Orchids of the West Indies: predictability of diversity and endemism

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1 Journal of Biogeography (J. Biogeogr.) (2007) 34, ORIGINAL ARTICLE Orchids of the West Indies: predictability of diversity and endemism J. D. Ackerman 1 *, J. C. Trejo-Torres 2 and Y. Crespo-Chuy 1 1 Department of Biology, University of Puerto Rico, PO Box 23360, San Juan, PR , USA and 2 Cuidadanos del Karso, 497 Emiliano Pol (Box 230), Urbanización La Cumbre, San Juan, PR , USA ABSTRACT Aim We examined phytogeographical patterns of West Indian orchids, and related island area and maximum elevation with orchid species richness and endemism. We expected strong species area relationships, but that these would differ between low and montane island groups. In so far as maximum island elevation is a surrogate for habitat diversity, we anticipated a strong relationship with maximum elevation and both species richness and endemism for montane islands. Location The West Indies. Methods Our data included 49 islands and 728 species. Islands were classified as either montane ( 300 m elevation) or low (< 300 m). Linear and multivariate regression analyses were run to detect relationships between either area or maximum island elevation and species richness or the number of island endemic species. Results For all 49 islands, the species area relationship was strong, producing a z-value of 0.47 (slope of the regression line) and explaining 46% of the variation. For 18 relatively homogeneous, low islands we found a non-significant slope of z ¼ )0.01 that explained only 0.1% of the variation. The 31 montane islands had a highly significant species area relationship, with z ¼ 0.49 and accounting for 65% of the variation. Species numbers were also strongly related to maximum island elevation. For all islands < 750 km 2, we found a small-island effect, which reduced the species area relationship to a non-significant z ¼ 0.16, with only 5% of the variation explained by the model. Species area relationships for montane islands of at least 750 km 2 were strong and significant, but maximum elevation was the best predictor of species richness and accounted for 79% of the variation. The frequency of single-island endemics was high (42%) but nearly all occurred on just nine montane islands (300 species). The taxonomic distribution of endemics was also skewed, suggesting that seed dispersability, while remarkable in some taxa, is very limited in others. Montane island endemics showed strong species area and species elevation relationships. Main conclusions Area and elevation are good predictors of orchid species diversity and endemism in the West Indies, but these associations are driven by the extraordinarily strong relationships of large, montane islands. The species richness of low islands showed no significant relationship with either variable. A smallisland effect exists, but the montane islands had a significant relationship between species diversity and maximum elevation. Thus, patterns of Caribbean orchid diversity are dependent on an interplay between area and topographic diversity. *Correspondence: J. D. Ackerman, Department of Biology, University of Puerto Rico, PO Box 23360, San Juan, PR , USA. jdackerman@uprrp.edu Keywords Biodiversity, Caribbean, habitat diversity, island biogeography, Orchidaceae, small-island effect, species area relationships, species elevation relationships, species richness. ª 2007 The Authors Journal compilation ª 2007 Blackwell Publishing Ltd doi: /j x

2 J. D. Ackerman, J. C. Trejo-Torres and Y. Crespo-Chuy INTRODUCTION The West Indies is one of the largest and most complex archipelagos in the world. There has been much debate over tectonic movements, continental connections, and the rise and fall of sea levels over geological time (Iturralde-Vinent & MacPhee, 1999). These issues, of course, have a direct bearing on the interpretation of distribution patterns of the rich flora and fauna. Although many taxa are shared with neighbouring continental areas, considerable autochthonous speciation has occurred (Hedges, 1996), often obscuring biogeographical relationships, particularly in the absence of phylogenies (Santiago-Valentín & Olmstead, 2004). This is certainly true for West Indian Orchidaceae, which is probably the most diverse plant family in the region. Biogeographical analysis of the orchid family in the West Indies and representative surrounding continental areas showed that the floristic affinities of shared species are not dependent on geographical proximity (Trejo-Torres & Ackerman, 2000). Instead, geomorphological and physiographical characteristics appeared to be more important determinants of orchid species composition on islands. The orchid floras of low, generally calcareous islands were found to be species-poor and weakly associated, whereas those of the montane islands were more species-rich and floristically similar to one another. The lack of distance relationships for the region s orchid floras may be explained by the long-distance dispersal capabilities of orchids, given that nearly all have dust-like and wind-dispersed seeds (Arditti & Ghani, 2000). With ecological conditions perhaps playing a major role in the floristic affinities and distributions of orchid taxa, we explored geographic relationships further by asking whether or not island size and topography were associated with species diversity and endemism (Rosenzweig, 1995; Ricklefs & Lovette, 1999). We proposed that, for montane islands, there should be strong relationships between species numbers, island area and maximum island elevation. We also expected that area and elevation relationships should be strongest for island endemics. While the elevation has a direct relationship with habitat diversity and can be viewed as a surrogate for it, island area can, but does not always, affect species richness independently of habitat diversity (Simberloff, 1976; Boström & Nilsson, 1983 in Rosenzweig, 1995; Triantis et al., 2003). Thus, if area per se is correlated with orchid species diversity, then species area relationships for orchid floras of low, relatively flat islands should be significant as well as for montane islands. METHODS We enumerated all orchid species for 54 islands or island groups of the West Indies (Tables 1 & 2) based on published articles and floras. Many separate islands of the Bahamian archipelago were excluded because either orchid distribution records were not specific enough to compile species lists for individual islands (e.g. Correll & Correll, 1982) or area and elevation were not readily available for them. We relied heavily on a published Table 1 Montane islands of the West Indies with elevation equal to or greater than 300 m and the number of orchid species on each. Island Area (km 2 ) Highest Elevation (m) Total no. of spp. No. of endemics (%) Greater Antilles Cuba 105, (27) Gonave (0) Hispaniola 76, (36) Isla de la Juventud (11) Jamaica 10, (30) Puerto Rico (11) St. Croix (0) St. John (0) St. Thomas (0) Tortola (0) Tortue (0) Vieques (0) Virgin Gorda (0) Lesser Antilles Antigua (0) Barbados (0) Dominica (0) Grenada (0) Guadeloupe (5) Martinique (0) Montserrat (6) Nevis (0) Saba (0) St. Eustatius (0) St. Kitts (0) St. Lucia (0) St. Maartin (0) St. Vincent (0) Continental Islands Curaçao (0) Margarita (4) Tobago (0) Trinidad (3) Total 300 checklist (Trejo-Torres & Ackerman, 2000) and an orchid flora of the Antilles (Nir, 2000). Although we made changes because of minor differences in taxonomy and the publication of new species and island records since 2000, the orchid flora of the region is now well-enough known that the rank-order of species richness among the islands is not likely to change much. Island areas and maximum elevations were obtained principally from Microsoft Encarta Encyclopedia 99 (Microsoft Corporation ), Wikipedia ( Atlas de Cuba (Instituto Cubano de Geodesia y Cartografía, 1978), Atlas of the World (2nd edition, Oxford University Press, 1993), and a miscellany of government websites discovered through web searches. Islands were placed into one of two groups according to their physiographic complexity and suggested floristic affinities 780 Journal of Biogeography 34,

3 Where are the orchids? Table 2 Low islands (< 300 m elevation) of the West Indies and the number of orchid species on each. Island Area (km 2 ) Highest elevation (m) Total no. of spp. Bahamian Archipelago Cat (0) Great Exuma (0) Great Inagua (0) Long Island (0) New Providence (0) Greater Antilles Anegada (0) Cayman Brac (0) Culebra (0) Grand Cayman (13) Little Cayman (0) Mona (14) Lesser Antilles Anguilla (0) Barbuda (0) La Désirade (0) Marie-Galante (0) St. Barthelemy (0) Continental Islands Aruba (0) Bonaire (0) Total 3 No. of endemics (%) based on the parsimony analysis of orchid distributions reported by Trejo-Torres & Ackerman (2000), namely montane, or largely or entirely calcareous islands of low elevation. We noted that most of the islands in the latter category had an elevation of less than 300 m, a height that usually results in a rain-shadow effect and vegetation noticeably more mesic at higher elevations (e.g. compare Culebra 195 m elevation with Vieques 301 m elevation; Daly et al., 2003). Ricklefs & Lovette (1999) calculated a habitat diversity index for the islands of the Lesser Antilles. All islands lower than 300 m (our elevation data) had the lowest possible index (or nearly so), and none had an index that exceeded the indices of islands higher than 300 m. Species numbers, island areas and maximum island elevations were log-transformed. We ran linear regressions on jmp version (SAS Institute, ), and for the species area analyses we report the taxon-specific constant z, which is equivalent to the slope of the regression line. The anova indicates whether or not the regression slopes are significantly different from the mean with a zero slope. Because scale may influence the species area relationship, particularly in the form of the small-island effect (Gentile & Argano, 2005; Whittaker, 2000; Lomolino & Weiser, 2001; Triantis et al., 2006), we first regressed log species on log area using all islands together and visually inspected the resulting regression. We then repeatedly re-ran the analysis by progressively eliminating the next largest island until the relationship became unambiguously non-significant (P > 0.1). The island size eliminated that resulted in this change is our approximate threshold for any small-island effect. Data were divided into two categories for island topography (montane and low), and two for island size (above and below the threshold for the small-island effect). We asked whether or not there were significant species area and species elevation relationships for each island category for which we had a sufficient sample size. To reveal the strength of a combined model using both log (area) and log (elevation) as predictors, and to show to what extent the two predictor variables were correlated and contributed to the overall model we ran a multiple regression analysis. Finally, we performed species-area and species elevation regressions for endemic species of montane islands. RESULTS West Indies species richness The West Indies has c. 729 orchid species distributed among 134 genera. The larger islands, namely Cuba, Hispaniola, Jamaica, Puerto Rico and Trinidad, house the most species, with Hispaniola being the richest, accommodating 352 species (Table 1). The low islands generally have low species counts (Table 2). Thirteen genera, about 10% of the total, account for 63% of the species in the West Indies (Table 3). Lepanthes is the most diverse genus, with 119 species, followed by Pleurothallis (85) and Epidendrum (63), with all three genera also very species-rich in the mainland tropics (Pridgeon et al., 2005). Species area relationships of the West Indies The 49 islands in our data matrix showed a strong and significant species area relationship (Fig. 1). The regression produced a z-value of 0.47 (anova: F 1,47 ¼ 42.61, P < ), Table 3 Single-island endemism in species-rich genera of Orchidaceae in the West Indies. Only genera with at least 10 species are included. Numbers in parentheses indicate the percentage of endemics of the total number of West Indian species in the genus. Asterisks indicate genera endemic to the West Indies (including southern Florida). Genus Total no. of species in the West Indies Dendrophylax* 14 9 (64) Encyclia (54) Epidendrum (29) Habenaria 14 4 (29) Lepanthes (98) Lepanthopsis (86) Malaxis 12 5 (42) Maxillaria 20 1 (5) Pleurothallis (58) Psychilis* (88) Stelis 16 9 (56) Tolumnia* (78) Vanilla 12 1 (8) No. of single-island endemics (%) Journal of Biogeography 34,

4 J. D. Ackerman, J. C. Trejo-Torres and Y. Crespo-Chuy (a) (c) 1.6 All islands z = 0.47 All montane islands z = Log area (km 2 ) Log area (km 2 ) All low islands z = Log area (km 2 ) (b) (d) Montane islands < 750 km2 z = Log area (km 2 ) Figure 1 Species area relationships of orchids of the West Indies. (a) Linear regression using log(area) and log(number of species) for all islands: log(species) ¼ ) log(area), r 2 ¼ 0.48, F 1, 49 ¼ 42.61, P < The dashed line is the approximate threshold for the small-island effect (log(750 km 2 ) ¼ 2.9). (b) All montane islands ( 300 m elevation): linear regression, log(species) ¼ log(area), r 2 ¼ 0.65, P < (c) All low islands (< 300 m elevation): linear regression, log(species) ¼ log(area), r 2 ¼ 0.001, P ¼ (d) Montane islands < 750 km 2 : linear regression, log(species) ¼ log(area), r 2 ¼ 0.1, P ¼ and explained c. 48% of the variation. Visual inspection of the graph suggested that much scatter occurred at the small island sizes. By progressively eliminating the current largest island from subsequent regressions we found that the island size threshold to produce an unambiguously non-significant regression was at c. 750 km 2 (below this size: z ¼ 0.20, r 2 ¼ 0.05, F 1,36 ¼ 1.74, P ¼ 0.28; including the island at 750 km 2 : z ¼ 0.22, r 2 ¼ 0.08, F 1,37 ¼ 3.12, P ¼ 0.09). Species area relationships of montane islands Species area relationships were examined using 31 montane islands of the West Indies (Table 1). We found a highly significant relationship (anova: F 1,29 ¼ 54.99, P < ), producing a z-value of 0.49, with 65% of the variation explained by the model (Fig. 1). When we constrained island size to < 750 km 2, the species area relationship abruptly weakened (z ¼ 0.25, anova: F 1,19 ¼ 1, P ¼ 0.17; Fig. 1). In contrast, montane islands of at least 750 km 2 showed a strong species area relationship (z ¼ 0.42, anova: F 1,8 ¼ 13.27, P ¼ 0.007, r 2 ¼ 0.62; Fig. 2). Species elevation relationships of montane islands Maximum elevations of montane islands were good predictors of species richness. We examined species elevation relationships for all these islands and found significant slope (b ¼ 1.55, F 1,29 ¼ 50.91, P < ), with 64% of the variation explained by the model. For islands < 750 km 2, the strength of the relationship was much reduced (r 2 ¼ 0.29) but it remained significant (b ¼ 0.93, F 1,19 ¼ 7.62, P ¼ 0.01). For montane islands of at least 750 km 2, the species elevation relationship was extraordinarily strong (r 2 ¼ 0.79) and significant (b ¼ 1.19, F 1,8 ¼ 30.11, P ¼ ; Fig. 2). Species area relationships of low islands We had data for 18 low islands (Table 2) and found that species area relationships were virtually non-existent. The slope (z ¼ 0.01) was non-significant (F 1,16 ¼ 0.01, P ¼ 0.9), with only 0.09% of the variation explained by the model (Fig. 1). When we divided the data set of low islands by size, the species area relationship for those islands < 750 km 2 was (a) Log area (km 2 ) (b) Log maximum elevation (m) Figure 2 Species area and elevation relationships for montane islands greater than or equal to 750 km 2 in area. (a) Species area: linear regression, log(species) ¼ log (area), r 2 ¼ 0.62, P ¼ (b) Species maximum elevation: linear regression, log(species) ¼ ) log(elevation), r 2 ¼ 0.79, P < Journal of Biogeography 34,

5 Where are the orchids? virtually the same, because only one low island exceeded the threshold size (z ¼ 0.03, F 1,15 ¼ 0.04, P ¼ 0.84). Species elevation relationships of low islands Species richness on low islands was not related to maximum island elevation (b ¼ )0.05, r 2 ¼ 0.006, F 1,16 ¼ 0.09, P ¼ 0.77). Small, low islands show virtually the same relationship because only one low island exceeded the threshold size. Combined species area and species elevation relationships The multiple regression of all islands produced a result whereby the model explained 67% of the variation and the slopes of all parameters (except the intercept) were significantly different from zero (anova: F 2,46 ¼ 47.54, P < ). The two independent variables, log (area) and log (maximum elevation), were significantly correlated, although the coefficient was not high (r ¼ 0.35; P ¼ 0.009). Area contributed 48.8% to the model whereas maximum elevation accounted for 51.2%. Small Islands Our small islands were defined by a threshold for a nonsignificant species area relationship. To determine whether or not maximum elevation behaved in a similar fashion to island area, we first examined whether or not maximum elevation and island area were correlated. We found that, among all small islands, size and maximum elevation were not correlated (r ¼ 0.08; n ¼ 38; P ¼ 0.64). We then asked whether or not significant species elevation relationships existed for small islands. We found that this was a significant relationship (b ¼ 0.35, r 2 ¼ 0.25, F 1,36 ¼ 15, P ¼ 0.001). Endemics and their species area and species elevation relationships There are nine genera endemic to the West Indies, namely Antillanorchis (1 species), Atopoglossum (3), Broughtonia (6), Dilomilis (5), Fuertesiella (1), Neocogniauxia (2), Psychilis (15), Quisqueya (4), and Tomzanonia (1). Four other genera would be endemics as well, but a species in each has been found vacationing in southern Florida: Basiphyllaea (4 species), Tolumnia (26), Tetramicra (12), and Dendrophylax (10). If these are included among the endemic species, then c. 10% of the orchid genera in the West Indies are endemic to the region. The frequency of single-island endemic species is 42% and these species are non-randomly distributed among genera (Table 3). Of the five genera that are represented by more than 25 species, Lepanthes has the highest incidence of endemism (98% of 119 species), whereas Epidendrum has the lowest (29% of 63 species). The 13 largest genera (> 10 species in the West Indies) account for 91% of the 304 endemic orchid species in the West Indies. Only the montane group had a sufficient number of islands (9) with endemic species to warrant species area or species elevation analyses (Tables 1 & 2). Endemism was highest in the major islands of the Greater Antilles. Hispaniola has the most endemic species (126) and the highest percentage of endemism (36%). Both the species area and the species elevation relationships were strong (species area: z ¼ 0.80, r 2 adj ¼ 0.80, F 1,7 ¼ 33.84, P ¼ ; species elevation: b ¼ 2.16, r 2 adj ¼ 0.65, F 1,7 ¼ 16.16, P ¼ 0.005; Fig. 3). DISCUSSION How diverse are the orchids of the West Indies? The number of orchid species in the West Indies is not particularly high relative to adjacent tropical mainland areas. Panama has c. 950 species, distributed in an area about onethird of the land area of the West Indies, and Costa Rica has about 1200 species in an area even smaller than that of Panama (Dressler, 1993). Although the number of orchid species reported for the West Indies has been steadily increasing over the last few decades (Ackerman, 1996; Luer, 1998; Llamacho & Larramendi, 2005), new discoveries are unlikely to affect the well-established observation that islands contain fewer species than mainland areas of similar climates. What are the species area relationships? We had assumed that habitat diversity is dependent on island area. The larger the island, the more habitats one may expect. However, it is not so simple to distinguish the effects of area and habitat. In some systems, the number of changes in response to differences in area is independent of habitat Figure 3 Endemic species and area and elevation relationships for montane islands. (a) Endemic species area: linear regression, log(endemic species) ¼ ) log(area), r 2 adj ¼ 0.80, P ¼ (b) Endemic species maximum elevation: linear regression, log(endemic species) ¼ ) log(maximum elevation), r 2 ¼ 0.83, P ¼ (a) Montane island endemics z = Log area (km 2 ) (b) Montane island endemics b = Log maximum elevation (m) Journal of Biogeography 34,

6 J. D. Ackerman, J. C. Trejo-Torres and Y. Crespo-Chuy diversity, yet in others there is no such relationship (reviewed in Rosenzweig, 1995; Triantis et al., 2003). For plants, strong correlations of species number with habitat diversity are known to occur, and our data concur (e.g. Harner & Harper, 1976; Abbott, 1977; van der Werff, 1983). While vegetation is not uniform on low islands, these islands are generally calcareous, hot and dry. Species numbers did not vary significantly with the size of the low islands, whether one looked at all low islands or only those < 750 km 2. In fact, the regression models could only explain % of the variation. Obviously, there is a large unexplained component to species richness on low islands, a phenomenon so commonly encountered that it has been labelled the small-island effect (Lomolino & Weiser, 2001). Numerous factors may account for this effect, including low habitat diversity, isolation, stochastic events, natural and human-induced disturbance regimes, small target area for immigrants, and recent geological age (Triantis et al., 2006), to which we add, for these low islands, a small pool of species adapted to hot, dry conditions. The latter may be particularly relevant for orchids (e.g. Ackerman, 1995; Partomihardjo, 2003; Jacquemyn et al., 2005; Llamacho Olmo & Larramendi, 2005). In contrast, montane islands are much more heterogeneous, not only in their topography, but also in their age, geology, temperature ranges, and amount of precipitation, including the spatial and temporal distributions of rainfall. As expected, the species area relationship was strong for this group, but it was driven by the rich diversity of the larger islands. When we restricted the analyses to islands < 750 km 2, the small-island effect was also revealed for the montane group. However, these mountainous islands did show a very strong species elevation effect, suggesting that habitat diversity still reigns as an important factor, regardless of island size. Area has the potential to be a good surrogate for habitat diversity, but Trejo-Torres & Ackerman s (2000) parsimony analyses of orchid distributions and our present study suggest that for orchids this is very much dependent on the degree of island topography. Ricklefs & Lovette (1999) showed a strong correlation between maximum island elevation and vegetation diversity for the Lesser Antilles, and we expect the same for the Greater Antilles. We certainly anticipated that the greater the habitat complexity, the more orchids there would be, so it came as no surprise that maximum island elevation was an excellent correlate of orchid species richness, even for small montane islands. Thus, if maximum island elevation does indeed reflect habitat diversity accurately, then it may be the single most important overall factor across all island sizes in explaining orchid species richness in the West Indies. Endemism The low-elevation islands have been above water for a much shorter time than most of the islands with substantial topography (Iturralde-Vinent & MacPhee, 1999), and may have appeared only since the Pleistocene (Hedges, 1996). Although orchid speciation may occur rapidly (Gentry & Dodson, 1987; Ackerman & Ward, 1999; Tremblay et al., 2005), the first colonists are likely to have been the orchid equivalents to Diamond s (1974) supertramp species that readily disperse and arrive in habitats at high rates. Such high dispersability counters local selection or genetic drift, so speciation is not likely to occur if such taxa were the original colonists. As it turns out, the frequency of endemism in these low-lying islands is very low (Table 2) and insufficient for species area analyses. It remains to be seen whether low endemism is a consequence of supertramp traits, youthfulness of the islands, lack of habitat diversity, or simply of the fact that most orchids occur in tropical cool wet habitats while the low islands are hot and dry. The interplay between habitat heterogeneity and area is further revealed by the species area relationships of just the single-island endemics. We expected that islands of greatest habitat heterogeneity would yield the most endemics and total species. Our species area analysis of the nine montane islands with endemics produced a z-value greater than did the analysis of all species (z ¼ 0.80 vs. 0.49, respectively). Such a contrast is consistent with other studies (Wilson, 1988). If we assume that most endemics are products of autochthonous speciation events rather than remnants of a once widespread species, it seems that whatever is good for all orchid species is even better for speciation. We await phylogenetic analysis of West Indian taxa to provide insight into this problem. Taxonomic distribution of endemic species Taxa with low dispersal capabilities generally have higher rates of endemism (Croat & Busey, 1975; Holloway, 1977). The physical properties of most orchids are similar in that they have dust-like, wind-dispersed seeds. In our region, only Vanilla has zoochorous seed dispersal (we assume), and only one of 12 species is an island endemic. However, the distribution of endemism is clearly skewed among orchid genera of the West Indies (Table 3). Nearly all Lepanthes, the largest orchid genus in the region, are single-island endemics. In contrast, Epidendrum, the third most diverse genus in the Caribbean, had only 22% endemism. Evolutionary potential varies substantially among orchid taxa (Ackerman & Ward, 1999), which implies that the physical capacity to be carried by wind is not the only factor involved with diversification. For example, some Lepanthes species are restricted to a single, small watershed, which contrasts with Oeceoclades maculata (Lindl.) Lindl., a dispersal champion that has spread rapidly through the Neotropics, especially over the last four decades (Stern, 1988). Why is there such variation in the ability to disperse? Our two favourite hypotheses are the following. Dispersion is limited by seed production: Lepanthes is xenogamous, seed production is very low, few fruits are set, and the capsules are relatively small (Tremblay, 1998), whereas Oeceoclades maculata is autogamous, fruit set is relatively high, and the capsules are much larger (González-Díaz & Ackerman, 1988). The second hypothesis is that orchids with broad ranges and good colonizing capabilities are either 784 Journal of Biogeography 34,

7 Where are the orchids? catholic in choices of mycorrhizal symbionts or their fungal associates are widely available (Otero et al., 2004). Unfortunately, we have very little data to support any hypothesis. CONCLUSIONS The orchid flora of the West Indies has been and is still in a state of flux. The geography of the region has been dynamic and powered by volcanism, tectonic movements and fluctuating sea levels. In addition to the dramatic effects of these geological changes, we can surmise that the human impact has also been substantial in these islands, since much of the pre-columbian vegetation is gone, especially at low to middle elevations. A relatively high proportion of taxa appear to be treading water and maintaining populations by autogamy (Ackerman, 1985; Ortiz-Barney & Ackerman, 1999). Nevertheless, for some taxa, conditions for evolutionary diversification seem ripe. Not only do we have a number of species-rich genera (e.g. Lepanthes), with many endemics of very restricted distributions, but there are also some data to indicate that conditions for both natural selection and genetic drift exist for some of these taxa (Tremblay, 1997). We found the species area relationships for orchids of the West Indies to be dependent on topographic diversity and to a lesser extent on a threshold of island size. No significant species area relationships were detected for low islands, and the strong relationships for montane islands break down for islands less than 750 km 2 in area. For the small, low islands, there is a strong stochastic component to floristic composition, much like Linhart (1980) found on a smaller scale for the floras of several Caribbean atolls. On the other hand, a maximum elevation of islands of at least 300 m above sea level was a good predictor of orchid species richness even for small islands. Patterns of endemism also suggest that habitat diversity is important, and that seed dispersability, while remarkable in some taxa, is not without limit in others. ACKNOWLEDGEMENTS This study was supported by American Orchid Society and National Science Foundation grants to J.D.A. (DEB , HRD ). The authors thank Elvia Meléndez-Ackerman for statistical advice, and two anonymous referees for their insightful comments on an earlier version of the manuscript. REFERENCES Abbott, I. (1977) Species richness, turnover and equilibrium in insular floras near Perth, Western Australia. Australian Journal of Botany, 25, Ackerman, J.D. (1985) Pollination mechanisms of temperate and tropical orchids. Proceedings of the eleventh world orchid conference (ed. by K. Tan), pp Eleventh World Orchid Conference, Inc., Miami, FL. Ackerman, J.D. (1995) An orchid flora of Puerto Rico and the Virgin Islands. Memoirs of the New York Botanical Garden, 73, Ackerman, J.D. 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