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Vol. 7(4), pp. 2-5The McAllen International Orchid Society JournalApril 2006

Book Review - The Monkey's Bridge: Mysteries of Evolution in Central America

Eric Olsen

David Rains Wallace. San Francisco: Sierra Club Books, 1997. 277 pp. $14.00. ISBN 1-57805-018-9.

Fig. 1. Cover, The Monkey's Bridge

Book titles are to the world of publishing what the first flashes of color from a flower's bloom are to the world of orchids: the alluring promise of more and better things to come that is sometimes built on a lie. This book's title is a lie, invented by someone more interested in sales than accuracy; but it is a harmless lie. While The Monkey's Bridge only speaks of monkeys as one small part of Central America's fauna and is certainly not focused simian evolution, it is still a fascinating read. For anyone interested in the natural history of Central America or in the general topic of evolution, Wallace's book is well worth the time spent. For readers interested primarily in orchids, it must be noted from the outset that this book superficially has even less to do with orchids than with monkeys. Wallace mentions orchids only in passing and in general, never in specifics. Yet, this book has everything to do with the extraordinary variety of orchids in Central America and how they are connected to their South American, North American, and West Indian relatives.

Wallace's fundamental theme is that Central America has played a key role in the recent evolution of life in the Americas. When it first formed and joined South and North America approximately five million years ago, it connected two continents that had evolved separately for tens of millions of years before that. For many species, both flora and fauna, Central America became a bridge between the continents, allowing them to move into new territories. As each species moved onto and across the bridge, it came into contact with new environments, whether climatic, geographical, or biological. Native species, long resident in a particular area, were also affected since the newcomers competed for resources or altered the ecosystem in some way. All these new pressures forced many species (both native and colonizing) to adapt and, where local populations became isolated, brought about the formation of new species. Thus Central America can be seen as a sort of evolutionary vortex, stirring up the biology of the Americas and creating myriad new forms. This, according to Wallace, explains why "Central America is so crowded with life that it supports seven percent of the earth's species on less than one-half percent of its land" (p. xiii).

If Central America is acting as an evolutionary bridge between North and South America, then what would one expect in the world of orchids? What would one expect from an area in which evolution seems to be working on fast forward? First, one might expect to see few orchid genera common to both continents, with the overlapping genera centered primarily on Central America. Second, if Central America also acts as an evolutionary accelerator, one would expect it to have a few unique genera, seen only there.

To attempt to support these preliminary conclusions, this writer turned to his orchid library and began searching for material on geographic distributions of different orchid genera. The key reference in this search turned out to be Tom and Marion Sheehan's An Illustrated Survey of Orchid Genera. Unfortunately, the Sheehans covered only some 150 of the approximately 800 orchid genera worldwide. Still, of those 150, 97 were genera endemic to the Americas. While not a complete survey of American orchids, it would still serve as a reasonable proxy, useful for at least getting a sense of how the genera are arranged geographically. By examining the Sheehans' distribution maps and the accompanying notes, orchid genera were classified by their presence or absence in North America (defined here as the North American continent, minus southern Mexico, minus the tip of Florida), the West Indies (the Caribbean islands, plus the tip of Florida), Central America (the area from Panama to southern Mexico), and South America (the continent, plus Trinidad). The first results of this exercise can be seen in Table 1.

Table 1. Number of Genera Present in Different Regions of the Americas

Area# of Genera Present
North America (NA)6
West Indies (WI)33
Central America (CA)72
South America (SA)78

These first results did not account for overlaps in the ranges of various genera and so were not all that useful in supporting or denying the two evolutionary expectations given above. However, what was notable and a little surprising was that Central America housed nearly as many orchid genera as the much larger landmass of South America.

To examine the overlaps in genera, a second table (Table 2) was constructed.

Table 2. Distributions of American Genera by All Possible Combinations of Regions

NAWICASA# GeneraGenera
*** Orchid genera unique to a single area. ***
x 1Platanthera
x 6Broughtonia, Cattleyopsis, Domingoa, Polyrrhiza, Psychilis, Vanilla
x 9Barkeria, Bothriochilus, Erycina, Meiracyllium, Mexicoa, Nageliella, Osmoglossum, Rhyncholaelia, Rossioglossum
x17Acacallis, Anguloa, Baptistonia, Batemannia, Bifrenaria, Bollea, Cochlioda, Gomesa, Leptotes, Miltonia, Promenaea, Rodrigueziella, Scuticaria, Sophronitis, Stenia, Zygopetalum, Zygosepalum
*** Orchid genera present in 2 of the 4 areas, but not in the others. ***
xx 1Calopogon
x x 1Cypripedium
x x0
xx 1Calanthe
x x0
xx36Acineta, Ada, Aspasia, Cattleya, Caularthron, Chysis, Clowesia, Coryanthes, Cycnoches, Gongora, Hexisea, Huntleya, Koellensteinia, Laelia, Lemboglossum, Lockhartia, Lycaste, Masdevalia, Mendoncella, Miltoniopsis, Mormodes, Mormolyca, Neomoorea, Notylia, Odontoglossum, Oerstedella, Peristeria, Pescatorea, Phragmipedium, Polycycnis, Psygmorchis, Rodriguezia, Sobralia, Stanhopea, Trichocentrum, Trigonidium
*** Orchid genera present in only 3 of the 4 areas. ***
xxx 0
xx x0
x xx0
xxx22Arpophyllum, Brassavola, Brassia, Bulbophyllum, Catasetum, Cochleanthes, Comparettia, Cyrtopodium, Encyclia, Galeandra, Ionopsis, Isochilus, Leochilus, Maxillaria, Oncidium, Paphinia, Polystachya, Sarcoglottis, Schomburgkia, Stenorrhynchus, Trichopilia, Vanilla
*** Genera present in all of the Americas. ***
xxxx3Epidendrum, Eulophia, Habernaria

This second table is more revealing. In the Sheehan's survey, only three genera (Epidendrum, Eulophia, and Habernaria) are present in both North and South America (and also extend through Central America and the West Indies). Furthermore, only Epidendrum is restricted to the Americas (the other two genera have world-wide distributions and their arrival in the Americas probably pre-dates both the Central American land-bridge and, most likely, even the separation of the continents). Thus, of the 97 genera surveyed, only one is native solely to both continents.

From Table 2 it is also seen that Central America as well as the West Indies are meeting grounds between North and South American genera of orchids. Thirty six genera are shared between South and Central America; a further 22 are found in South America, Central America, and the West Indies; and a single genus (Cypripedium, if Mexipedium is included) is native to both North and Central America. Due to the overlap with large numbers of South American genera, the data apparently show both Central America and the West Indies acting as bridges, primarily serving South American orchids heading north.

The second evolutionary expectation was that Central America might have its own unique genera, shared with none of the surrounding areas. This is supported by the nine genera found only in Central America. Compare this with the seventeen solely South American genera. This could be a sign of Wallace's Central American evolutionary vortex, where the changes wrought by new and actively changing environments are powerful enough not only to encourage speciation, but also the formation of new genera. The West Indies with its own six unique genera also appears to be a place of rapid evolutionary change. Oddly though, only a single genus (Calanthe) is found exclusively in Central America and the West Indies. It would seem that the Central American and West Indies bridges probably have little direct communications between them and the movement of orchid populations is mostly south to north.

The genus distributions in An Illustrated Survey of Orchid Genera bring up other questions. For instance, Epidendrum ranges from southern Brazil, through the Amazon, up the Central American bridge, island-hops through the West Indies, and extends through the south-eastern United States, but why did it leave a gap on the Gulf of Mexico coast from about Tampico to Corpus Christi? The genus Laelia leaps even more strikingly from Brazil, skipping Venezuela and Colombia, and into Central America. Why these gaps? Are they areas where populations used to be, or do we simply not know enough about these areas to fill in the gaps?

In the final chapter of The Monkey's Bridge ("The Bridge of Cattle and Coffee"), Wallace talks about the role of man in altering the nature of the Central American bridge. He tells us how urbanization, agriculture, and roads are changing the landscape. He observes that for some species, this makes the bridge act as a biological filter that prevents movement; for others, it enhances the bridge, making passage much easier (for example, the rapid traversal of Africanized bees from South America in spite of many efforts to stop them). What questions might an informed orchid enthusiast, conservationist, or scientist ask given these observations? Is the passage of a particular orchid species impeded or assisted? How does it affect orchid pollinators? How does it affect the dispersal of orchid seeds? Every chapter in Wallace's complex mosaic of geology, climatology, botany, biology, history, and travelogue provides a rich context in which to ask these and many other questions. Like the land it describes, The Monkey's Bridge is a vortex; a vortex of ideas. Orchid lovers should read it with their favorite books on Central American orchids and watch their thoughts and ideas overflow and evolve.

Copyright © 2006 Eric Olsen