rainforest Communities
ELDREDGE BERMINGHAM AND CHRISTOPHER W. DICK
The biologists and natural historians of the nineteenth century had a prescient fascination with the species richness of lowland tropical forests (Bates 1864; Belt 1874; Wallace 1878). More than a hundred years later, we remain fascinated, yet not sufficiently close to understanding the origins and long-term maintenance of high species diversity in the tropical forest biome. Thus, a principal aim of this volume is to accelerate the understanding of tropical rainforests through increased recognition that evolutionary processes, in addition to ecological ones, strongly influence the species composition of local communities. As we will see in the chapters of part I, questions regarding the relative roles that ecology and evolution play in the assembly and persistence of tropical forest communities are deeply intertwined, and at a time when biodiversity is being lost at an unprecedented rate, they are among the most important questions in biology.
Although attitudes are changing rapidly, many ecologists have argued that history can be safely ignored. Simply put, because species interactions-principally competition-have rapid dynamics relative to climatic or geologic change, communities reach local equilibria fast enough to overwrite the trace of history (MacArthur 1965; Ricklefs 1989). However, diversity differences among communities occupying similar habitats in different regions (so-called diversity anomalies) provide a strong indication that history has a lasting effect on the species richness of local communities. For example, diversity anomalies between local tree communities in the rainforests of Africa and the Neotropics indicate that regional differences in species richness translate into differences in local species richness. As Richards (1952) noted, "whatever the true explanation for the poverty of the African rain forest, it can hardly be due to any ecological factor operating at the present day."
Diversity anomalies indicate that historical factors reach down through time, interacting with ecological processes to determine patterns of local diversity. But a strong role for historical/regional processes is difficult to reconcile with local processes operating on time scales several orders of magnitude shorter in duration. Ricklefs (1989) pointed out that the putatively weak force of historical/ regional processes might be reconciled with their apparent imprint on local diversity if the outcomes of competitive exclusion were prolonged to evolutionary time scales. The experiments of Gause (1934) and others notwithstanding, it may be rare in nature that one species excludes another in ecological time. The competitive equivalence of species is a principal assumption of Stephen Hubbell's neutral theory of biodiversity and biogeography (2001; Hubbell, chap. 4 in this volume). Under the neutral theory, species characterized by even modest population sizes persist for long periods of evolutionary history. Furthermore, even a small increase in the time course of competitive exclusion beyond the tens of generations often considered by ecologists (Ricklefs 1989, table 3) brings this local community process into temporal register with the regional process of dispersal, and even with speciation under Hubbell's (2001) point mutation mode of species formation.
Ricklefs (1987, 1989; Ricklefs and Schluter 1993b) has long argued that a major synthesis of ecology and evolution is necessary to adequately interpret the development of biological communities. Although we are still a long way from the desired synthesis, part I of this volume fosters a more dualistic approach to the investigation of tropical rainforest assembly, maintenance, and conservation. It is fitting that Ricklefs leads off part I, and the book, with a chapter presenting two phylogenetic methods aimed at providing insight into the role that historical factors have played in establishing regional and local differences in species richness. Phylogenetic analysis of species production, dispersal, ecological adaptation, and extinction in relation to geologic, geographic, and environmental history is key to understanding regional differences in diversity and, in turn, local diversity anomalies. Sister-taxon phylogenetic analysis provides one profitable direction of investigation because differences in the contemporary species richness of sister clades located in different regions must have resulted from differences in the net rate of diversification (speciation minus extinction). Thus, sister-taxon phylogenetic comparison provides a means for identifying differences in morphology, ecology, or geographic distribution that set regional differences in species number. The second method advocated by Ricklefs is increased use of phylogenetic analysis to separate the contributions of time and speciation rate to species diversity in order to determine the relative ages of clades constituting regional species pools or local ecological communities.
Phylogeny and species identification (taxonomy) also play a role in assessing the relative importance of ecological drift in establishing species richness and relative abundance. According to Hubbell's neutral theory (2001), described in chapter 4, local diversity is controlled primarily by rates of species diversification and by the size of the metacommunity, defined as the "evolutionary-biogeographic unit within which most member species spend their entire evolutionary lifetimes" (Hubbell 2003). But this is a challenging area of inquiry, particularly to the degree that species form according to the point mutation mode, thus yielding species that are not sufficiently divergent from their parent to be recognizable. The evolutionary unit is the individual, and lineages have no assigned probabilities of speciating or going extinct, as in conventional neutral models of phylogenetic reconstruction (Raup et al. 1973; Gould et al. 1977; Nee, Mooers, and Harvey 1992; Nee, May, and Harvey 1994). Rather, the probability of speciating or going extinct is determined by the relative abundance of a lineage, leading to the prediction that most regionally abundant species are old compared with rare species, and are far more likely to have produced daughter species. Thus, the loss of species through competitive exclusion might not occur much more rapidly than the gain of new species in a community, and both of these processes might occur on much longer time scales than apparent in simplified model systems or microcosms.
Because ecological drift is a stochastic process, samples of local communities separated in time (or space) should have low correlations between their species compositions. In the absence of changes in metacommunity size or in the speciation rate, local species richness should stay the same and should represent a genuine steady state between speciation and extinction (Hubbell 2003). Assessments of temporal changes in species diversity and community membership based on the fossil record are often faulted owing to holes in the record. In chapter 5, John Flenley presents a simple but useful test of the quality of plant pollen and spore fossil data by establishing that the well-documented increase in tree species diversity with decline in latitude can be recovered with a high degree of reliability from the arboreal pollen record. This is a fair test of palynological data quality, given that the latitudinal diversity gradient must be a relatively persistent feature of earth history because it is expressed at multiple taxonomic levels (species, genera, and families). Having verified that the pollen record provides a reasonable proxy for species richness, Flenley sets out to test whether tropical rainforest tree species diversity has increased or decreased during the Pleistocene. Such a test can falsify diversification hypotheses predicting either positive or negative change in species diversity due to climatic fluctuations.
Pollen records from only two sites, Borneo and Amazonia, meet the sedimentary and chronological criteria necessary to assess changes in local species number over time. The Borneo site showed virtually no change in species number over time, and the pollen taxa in the Miocene and Holocene pollen records were almost identical, whereas the Amazonian site revealed that palynological richness was approximately halved in the Holocene compared with the Miocene. Keeping in mind caveats regarding taxonomic resolution and sample quality between time intervals, the apparent lack of species turnover between the Miocene and Holocene records at the Borneo site falsifies ecological drift and suggests that non-stochastic processes (e.g., niche assembly) have stabilized community composition. A similar pattern of community stability for a temperate forest was observed across a 10,000-year pollen record of red maple, birch, beech, ash, oak, hemlock, and elm trees from cores of lake sediments in southern Ontario (Clark and McLachlan 2003). If the time scale of species turnover through ecological drift is generally longer than the time scale of environmental change at a particular location (Ricklefs 2003), it follows that the palynological record can more easily be used to reject the neutral theory than to support it. Thus, the decline in species number at the Amazonian site documented by Flenley could be considered evidence of ecological drift only in the absence of environmental change over the time interval assessed by the pollen record.
In chapter 6, Paul Colinvaux holds our focus on the Amazon basin during a period of dynamic environmental change and marshals pollen, temperature, C[O.sub.2], and geomorphological evidence to advance two important points. The first is that the lowland South American rainforest was a stable formation in the face of Pleistocene climate change. In other words, the forest was never fragmented into the refugia envisioned by Haffer (1969). Second, both species composition and species population sizes varied with climate in Amazonian rainforests. The Amazon pollen record demonstrates the penetration of some montane tree species into the lowland rainforest and hints at changes in the relative abundances of lowland species. Thus, the detailed species composition of Neotropical lowland plant communities varied, but there was no community-level replacement, and what is forested now was forested during the Pleistocene glacial periods. The evidence of changes in species composition and relative abundance presented by Colinvaux is consistent with the species turnover predicted by Hubbell's ecological drift model, but environmental fluctuations can produce the same patterns.
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Craig Moritz, Christopher W. Dick, and Eldredge Bermingham
Part I. Evolutionary and Ecological Determinants of Tropical Rainforest Diversity
2. Overview: The History and Ecology of Tropical Rainforest Communities
Eldredge Bermingham and Christopher W. Dick
3. Phylogenetic Perspectives on Patterns of Regional and Local Species Richness
Robert E. Ricklefs
4. Large-Scale Diversity and Species-Area Relationships in Tropical Tree Communities under the Neutral Theory
Stephen P. Hubbell
5. Palynological Richness and the Tropical Rainforest
John R. Flenley
6. The Pleistocene Vector of Neotropical Diversity
7. The History of Amazonian Mammals: Mechanisms and Timing of Diversification
James L. Patton and Maria Nazareth F. da Silva
8. Biogeography and Diversification of African Forest Faunas: Implications for Conservation
Jon Fjeldså, Michelle K. Bayes, Michael W. Bruford, and Michael S. Roy
9. Evaluating the Divergence-with-Gene-Flow Model in Natural Populations: The Importance of Ecotones in Rainforest Speciation
Thomas B. Smith, Robert K. Wayne, Derek Girman, and Michael W. Bruford
10. Geologic, Evolutionary, and Ecological Bases of the Diversification of Neotropical Butterflies: Implications for Conservation
Keith S. Brown Jr.
11. Dynamic Landscape Models for Tropical Rainforests
Brendan Mackey and Wengui Su
12. Understanding and Conserving Tropical Diversity: Perspectives from Barro Colorado Island
Egbert Giles Leigh Jr. and Ira Rubinoff
13. Landscape Heterogeneity and Species Diversity in Amazonia
Kalle Ruokolainen, Hanna Tuomisto, and Risto Kalliola
14. Spatial Changes in Tree Composition of High-Diversity Forests: How Much Is Predictable?
Richard Condit, Salomón Aguilar, Andrés Hernández, Rolando Pérez, Suzanne Lao, and Christopher R. Pyke
15. The El Niño Southern Oscillation Influences Tree Performance in Tropical Rainforests
S. Joseph Wright
Part II: A Multidisciplinary Perspective on an Entire Rainforest System: The Australian Wet Tropics
16. Overview: Rainforest History and Dynamics in the Australian Wet Tropics
17. The Origin and Evolution of Australia's Eastern Highlands
18. The Origins and Tertiary History of Australian "Tropical" Rainforests
David R. Greenwood and David C. Christophel
19. Patterns and Causes of Vegetation Change in the Australian Wet Tropics Region over the Last 10 Million Years
A. Peter Kershaw, Patrick T. Moss, and Russell Wild
20. Effects of Quaternary Climate Change on Rainforest Diversity: Insights from Spatial Analyses of Species and Genes in Australia's Wet Tropics
Christopher J. Schneider and Stephen E. Williams
21. Mosaic Macroevolution in Australian Wet Tropics Arthropods: Community Assemblage by Taxon Pulses
Patrice Bouchard, Daniel R. Brooks, and David K. Yeates
22. Biodiversity of the Freshwater Invertebrates of the Wet Tropics Region of Northeastern Australia: Patterns and Possible Determinants
Richard G. Pearson
23. Dynamics of Seedling Recruitment in an Australian Tropical Rainforest
Joseph H. Connell, Igor Debski, Catherine A. Gehring, Lloyd Goldwasser, Peter T. Green, Kyle E. Harms, Peter Juniper, and Tad C. Theimer
24. The Theory and Practice of Planning for Long-Term Conservation of Biodiversity in the Wet Tropics Rainforests of Australia
Nigel E. Stork
Part III: Rainforest Futures
25. Overview: Processes, People, and Prospects for Tropical Rainforests
Craig Moritz, Christopher W. Dick, and Eldredge Bermingham
26. Evolutionary Approaches to the Conservation of Tropical Rainforest Vertebrates
Craig Moritz and Keith R. McDonald
27. Parks, People, and Policies: Conflicting Agendas for Forests in Southeast Asia
28. The Future of the Amazon
William F. Laurance, Scott Bergen, Mark A. Cochrane, Phillip M. Fearnside, Patricia Delamônica, Sammya Agra D'Angelo, Christopher Barber, and Tito Fernandes
List of Contributors