Distribution of Anthurium
Anthurium ranges from Mexico (Tamaulipas on the Caribbean slope and Nayarit on the Pacific slope) to northern Argentina and Paraguay. Species diversity is greatest in the low and middle elevations of northern South America, Panama, and Costa Rica, with less important centers in the mountains of southern Mexico and southeastern Brazil. While the upper Amazon forests of Bolivia and especially Peru, Ecuador, and Colombia are rich in species, the lower Amazon is relatively poor in species.
Distribution of Central American Anthurium
Because of the large size of the genus and its taxonomic difficulty, revisionary work on Anthurium must progress in stages. There are approximately 219 species of Anthurium in Mexico and Central America (including Panama). This paper, the first of a series, will deal with 122 Mexican and Middle American taxa, but will exclude a revision of the Panamanian species, which number 152 species alone. The latter will be presented in Part II of this paper. Owing to the great morphological variability in most parts of Anthurium, it has been very difficult to prepare a key to large numbers of species. In addition, the Panamanian species arc not yet sufficiently well known to complete a key to the species occurring in Panama. This is due to the fact that Panama has almost as many species as the rest of Central America and Mexico put together.
Most of these species have been discovered only in recent years and many are still poorly known. The Costa Rican Anthurium flora is more closely related to that of Panama than to that of the remainder of Central America, but it is relatively well known (Croat & Baker, 1979), and thus will be included here.
Although Nicaragua is beginning to receive serious botanical attention as the result of the Flora of Nicaragua Project conducted by W. D. Stevens (Missouri Botanical Garden), its aroid flora is proving to be relatively poor. Most Anthurium species collected in Nicaragua represent species of Costa Rica and Panama that reach their limit in Nicaragua. These are found principally in the very wet southeastern corner in the Department of Zelaya. Examples of these are A. acutangulum Engl., A. hacumense Engl., A. ochranthum C. Koch, A. ramonense Engl. ex K. Krause, A. spathiphyllum N. E. Brown, and A. upalaense Croat & Baker. Other species are basically Mexican species that just reach Nicaragua from the north. Examples of these are A. huixtlense Matuda and A. schlechtendalii Kunth.
Additional Nicaraguan Anthurium species include wide ranging species that range from Mexico or Middle America to South America, such as A. cubense Engl. (also in West Indies), A. clavigerum Poepp., A. friedrichsthalii Schott, A. gracile (Rudge) Lindl., A. interruptum Sodiro, A. microspadix Schott, A. obtusilobum Schott, A. ravenii Croat & Baker, A. scandens (Aubl.) Engl., and A. trinerve Miq. Still others are species that range throughout Central America, such as A. bakeri Hook. f. and A. pentaphyllum (Aubl.) G. Don var. bombacifolium (Schott) Madison. These are also expected to be found in Colombia. The only endemic species of Anthurium known from Nicaragua is A. beltianum Standl. & L. 0. Wms.
To date, only 25 species are known for Nicaragua and, although it is certain that more species will be added to the flora with the ongoing collecting by Stevens and his associates, there is little doubt that species diversity of Anthurium is greater south of the San Juan depression, which separates Costa Rica and Nicaragua. Costa Rica, although appreciably smaller, has approximately 65 species of Anthurium (Croat & Baker, 1979).
Panama is even richer with 152 species. Nicaraguan forests that I have investigated were poor in species. For example, the cloud forest on Volcán Mombacho east of Lake Nicaragua had only a few species of aroids. Even the much wetter cloud forest regions between Jinotega and Matagalpa produced relatively few species. In a comparable cloud forest of Costa Rica or Panama, many species can be encountered in a small area.
The remainder of Middle America is, like Nicaragua, relatively poor in species. Honduras has only 13 known species with an additional four species, namely A.friedrichsthalii Schott, A. microspadix Schott, A. schlechtendalii Kunth, and A. trinerve Miq., expected there. Like Nicaragua, Honduras is expected to have only a single endemic species, namely A. lancetillense Croat.
Belize has no more than eight to ten species (none of them endemic) but Guatemala, with about 25 species, is richer.
Although the greater number of species in Guatemala can perhaps be attributed in part to the fact that it has been much better explored than the remainder of Middle America, the chief reason for its greater number of known species is that it shares with Mexico a number of typically Mexican species. These include A. chiapasense Standl., A. huixtlense Matuda, A. montanum Hemsl., A. seleri Engl., and A. titanium Standl. & Steyerm. A number of other species from Chiapas are to be expected in Guatemala. These include A. berriozabalense Matuda, A. chamulense Matuda, A. cordatotriangulum Matuda, A. nakamurae Matuda, A. ovandense Matuda, and A. pedatoradiatum Schott.
Still, Guatemala is much richer in species in its own right than the remainder of Middle America, with three endemic species. These are A. armeniense Croat, A. parvispathum Hemsl., and A. retiferum Standl. & Steyerm. However, all of these might possibly also be discovered in Chiapas with further collecting there.
Mexico, with 41 known taxa and an additional four or five species to be expected, has a relatively rich Anthurium flora with 26 endemic taxa. Some of the endemic taxa are relatively widespread within Mexico, such as A. andicola Liebm., A. chiapasense Standl., A. schlechtendalii ssp. jimenezii (Matuda) Croat, A. nizandense Matuda, A. ovandense Matuda, A. pedatoradiatum Schott, and A. podophyllum (Cham. & Schlecht.) Kunth. The majority, however, arc relatively narrow endemics, mostly known only from northern Oaxaca (seven taxa, A. cerropelonense Matuda, A. chamulense ssp. oaxacanum Croat, A. longipetalum Matuda, A. machetioides Matuda, A. nelsonii Croat, A. subovatum Matuda, A. umbrosum Liebm., A. yetlense Matuda); northern Chiapas (four taxa, A. berriozabalense Matuda, A. clarinervium Matuda, A. leuconeurum Lem., A. lezamae Matuda); or southeastern Chiapas (two taxa, A. cordatotriangulum Matuda, A. nakamurae Matuda).
Many of the Mexican species appear closely related, and their current isolation is perhaps due to changes in the environment brought about by increasingly arid conditions and the formation of broad expanses of intervening desert or extremely arid areas. Thus, for example, A. halmoorei Croat, A. schlechtendalii ssp. jimenezii (Matuda) Croat, and A. nizandense Matuda all appear to have been derived from A. schlechtendalii Kunth, a common species from the Caribbean slope (or from a common ancestor). Speciation here would appear to have resulted from the onset of arid conditions with the resulting isolation of these species in specific parts of western Mexico. Similar isolation of species has occurred in Chiapas, where relatively close species are now endemic to northern Chiapas or to southeastern and southwestern Chiapas.
That Mexico and Middle America were isolated from Costa Rica and Panama during the evolution of many Mexican Anthurium species seems obvious from the fact that only six Mexican species, including A. flexile Schott, A. microspadix Schott, A. pentaphyllum var. bombacifolium (Schott) Madison, A. salviniae Hemsl., A. scandens (Aubl.) Engl., and A. trinerve Miq., reach Costa Rica. The same six species extend also to Panama and the latter two range throughout much of South America as well. The situation in Guatemala is little better with only ten species reaching Costa Rica. These include (in addition to the Mexican species already mentioned) A. bakeri Hook. f., A. cubense Engl., A. gracile (Rudge) Lindl., And A. interruptum Sodiro. The three latter species are also present in South America.
Further evidence of this isolation can be seen in the distinctive nature of many Mexican species and the predominance of D-shaped or broadly sulcate petioles that are uncommon elsewhere in Anthurium. This feature is exhibited in several leaf forms, including species with oblong leaf blades and ovate-cordate blades, as well as in pedatisect species, such as A. podophyllum (Cham. & Schlecht.) Kunth and A. pedatoradiatum Schott. The Mexican species in this group will be referred to as the Anthurium andicola alliance. They may ultimately prove to be a section in their own right, but final judgment on this will be deferred until further work can be done with the South American species of the genus.