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Tropical Forest Biodiversity: Distributional Patterns and Their Conservational SignificanceAuthor(s): Alwyn H. GentrySource: Oikos, Vol. 63, Fasc. 1 (Feb., 1992), pp. 19-28Published by: Wiley on behalf of Nordic Society OikosStable URL: http://www.jstor.org/stable/3545512 .

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OIKOS 63: 19-28. Copenhagen 1992

ITropical forest biodiversity: distributional patterns and their conservational significance

Alwyn H. Gentry

Gentry, A. H. 1992. Tropical forest biodiversity: distributional patterns and their conservational significance. - Oikos 63: 19-28.

Phytogeographical knowledge of two major patterns important to conservational planning - the distribution of diversity and endemism in tropical forests - are summarized. High diversity forests occur on all three continents and are concentrated in lowland areas with high and evenly distributed rainfall, but with greatest diversity usually occurring in northwest South America forests. Tree and liana species richness is greatest in upper Amazone and non-tree species richness greatest in the northern Andean foothills and southern Central America, suggesting conservational priority for these areas. Endemism is only partly correlated with diversity and is concentrated in isolated patches of unusual habitat, in cloud forests, in topographically dissected montane areas, and on continental fragment islands, areas which also deserve conservational priority. Since different taxa show different distributional patterns, herbs and epiphytes, as well as trees and large vertebrates, must be considered in tropical conservational planning.

A. H. Gentry, Missouri Botanical Garden, P. 0. Box 299, St. Louis, MO 63166-0299, USA.

Tropical forests are famous for being the most species rich ecosystems on earth. To a biologist this concentra- tion of diversity is exciting and challenging; but it is also a kind of scientific millstone around his neck because, for many groups, most notably insects, our level of taxonomic knowledge is inadequate to cope with such overwhelming diversity. The world's tropical forests are disappearing at alarming rates, yet the cataloguing of their constituent species that would be the obvious first step in understanding and conserving them is made impossible by the dearth of taxonomic expertise.

It is no accident that a disproportionate amount of ecological theory and strategy for conservation are based on birds, taxonomically the best known major group of organisms. The broad aspects of bird distributions are now well enough understood that the major concentrations of bird endemism and diversity can be reasonably well documented merely by plotting the known distributions of the individual species. But how representative are birds of other taxa and especially of other trophic levels such as plants?

Accepted 9 April 1991 ?D OIKOS

Vascular plants contribute most of the structure and biomass to a rainforest and are surely the single most fundamental rainforest component. As sessile auto- trophic, mostly very long-lived organisms they may be subject to very different evolutionary and ecological constraints than are birds. Clearly direct data on distribution and diversity patterns of tropical plants are needed if we are to make wise conservational decisions. The level of knowledge of the taxonomy of vascular plants, intermediate between that of birds and insects, is far from complete, but adequate to provide rather complete and detailed distributional data for many parts of the world.

An obvious approach to conserving plant biodiversity is to map distributional patterns and look for concentrations of diversity and endemism. This is exactly what has been done in North America and Europe where organizations like the Nature Conservancy have devel- oped data banks of such distributional patterns and focused their conservational attention on areas where rare species are concentrated. With such data Elias

2* OIKOS 63:1 (1992) 19



(1977) could confidently list 90 plant species as having become extinct in the US, with the rate of extinction demonstrably accelerating. The Nature Conservancy currently lists 84 extinct or probably extinct full species (Morse 1990), mostly from California. Using this data base, Gentry (1986a) was able to show that the concen- tration of locally endemic US species is strongly focussed, not only in California and Hawaii, but also in northern Florida.

State of tropical floristic knowledge Unfortunately the much greater biodiversity of tropical forests is also much more poorly known. While several large scale monographic projects which include distributional data are underway, they are woefully inadequate both as to the available data base and as to the level of effort currently being expended. For example, it has recently been estimated that at current rates the Flora Neotropica monograph series will be completed in 300- 400 yr (Prance 1977, Mori, pers. comm.). Worse, even the most recent monographs quickly become out of date as new collections continue to be made. For example, from additional collections made during the 15 yr subsequent to his publication of the Flora Neotropica Chry- sobalanaceae monograph (Prance 1972), Prance (1989) described 67 new species of that family. Data on local concentrations of diversity and endemism extracted from tropical monographs must be interpreted with ex- treme caution, since apparent patterns may reflect no more than collection artifacts. Even though many species are known from single collections or single localities, it is difficult to know whether their distributions are truly restricted. A typical example is the 19 species of presumably locally endemic Bignoniaceae known only from single collections or collection localities whose second known collection site was at least halfway across Amazonia from the first collection site (Gentry 1979). Far from being locally endemic, they were merely inad- equately collected. Nelson et al. (1990) have shown that the maps of concentrations of endemism on which plans for conservation of Brazilian Amazonia were based are exactly coincident with maps of collection density.

A few examples from recent field work in Amazonian Peru will serve to give an idea of how incomplete our knowledge is. Memora pseudopatula, discovered in 1976, is the commonest climbing species in the seasonally inundated forests near supposedly well-collected Iquitos (Gentry 1981). Styloceras brokawii, the first lowland species of its Andean genus (Gentry and Foster 1981), is locally so common in parts of Manu Park that it was already infamous among local primatologists for impeding their progress through the forest long before we described it. At the same locality Caryodaphnopsis fosteri is the commonest Lauraceae (and one of the commonest tree species); described in 1986, it belongs

to an Asian genus only recently reported for the West- ern Hemisphere (Werff and Richter 1985, Werff 1986), yet it is widespread elsewhere in Amazonian Peru as are several undescribed congeners. This "Asian" genus is now known to include 9 neotropical species, at least one of which is an important timber tree. Over the last three years, 72-74% of the round wood used in construction at Iquitos has been from "aceite caspi" which turns out to be a species of Caraipa new to science (Vasquez 1991b); the main construction timber at Jenaro Her- rera, on the Rio Ucayali, is a new species of Haplo- clathra (a genus also new to Peru) so distinctive that we originally thought it might represent a new genus (Vas- quez 1991a).

The situation is even worse in some other parts of the world. In the cloud forests along the base of the Andes I suspect that each ecologically isolated patch of ridge top cloud forest may have ca. 20% undescribed species (Gentry 1986a). For example, 92 new species were discovered during the preparation of the Florula of the 1 km2 Rio Palenque Field Station in western Ecuador (Dodson and Gentry 1978 and unpubl.).

Given this general lack of knowledge it seems likely that we should concentrate on looking for patterns rather than focussing on individual species, in order to make possible rational conservation in the face of such incomplete knowledge. To what extent can we elucidate distributional patterns that are shared between different plant taxa?

Phytogeographic patterns We can explore tropical phytogeographic patterns from a variety of perspectives. One feasible approach that has direct conservational consequences is to look for concentrations of diversity. This is essentially the approach advocated by Mittermeier and colleagues (1990) at Conservation International, who have emphasized conservational efforts in a dozen "megadiversity" countries.

While the "megadiversity" approach is at the national level and focuses on better known vertebrate taxa, a more local approach to cataloging concentrations of diversity may be more appropriate for tropical plants. One of the biggest advantages of this kind of local focus is that particular plant communities can be sampled and their species richness quantified even in the absence of complete identification. Much of the diversity of tropical forests is vested in taxonomically difficult and poorly collected families of large trees like Sapotaceae and Lauraceae, many of whose species remain undescribed. The only way to get an idea of local diversity is often by ecological sampling, where incompletely iden- tified taxa can at least be assigned to morphospecies. Once we know which areas have the greatest diversity

20 OIKOS 63:1 (1992)



we might logically single these out for conservational emphasis.

It is abundantly clear that within-community (alpha) diversity of tropical forests varies dramatically from place to place. From the use of inventory techniques, we have begun to accumulate a reasonable data base for assessing patterns of local diversity of woody plants. The broad outline of these diversity patterns has long been known but some of the emerging details are sur- prises, and a few, like the relationship between species richness and soil fertility, remain hotly debated. In general the most diverse forests occur in lowland tropical areas with high and evenly distributed rainfall.

The relationship between nutrient availability and species richness is more controversial. Rich soil forests may be the most species-rich, presumably related to high productivity and/or stochastic effects associated with high turnover (Gentry 1988b, Adams 1989, Adams and Woodward 1989). However, there are dissenting opinions which hold that low-fertility soils generally have greater species richness (Huston 1979, 1980) or that intermediate fertility soils have the most diverse forests (Ashton 1991). Overall it is probable that soil fertility has a relatively minor direct effect in determin- ing species richness (Stark et al. 1991).

In the Neotropics the greatest a-diversity is generally in upper Amazonia. Birds, mammals, reptiles, and but- terflies as well as trees, all have their highest known a-diversity in the world at some site in Amazonian Peru and the most diverse site for amphibians is in Amazo- nian Ecuador (Gentry 1988a). The world record and runner-up for trees are two 1 ha sample sites from the aseasonal lowland forest near Iquitos, Peru, both with nearly 300 species of woody plants ' 10 cm diameter. Only for plants ' 2.5 cm diam. in 0.1 ha samples is the world record diversity not in Amazonia; for this kind of sample the pluvial forests of coastal Colombia are the most species rich, although several upper Amazonian samples (and a few Asian ones) are nearly as speciose (Gentry 1986b, 1988b).

In Asia the richest forests are in Borneo and adjacent Peninsular Malaysia (Ashton 1977, Whitmore 1984, Appanah et al. 1991); in Africa the most species-rich forests, at least for woody plants, are in the wetter areas of west Central Africa, from near the base of Mt. Cam- eroun (Gentry 1992) southeast into Gabon (Reitsma 1988, Gentry 1992); in Madagascar in the northeast around the Bay of Antongil (Gentry 1988c, 1992). Ap- parently no forests in Central America, West Africa, the Indian subcontinent or Australia are as diverse as the richest South American, Afro-Madagascan, and Malesian ones. In general, levels of intracommunity diversity in all three main tropical regions seem remark- ably similar for woody plants.

Perhaps an appropriate conservation strategy for tropical forests should just focus on trees, which tend to be relatively well-known taxonomically and many of which prove to be relatively wide-ranging. However,

tropical forests are also rich in non-tree species, even though this is often overlooked. Indeed one reputed aspect of tropical forests is that their flora is predominantly composed of large woody plants, whose diversity is only apparent at relatively large scales. Thus there have been suggestions that at small scales of 0.1 ha or less, comparable to those used for sampling temperate zone vegetations, the diversity of some non-tropical vegetations might equal or exceed that of tropical forests. As recently as 1988, Mooney could review the literature and conclude that while "tropical systems are probably among the world's richest, ...the vegetation of Mediterranean-climate regions is also quite rich ... (and some) Mediterranean-climate vegetation is as rich as any found on Earth".

While it is true that tropical forests have uniquely high tree species richness, our data from western Ecua- dor (Gentry and Dodson 1987a) would appear to dem- onstrate conclusively that wet tropical forests are far more diverse than any extratropical vegetation, even at scales of 0.1 ha or less, and even for non-trees. Indeed the Rio Palenque 0.1 ha sample would be the richest yet sampled in the entire world even if all tree species (including their juveniles) were excluded. When habit representation in complete local florulas is compared, less than 1/4 of the species at most neotropical sites are trees, although this number may be higher in central Amazonia where 68% of the species in an incomplete local list are trees (Prance 1990, Gentry 1990b). Epi- phytes account for 12-16% of the species at moist forest sites and about 1/4 in wet forests. Up to 50% of the plant species at individual sites are herbs and shrubs (Foster 1990, Gentry 1990b). Clearly any rational plan to conserve the biodiversity of tropical forests must take into account the numerous species of non-tree habit groups.

Although few data are available for paleotropical forests, non-trees also seem to predominate in most of them. For example at Makokou, Gabon, only 34% of the species are trees potentially > 10 cm dbh (Gentry and Dodson 1987b), whereas climbers constitute a quarter of the species and herbs and shrubs another 37%. Hladik (1986) pointed out that there are more species of shrubs and small trees and as many of lianas at Mako- kou than of trees > 10 m tall. In a set of very small (0.01 ha) plots in Gabon Reitsma (1988) found that 23-30% of the species were herbs and subshrubs and 27-34% lianas.

Southeast Asian forests may have a higher proportion of trees (Janzen 1977), but in the only published local florula Corlett (1990) found 19% of the species in the Bukit Timah Reserve, Singapore, to be climbers and only about half of the flora to be composed of trees and shrubs together. The largest families at Bukit Timah included predominantly herbaceous ferns and orchids, and predominantly shrubby Rubiaceae, along with mostly arborescent Euphorbiaceae. In, a survey of fer- tile plants in the understory at Pasoh, Malaysia (A.

OIKOS 63:1 (1992) 21



Tab. 1. Number of species (' 2.5 cm dbh) shared by 0.1 ha samples of adjacent central African forests on different (Cam- eroun) and similar (Gabon) substrates. (Compare with Ama- zonian data from Gentry 1988b: Table 3).

Southwest Cameroun Korup Mt. Cameroun

Korup (poor skeletal soil) 139 20 Mt. Cameroun (volcanic soil) 128 Gabon

Makokou No. 1 Makokou No. 2 Makokou No. 1 135 60 Makokou No. 2 116

Gentry and J. LaFrankie, original data), we found 39 species in a 500 x 5 m transect, a value similar to the average for comparable neotropical sites (Gentry and Emmons 1987).

In Latin America diversity of non-tree taxa is almost inversely correlated with diversity of tree taxa (Gentry 1982). Predominantly woody plant families have their distributional centers in Amazonia, predominantly herbaceous and epiphytic families in the cloud forests along the base of the northern Andes and adjacent Central America (Maas 1977, Gentry 1982, Andersson 1989, Luteyn 1989). Thus at a regional level a very different set of areas is suggested for conservational emphasis by the epiphytic and non-woody taxa than by the tree and liana taxa whose generally Amazon-centered distributions have been more instrumental to date in developing conservational strategy. At a more theoretical level it is quite likely that different conservation strategies might be appropriate for small short-lived herbs than for large, long-lived, widely spaced, predominantly outcrossing trees.

Endemism Understanding patterns of endemism requires a much stronger data base and better taxonomic resolution than does establishing broad scale diversity patterns. Unless a tropical genus or family has been recently monographed, many of its apparently endemic species often turn out to represent taxonomic artifacts resulting from careless or parochial taxonomy rather than true endemics. Conversely many endemic species have not yet been described, especially in the large and taxonomically difficult genera which tend to contain a disproportionate share of endemic species. Worse, any locally endemic taxa have not yet been discovered in those large areas of the tropics which have not even been cursorily explored botanically.

Despite such constraints, we have sufficient data to know that endemism is far from randomly distributed and to suggest, even in the absence of complete taxonomic data, where areas with unusually high endemism are concentrated. For example, in Latin America the

greatest concentrations of local endemics are found in four situations - 1) isolated patches of unusual habitat, especially in Amazonia; 2) cloud forest ridges, especially along the lower slopes of the Andes and in adjacent southern Central America; 3) isolated dry inter- Andean valleys, and similarly dissected central Mexican valley systems; 4) the two largest Antillean islands, Cuba and Hispaniola.

Habitat specificity It is now well-established that many neotropical plant species are habitat specialists, especially in Amazonia (Prance 1982, Gentry 1986a, 1988b, 1990a). Moreover, at least in Amazonia, a disproportionate part of the locally endemic plant species are restricted to patches of unusual habitat (Gentry 1991b). This suggests that much tropical plant speciation may have involved spe- cialization for marginal habitats, a model proposed by Gentry (1986a, 1989) as an alternative to the wellknown but increasingly controversial Pleistocene Refuge model. A good example of how this process works is Phryganocydia (Bignoniaceae), a small well-defined genus that consists of three accepted species (Gentry 1983). A wide-ranging wind-dispersed species, P. corymbosa, has given rise to two locally endemic water- dispersed derivatives with wingless seeds in the swamps of the Magdalena Valley (P. uliginosa) and the Pacific coast mangroves of northern Choco and southern Cen- tral America (P. phellosperma). A third derivative, with relatively thick subwinged seeds and white instead of magenta flowers, occurs in the seasonally inundated varzea and tahuampa forests along the Amazon. The taxonomic status of this unnamed form remains unre- solved, but whether or not it is specifically distinct, it clearly represents incipient speciation similar to that which gave rise to P. phellosperma and P. uliginosa. Typical P. corymbosa also occurs in Amazonia but not in seasonally inundated forest. Selection for water-dispersed seeds in the varzea habitat, presumably due to interrupted gene flow across strong habitat gradients (cf., Endler 1982a, b), seems to be taking place under today's climatic regime and without intervention of Pleistocene refugia nor any large scale allopatry or vi- cariance event.

There seems little doubt that much (perhaps most) woody plant evolution in Amazonia (and elsewhere in the Neotropics) reflects strong selection for adaptation to specific kinds of substrates (Gentry 1988b, 1989, 1990a). For example, at Tambopata, Peru, most of the species in a series of 1-ha tree plots in different forest types occur only in one type of forest (Gentry 1988b). A good extra-Amazonian example is the Bajo Calima area of western Colombia where, on an unusual white clay soil virtually lacking in phosphorus, locally endemic species of many different families have similar large

22 OIKOS 63:1 (1992)



sclerophyllous leaves with brownish or tannish pubes- cent undersurfaces. Indeed at least nine Bajo Calima species have the largest leaves of any member of their respective families in the entire world and many more have the largest leaves in their genera (Gentry 1986b). Ashton (1977) and Whitmore (1984) have also emphasized that many tropical Asian tree species tend to be substrate specific. The same pattern seems to hold in tropical Africa where there is a distinctive riverine forest community at Makokou, Gabon, and where there is only 8% overlap between 0.1 ha samples on different substrates near Mt. Cameroun (Table 1).

Local endemism is often associated with speciation on unusual substrates, especially in Amazonia. For the Big- noniaceae as a whole, about 3/4 of the locally endemic (= ranges < 50, 000 km2) Amazonian species are specialists in such habitats as white sand savannas, swamps, seasonally inundated varzea (tahuampa) forest, igapo (sensu Prance 1980: = black-water tahuampa), lime- stone outcrops, laja outcrops, or peripheral cloud forests. Only 12 of the 48 locally endemic Amazonian species of Bignoniaceae occur in terra firme forest (Gentry 1986a). This same situation is prevalent in other woody Amazonian families. For example, Prance 1982) suggests that more than 2/3 of the endemic Chry- sobalanaceae species of the Guayana-Venezuela region are habitat specialists. Renner's (1990) analysis of distribution patterns and modes of speciation in several me- lastom genera shows similar patterns, with Macairea, the most speciose of the analyzed genera, concentrated in savannah habitats in the Guayana area and having much more local endemism than the other genera. Thus from a conservational viewpoint it is important to include in a system of preserves those patches of unusual substrate where speciation has given rise to many local endemics.

Cloud forest ridges There is mounting evidence that many of the taxa of herbs and epiphytes that are concentrated in the An- dean foothill cloud forests are strikingly prone to local speciation (Gentry 1986a, 1989, Gentry and Dodson 1987a). Local endemism is very high in these "Andes- centered" groups and the genera tend to be exception- ally large and speciose (Gentry 1982, 1986a). The small amount of evidence available suggests that endemism can occur on extremely local scales in ecologically isolated cloud forests as small as 5-10 km2 (Gentry 1986a). In such ridge-top forests as Centinela, Ecuador, which is only 350 m higher than the adjacent valley that sep- arates it from the Andean Cordillera, and Cerro Tacar- cuna, on the border of Panama and Colombia, the level of local endemism is apparently 10-24% (Gentry 1986a). A good example of this kind of explosive speciation is Gasteranthus with a quarter of its world total 24

species endemic to the environs of Centinela (Gentry and Dodson 1987b).

It is likely that an entirely different evolutionary mode is operating in these areas, the exceedingly dy- namic speciation perhaps mediated more by genetic transilience associated with genetic drift in small founder populations in a kaleidoscopically changing milieu than by fine-tuned selection of the type suggested above for lowland rain forest. The process is similar to the founder-effect-mediated speciation discussed by Carson and Templeton (1984) for Hawaiian Drosophila. In terms of Templeton's (1989) cohesion species concept, genetic drift gives rise in different founder populations to different sets of demographically exchangeable al- leles. We suspect that at least in orchids, where this kind of process is complemented by intricate revolutionary interactions, speciation can take place in as little as 15 yr (Gentry and Dodson 1987b).

In the cloud forest areas, unlike the lowland rainforest, speciation appears to be an altogether open-ended phenomenon without the slightest hint of any kind of ecological equilibrium or limit on species diversity (Gentry 1989). If these patterns are correctly understood, they have profound conservational implications. We may expect that local endemism is a common occur- rence in cloud forest situations. If other insignificant ridges like Centinela similarly have up to a hundred endemic species, most of them undescribed, we are faced with a truly daunting conservational task if we want to save this potentially significant fraction of the earth's biodiversity.

Nor is this kind of local endemism restricted to the Neotropics. Similar phenomena appear to occur in the East African mountains where Lovett (1988 and pers. comm.) is discovering dozens of locally endemic new species, in marked contrast to the usual pattern of wide- ranging African species; for example 18 of the 20 known species of horticulturally important Saintpaulia are endemic to eastern Tanzania. New Guinea, with its exten- sive montane cloud forests, is the closest paleotropical equivalent of the Andes, and local endemism, especially among herbaceous and epiphytic taxa is apparently rife as well (Johns, pers.comm.). For example, 155 of New Guinea's 157 Rhododendron species are endemic, and these constitute well over half of all the Malesian species of the genus (Sleumer 1966).

Given our current levels of knowledge (or lack thereof), the only conservationally responsible solution would appear to be to attempt to save all northern Andean cloud forests and the equivalents if we are to avoid significantly eroding the world's biodiversity.

Dry montane valleys Another situation where local endemism is concentrated in the tropics is in isolated valleys in broken

OIKOS 63:1 (1992) 23



.file~~~~~~~,

Fig. 1. Distribution of Tecoma (Bignoniaceae) showing local endemism associated with inter-Andean Valleys in Peru and Bolivia. Not shown are the two African species and widespread T. stans (L) Juss. ex HBK. which ranges from the southern United States to Argentina. From Flora Neotropica Monograph 25(2) (Gentry 1991c).

mountainous terrain. This is especially pronounced in the inter-Andean Valleys, from southwest Ecuador to Bolivia (e.g., Berry 1982). A good example of this pattern is Tecoma, which has one species widespread from the southern United States and the Antilles to Argentina, two more rather widespread in southeastern Africa, and ten locally endemic species concentrated in the Central Andes (Gentry 1991c). Although the phenomenon of local endemism in dry valleys is strongest in the Central Andes, it is also prevalent in southern and central Mexico (Neill 1988, Rzedowski and Calderon 1987, 1989).

While there are similarities between the speciation patterns in dry dissected terrain and those in similarly dissected cloud forest, there are also differences. In- stead of six species of the same genus locally endemic on the same ridge as in Gasteranthus on Centinela, dry- valley endemics tend to be isolated from their congeners, with closely related species occurring allopatrically in adjacent valleys. Tecoma, for example, has a pair of species in each inter-Andean valley, one hummingbird- pollinated and one bee-pollinated; otherwise the locally

distributed species are strictly isolated from each other (Fig. 1).

Regional endemism Certain phytogeographic regions have much higher levels of endemism than do others. For example, based on a sample of 8117 recently monographed species, Gentry (1982) concluded that regional endemism in the nine neotropical phytogeographical regions he recognized varies from 73-77% in Amazonia, coastal Brazil, the cerrado/caatinga dry vegetations, and the Guayana Highlands to 54-60% in the southern Andes, northern Andes, Central America, and the West Indies, to only 24% in northern Venezuela/Colombia. Taking into account the different numbers of species in the different regions, the same sample of 8117 taxa indicates that the greatest numbers of endemic species (840-1390) occur respectively in Amazonia, Central America (including

24 OIKOS 63:1 (1992)



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Fig. 2. Endemism by phytogeographic region. (Data from Gentry 1982: table 7). Top number is estimated number of endemic plant species from that region (number of species in monographed taxa Z 10). Bottom number is percent endemism (% of monographed species occurring in that region that are endemic to it). Guiana lowland subregion included in Amazonia except that number of ";Amazonian" species restricted to Guiana subregion indicated parenthetically.

Mexico), coastal Brazil, and the northern Andes; the southern Andes and cerrado/caatinga had 676-683 endemic species, while the West Indies had 424, the Guayana Highlands 168, and northern Venezuela/ Colombia 153. Since this sample represents about 10% of the neotropical flora, multiplication of these numbers by ten should approximate the actual number of endemic species in each region (Fig. 2). Some smaller regions also have significant endemism. About 20% of the Choco region plant species are apparently regional endemics (Gentry 1986a). In a separate analysis we have estimated that about 20% of the ca. 6300 vascular plant species of western Ecuador below 900 m, or ca.

1500 species, are endemic to this relatively small region (Dodson and Gentry 1991).

Thus within Latin America areas like coastal Brazil, coastal Ecuador, and the Choco region, which have both high overall numbers of species and high levels of endemism, are especially significant from a conservational perspective. These are precisely the areas that have been singled out by Myers (1988) as evolutionary (and extinction) "hot spots" and on which much conservational focus is now being focussed. In addition to the neotropical areas, Myers (1988) used similar analyses to single out parts of Madagascar, the eastern Himalayas, Peninsular Malaysia, northern Borneo, the Philippines,

OIKOS 63:1 (1992) 25



and New Caledonia as similar conservational "hotspots".

Islands In a symposium honoring one of the co-founders of Island Biogeography (MacArthur and Wilson 1967), some comments on islands are especially appropriate. No distributional patterns have been more susceptible to the kinds of analyses and predictions upon which conservational decisions might be based than those of islands. It is noteworthy that while several of the proposed conservational "hotspot" regions are islands, none are oceanic islands. While oceanic islands like the Hawaiian archipelago may have extremely high per- centages of endemism, their overall flora is so depauperate, as predicted from island biogeographical theory (MacArthur and Wilson 1967), that the overall number of species at risk tend to be relatively few. On the other hand, continental fragment tropical islands like Mada- gascar, New Caledonia, the Philippines, and Borneo often have both high numbers of species and high rates of endemism. However, at least for plants, these patterns tend to be much more complicated than indicated by the size/distance parameters of island biogeographical theory, and local ecology plays a prominent role. For example, additional field work in the Galapagos subsequent to publication of the Flora (Wiggins and Porter 1971), produced a significantly different data set suggesting a much more important role for ecology as a determinant of diversity than had been originally sup- posed (Werff 1983). Moreover, an island's history and size may affect different organisms very differently. For example, New Caledonia, which has been separated since the late Cretaceous from the larger Gondwanan land masses, is of outstanding botanical interest, with 1400 endemic plant species and the highest rate of plant endemism in the world (89%) (Morat et al. 1984, Myers 1988), but is of minimal interest from the point of view of ornithologists or mammalogists.

The distributions of diversity and endemism on different parts of the same island may also be quite different, nor do they always vary in the same way on different islands. In New Guinea, Madagascar, and New Caledo- nia, the bulk of the diversity and endemism, and thus of conservational interest, lies in the moist forests. How- ever, in the Greater Antilles, the moist forest is generally depauperate, low in endemism, and conservationally relatively unimportant. In the Antilles, diversity is as great or greater in dry as in moist forests (quite unlike most other regions), and endemism is higher in dry than in moist forests (over 1/3 Antillean endemic species vs < 115 endemics in moist forests in my samples), though even higher in montane cloud forests and especially on unusual substrates like serpentine.

Ecological islands The "hot spot" continental areas also have island-like features. Each of them constitutes a kind of ecological island, geographically separated from other patches of similar vegetation. The ca. 75% endemism rates for areas like coastal Brazil and the cerrado/caatinga forma- tions are as high as on islands and the numbers of species involved are much greater. Thus from the conservational standpoint it is no more relevant to the conservational status of the species of coastal Brazil or western Ecuador whether large sections of Amazonia remain intact than would be the preservation of North American or European forests. Thus analyses based on application of the principles of island biogeography to amount of extant rain forest at a continental level (Sim- berloff 1986) are to some extent irrelevant. When treated as specific islands, according to the rule of thumb that a 90% loss of area equilibrates to a 50% loss in species richness, it seems likely that major extinctions should already have occurred in an area like coastal Brazil which today retains only 5% of its original forest, with perhaps 2% in primary form (Myers 1988), and even this highly fragmented. While there have undoubt- edly been some extinctions, to date, they are surely far fewer for plants than would be predicted by island biogeography. Indeed species not collected since the past century have a disconcerting tendency to turn up not only alive and well but even as common components of patches of forest right outside Rio de Janeiro. Examples from Bignoniaceae are Adenocalymna subsessilifolium and Tabebuia pedicellata. Once again we are faced with the problem of separating the effects of inadequate collecting from biological ones. Similarly in lowland western Ecuador a loss of about 92% of the forested area, mostly in the last two decades, has thus far re- sulted in only ca. 100 known or suspected extinctions of endemic plant species, most of these on a single ridge- top (Gentry 1986a, Dodson and Gentry 1991), although we suspect that many other never-described species may also have gone extinct on other now-deforested ridges. Perhaps many plants are capable of surviving rather long periods in even very small habitat fragments. Again the key point is that we need more ground truth data.

Conservational implications Patterns of diversity and endemism are largely non- coincident; thus conservation of a few major centers of megadiversity will give a most inadequate representation of a region's plant species. Moreover, different kinds of plants share very different patterns of endemism, and even a megadiversity-based conservational scheme would have to make very different conservational priorities, for example, for woody plants than for

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epiphytes. Since many tropical forest plants, and especially many of the locally endemic taxa, are herbs, shrubs, or epiphytes, the optimal conservational plan for plants might well focus on preserving many small areas rather than the fewer large areas that have been advocated by many zoologists and that might also be best for trees. In Amazonia conservation of isolated patches of unusual substrate is especially important for preserving local endemics; in the mountainous areas of Central America and western South America, preservation of topographically and ecologically dissected terrain including representation of as many isolated ridges and valleys as possible, should have the highest priority. By extrapolation parts of the east African mountains and New Guinea are likely to merit similar emphasis. Large continental islands and island-like regions of habitat have high concentrations of endemism and deserve special conservational attention, although the picture is more complicated than indicated by island biogeographical theory, with endemism concentrated in different vegetation types on different islands.

Perhaps the most obvious conservational conclusion is that a far more concerted collecting effort is needed in tropical forests if we are to truly hope to catalogue and conserve their plant species. Even though obvious, this is an expensive proposition and conservationists have been reluctant to allocate scarce resources to the needed documentation. In lieu of adequate inventories, we are reaching the point of being able to extrapolate from theoretical understanding of population parameters and distributions of the types elucidated by Ehrlich and Wilson. Whether we act on actual data or perceived patterns, it is imperative that tropical forest conservation not be delayed. We have already lost many species and are facing the loss of much of the earth's biodiversity in the very near future. Nor is preservation of this biodiversity a quixotic exercise in conservation ethics or esthetics. It is becoming increasingly clear that the very biodiversity that enchants biologists and daunts conservationists is also of tremendous direct economic importance (e.g., Peters et al. 1989, Vasquez and Gen- try 1990). Indeed it seems likely that the only alternative to massive social and economic disintegration and ever-increasing misery in much of the third world lies in finding ways to combine the preservation and sustained utilization of the biodiversity of tropical forests (Gentry 1991c).

Acknowledgements - I thank the Mellon Foundation and the National Geographic Society for supporting the field work on which this analysis is largely based, P. Berry and S. Renner for review comments, and G. Yatskievich, L. Woodruff and T. Wendt for providing relevent data.

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Nordic Society Oikos - PlantSystematics.org · 2013-09-27 · For example, it has ... conservational efforts in a dozen "megadiversity" coun- tries. While the "megadiversity" approach