Macroclinium, Barbosa-Rodrigues, 1882

Fernández-Concha, German Carnevali & Cetzal-Ix, William, 2012, A new species of Macroclinium (Orchidaceae: Oncidiinae) from Andean Venezuela with brief comments on the biogeography of the genus, Phytotaxa 40, pp. 12-20 : 16-17

Biogeography of Macroclinium

Pupulin (1997) indicated that of the 38 species of Macroclinium known at that time, 65% were endemic to a single country, and only five species ranged over three or more countries. Table 2 presents an update of species numbers and distributions.

Geographically, the Andean countries of northwestern South America are the centre of diversity for the genus with 28 species, but Costa Rica in Central America is the individual country with more recorded species (11), followed by Peru (10 species), and Colombia and Ecuador (nine species each). After Pupulin’s (1997) revision, three additional species have been described ( Pupulin 2001, Campacci & Silva 2009). Thus, the genus includes now 42 species, which have been registered from 16 countries, but in seven of these there are currently records of fewer than two species.

Table 2 and Appendix 1 display the diversity of the genus in several major Neotropical biogeographical areas and countries. For the purposes of this comparison, Megaméxico is defined as in Rzedowski (1991), whereas Costa Rica and Panama are considered as the remaining portion of Central America south of Megamexico; it features a distinctive, highly endemic flora, which is otherwise floristically closer to South America ( Morrone 2006). The Chocó region also has a highly diverse and endemic-rich flora that shares elements with both the Andean region and Costa Rica-Panama ( Gentry 1986). Here it is defined as a narrow band of perhumid vegetation associations along the western rim of the Andes from southwestern Panama southward into northwestern Ecuador, essentially at low to intermediate elevations ( Morrone 2006). The Amazon region here is defined as the Amazon River drainage at elevations below 500 m (rarely to 900 m). The Andean Region is understood here as the slopes and peaks of this mountain range at elevations above 500 m. The Guayana region refers to the granitic and quartz outcrops and derived soils in southwestern Colombia, southern Venezuela, and the countries of Guyana, Suriname and French Guiana (the provinces of Guyana, Humid Guyana and Roraima of Morrone, 2006). The Caatinga region is defined as the dry areas of northeastern Brazil, as circumscribed in Morrone (2006).

Table 3 presents the diversity of the genus by country as well as the species that are endemic to each. The genus Macroclinium is most diverse along a relatively narrow strip of tropical cloud forests ranging from central Costa Rica into Panama southward into the northwestern Andes of Colombia, Ecuador, and Peru, where most of the species occur. There is a secondary centre of diversity in the western Amazonian region with 12 species. However, with the exception of 3-4 species, these are all restricted to the low-lying Amazonian slopes of the Andes, where there is a continuum of narrowly distributed species, discontinuously arranged along an elevational gradient. Thus, there are apparently no sharp limits between the Macroclinium biotas of the two biogeographical areas. There are only a few species of Macroclinium as we go eastward, away from the Andes, and these are widely distributed, e.g. M. mirabile and M. wullschlaegelianum , ranging into the Guayana region. Absence of the genus is noteworthy in central and southestern Brazil, and there is low diversity in the Guayana region and Megamexico. The genus is totally absent from the West Indies.

Biogeographically, the genus presents the typical pattern described by Gentry & Dodson (1987) of high epiphytic diversity in the northwestern Andean region. This is most likely explained by the “evolutionary explosion” hypothesis of Gentry (1982), in which swarms of closely related species occur closely packed together in small areas. This is probably due to the existence of many favorable microniches (due to high rainfall and even tropical temperatures) associated with the complex orography of the area and intermediate levels of disturbance (due mostly to landslides and volcanic activity). All this favors a scenario of repeated events of colonizations and local extinctions with genetic bottlenecks and “shifting balance” founder events ( Templeton, 1980), prompting active speciation. Macroclinum species tend to feature narrow distributions and typically occur as isolated populations surrounded by large areas where the genus is apparently absent, a populational feature that allows for the biogeographical pattern described above. However, since the plants are small and inconspicuous, they are frequently overlooked and undercollected, possibly obscuring underlying biogeographical and ecological patterns.