GEOGRAPHICAL DISTRIBUTION AND ENDEMISM
The P. subg. Philodendron flora in Central America is both rich and diverse but heavily concentrated in the southeastern part near South America. Despite this, relatively few species actually enter South America. Most species which do enter the South American continent range along the Pacific slope into northwestern Colombia and Ecuador with relatively few occurring east of the Andes and even fewer entering the Amazon basin.
As was shown true with Anthurium (Croat, 1983, 1986, 1986a) species diversity of P. subg. Philodendron shows a general diminution from Mexico to Middle America, followed by a marked increase approaching South America (Appendix 4. Sectional Classification of Philodendron). Distribution of Philodendron in Central America). Mexico has 21 taxa, Guatemala 15 taxa, Belize 9, El Salvador 5, Honduras 13, Nicaragua 18, Costa Rica 49, and Panama 80 taxa respectively. Thus species diversity is greatest in Costa Rica and Panama, especially the latter, with 49% and 80% of the total Central American species respectively. Collectively the number of species in these two countries comprise 90% of total species.
Fifty-six species (61 taxa) of P. subg. Philodendron (nearly 59% of the total) are endemic to Costa Rica and Panama. Endemism is particularly high in Panama where 40 taxa (36 species) of the 80 taxa (50%) are endemic. In Costa Rica 7 of 49 taxa (48 species) (18%) are endemic. Mexico has a higher rate of endemism with 7 of 21 (20 species) (33%) endemic. In Middle America little endemism occurs. With the exception of Belize, which has one endemic, P. dwyeri, no other country in Middle America has any endemic species. Mexican endemics are P. basii, P. breedlovei, P. dressleri, P. radiatum var. pseudoradiatum, P. hederaceum ssp. oxycardium, P. sousae, P. subincisum. Costa Rican endemics are P. aromaticum, P. auriculatum, P. chirripoense, P. cotobrusense, P. dominicalense, P. microstictum, and P. wilburii var. wilburii.
The overall range of Central American Philodendron reflects the endemic nature of the genus. Only 26 species (27 taxa) (a total of 26% of all Central American species of P. subg. Philodendron) range into South America, seven species (7% of the total) only to Colombia but to no other country (Table 3). These are: P. heleniae, P. ligulatum var. ligulatum, P. malesevichiae, P. mexicanum, P. pseudoauriculatum, P. radiatum var. radiatum and P. squamicaule.
Fourteen taxa occur in Ecuador, all but one of them, P. jacquinii, ranging along the Pacific slope of the Andes. These are: P. brunneicaule, P. dodsonii, P. grandipes, P. hebetatum, P. platypetiolatum, P. purpureoviride, P. hederaceum ssp. kirkbridei, P. hederaceum ssp. hederaceum, P. squamipetiolatum, P. scalarinerve, P. tenue, P. tripartitum, and P. verrucosum. Of these 12 taxa, four of them skip Colombia or have not yet been collected there. These include: P. brunneicaule, P. dodsonii, P. platypetiolatum, P. purpureoviride, and P. hederaceum ssp. kirkbridei. Philodendron dodsonii, which also is absent from Panama, is particularly unusual.
Seven species, P. brevispathum, P. fragrantissimum, P. hederaceum, P. jacquinii, P. sagittifolium, P. strictum, and P. tenue, range to Venezuela. The range of P. brevispathum, P. fragrantissimum, and P. hederaceum also extend into the Amazon drainage while the others occur either on the northern slope of the coastal cordillera or otherwise in the drainage of the Orinoco River Basin. Curiously only four species, P. brevispathum, P. fragrantissimum, P. hederaceum, and P. verrucosum, occur east of the Andes and also in the Amazon drainage. Three additional species, P. jacquinii, P. strictum, and P. tenue occur east of the Andes but exist only along the Cordillera de Merida, the Cordillera de la Costa or in the northern part of the Guiana Highlands and within the drainage of the Río Orinoco not in the Amazon basin.
The only truly widespread Central American species is P. hederaceum which occurs virtually throughout the neotropics, and is one of only two species (the other being P. verrucosum) that occurs on both slopes of the Andes. Philodendron fragrantissimum is probably the next most widespread species, ranging from Belize to the West Indies and into South America to the Guianas, northern Brazil and to southern Peru.
Further collecting in Colombia, especially along the western slope of the Andes will probably change these statistics but the figures most likely reflect the realities of life zone ecology and geologic history of the area rather than under-collecting. Since relatively few species of Araceae are known to occur at lower elevations on both the eastern and western side of the Andes, it can probably be presumed that the evolution of the respective Amazonian and Pacific coastal floras occurred independently after the Andes began to arise toward the end of the Cretaceous (Raven & Axelrod, 1974). The relatively few truly wide-ranging species, i.e., those ranging from Mexico to Brazil appear to attest to this isolation. In Central America only one species, P. hederaceum really falls into such a category and it is also common in the West Indies indicating that it may have an ancient origin. The high rates of endemism in Costa Rica and Panama as well as Mexico perhaps reflects the isolation of these areas during periods when the oceans were at much higher levels than they are today and when the area that is now central Panama and Costa Rica was subsequently disconnected from South America. Much of the present area of Central America was submerged during early times. At the close of the Tertiary, 800,000 years ago, the sea level was about 100 m higher than today (Holmes, 1969). The land mass of what is now Central America began to emerge as a series of islands during the Oligocene with further uplifting during the Middle Miocene. It was not until the Upper Miocene and Pliocene that the final portions of the isthmus of Panama emerged above sea level (Torre, 1965) and the final connection of Central andíouth America was about 5.7 million years ago. In order to put these geological events in relation to the modern aroid flora it should be noted that even during this era, precursors to the existing flora probably already existed since the angiosperm floras of the Oligocene were believed to have consisted almost entirely of existing genera and the floras of the Oligocene and Pliocene probably already had existing species (Takhtajan, 1969).
Just as important as geology from the standpoint of the isolation of the Central American aroid flora, are ecological reasons which would cause Central American species to be isolated from those of South America. Much of eastern Panama consists of broad expanses of Tropical moist forest(T-mf) with other, generally smaller areas of Premontane wet (P-wf) and Tropical wet forest (T-mf). In contrast to Panama, much of the area of northwestern Colombia in the Department of Chocó, consists of much wetter pluvial forest with annual precipitation often exceeding 11,700 mm in some parts of the region (Gentry, 1982). This broad band of pluvial forest with its own suite of unique endemic species no doubt acts as a barrier for species from regions with lesser rainfall amounts. It probably also accounts for the Panamanian and Costa Rican species which skip the wettest areas of northwest South America but reoccur in the relatively drier areas of mesic western Ecuador.
Just as the Central American P. subg. Philodendron flora is rather isolated from South America, there is a certain amount of isolation within different parts of Central America. In comparison to Mexico and Costa Rica-Panama, Middle America (Table 6) has low species diversity with Guatemala having only 15 species, Honduras 13 species and Nicaragua 18. Most of the species in Honduras, excepting P. anisotomum, P. mexicanum, and P. warszewiczii, are shared with Nicaragua. Those species shared between the Honduras and Nicaragua are: P. angustilobum, P. fragrantissimum, P. hederaceum ssp. hederaceum, P. jacquinii, P. jodavisianum, P. radiatum var. radiatum, P. rothschuhianum, P. sagittifolium, P. smithii, P. tenue, P. tripartitum, and P. wendlandii. Nicaragua has seven additional species not shared with Honduras: P. brevispathum, P. grandipes, P. immixtum, P. ligulatum var. ligulatum, P. platypetiolatum, P. pterotum, P. tenue, and P. wendlandii. All of the latter are shared with Costa Rica and Panama. Guatemala shares only about half of its species with Honduras and Nicaragua, namely P. fragrantissimum, P. jacquinii, P. jodavisianum, P. radiatum, P. sagittifolium, P. smithii, and P. tripartitum. Alternatively its other species are shared only with Mexico (or rarely with Costa Rica and Panama, e.g., P. mexicanum. These are: P. advena, P. anisotomum, P. glanduliferum, P. mexicanum, P. purulhaense, P. verapazense, and P. warszewiczii.
The low species diversity and the very low endemism in Middle America is perhaps explained by its being more remote from existing large land masses to the north and to the south, leaving it isolated from the independent evolution that must have been taking place in both of these larger areas (see below for a discussion of the possible origins of the respective species in the Central American Philodendron flora). There is strong evidence, at least based on the distribution of modern aroid species that the northwestern part of Middle America may have been isolated from Costa Rica in the area of the San Juan Depression. Many of the species that occur in Costa Rica or Panama enter into Nicaragua in only a small area in the southeastern part of the country. Though the existing flora of Guatemala does not reflect isolation from Mexico to the same degree, it is possible that the more elevated portions of Guatemala, Nicaragua and Honduras were isolated from major portions of Mexico at the Isthmus of Tehuantepec.
Certainly the Mexican aroid flora appears to be quite isolated, even when compared to the western parts of Middle America (here defined as Guatemala to Nicaragua). In addition to having one-third of its species endemic (7 species), relatively few Mexican species of Philodendron range throughout Central America. Aside from the aforementioned P. hederaceum, only P. jacquinii, P. jodavisianum, P. radiatum, P. sagittifolium, and P. tripartitum range to northern South America. Two additional species, P. anisotomum and P. mexicanum, range as far as Panama. Several species, P. advena, P. glanduliferum var. glanduliferum, and P. verapazense range only to Guatemala. Philodendron purulhaense and P. warszewiczii range to Honduras and P. smithii ranges to Nicaragua.
Taken together Honduras and Nicaragua have 21 species of Philodendron. Of these, 8 species, P. advena, P. anisotomum, P. mexicanum, P. radiatum, P. sagittifolium, P. smithii, P. tripartitum, and P. warszewiczii appear to be of Mexican origin or in the case of the more widespread and variable species, namely P. radiatum, P. sagittifolium, and P. tripartitum, they may have originated in Panama or Costa Rica and ranged to both Mexico and South America. Certainly in terms of morphological variation, all of these species are much more variable in Panama and Costa Rica than they are further to the north. Philodendron jacquinii has a circum-Caribbean distribution, indicating that it may be of West Indian origin. It is difficult to determine the origin of P. hederaceum given its pantropical distribution. Three species, P. brevispathum, P. fragrantissimum, and P. tenue probably originated in South America considering their widespread distribution there. Philodendron platypetiolatum, ranging from Ecuador and barely entering Nicaragua, may be another derivative from South America as well. The remainder, P. angustilobum, P. grandipes, P. immixtum, P. jodavisianum (ranging to barely to Chiapas and rare there), P. ligulatum var. ligulatum, P. pterotum, P. rothschuhianum, and P. wendlandii are probably of Panamanian or perhaps of Costa Rican origin.
The remaining Costa Rican and Panamanian species not already discussed above appear not to have strong affinities for South
America and clearly did not originate in areas of Middle America. Much of the flora of adjacent Nicaragua is closely related to Costa Rica but most of the species shared with Costa Rica enter only the wettest southeast portion of the Nicaragua. Except for those rather widespread species mentioned above, i.e., P. brevispathum, P. fragrantissimum, and P. tenue, as well as P. platypetiolatum (already discussed), there are relatively few species likely to be of South American origin. Philodendron verrucosum is almost certainly a South American species since it is relatively widespread there, occurring on both sides of the Andes. In addition it seems to have more related species in parts of South America especially in the Andes of western Colombia.
A few species, such as P. brunneicaule, P. dodsonii, and P. purpureoviride, which occur in Ecuador but not Colombia, are just as likely to have originated in South America as in Central America. The same is true of P. strictum which is known from eastern Venezuela and has relatives in the Andes of central Colombia as well as P. hebetatum, which is rather widespread in western South America as far south as Ecuador. Moreover, the latter apparently does not even reach Costa Rica, making the case for a South American distribution even more likely.
Philodendron grandipes, known from western South America as far south as Ecuador, might conceivably be of South American origin despite being very widespread and common as far north as Nicaragua. More probably of South American origin is P. heleniae, which is common in Colombia and ranges only to western Panama. Moreover, it seems to have a close relative on the eastern slopes of the Andes.
Three species with scaly parts, P. malesevichiae, P. squamicaule, and P. squamipetiolatum are all moderately rare in Panama and though also still poorly known in Colombia, all are more likely to have originated in northwestern Colombia where there are several other relatives with scaly parts.
Some species, such as P. ligulatum var. ligulatum, P. pseudoauriculatum, and P. scalarinerve either barely enter Colombia or are rare there and are more likely therefore to have originated in Central America.
All the species not yet discussed occur in Panama and 11 of them occur also in Costa Rica but nowhere else. These are P. alticola, P. bakeri, P. brenesii, P. cotonense, P. crassispathum, P. cretosum, P. davidsonii var. davidsonii, P. findens, P. grayumii, P. lentii, P. schottianum, P. sulcicaule, and P. thalassicum are shared between Cost11a Rica and Panama and occur nowhere else.
The Central American species not yet mentioned are all endemic to Panama. These are: Philodendron albisuccus, P. annulatum, P. antonioanum, P. brewsteriense, P. chiriquense, P. clewelii, P. coloradense, P. correae, P. davidsonii var. bocatoranum, P. dolichophyllum, P. edenudatum, P. ferrugineum, P. folsomii, P. fortunense, P. gigas, P. granulare, P. hammelii, P. jefense, P. lazorii, P. lentii, P. ligulatum var. ovatum, P. ligulatum var. heraclioanum, P. llanoense, P. madronoense, P. morii, P. niqueanum, P. panamense, P. pirrense, P. pseudoauriculatum, P. roseospathum var. roseospathum, P. roseospathum var. angustilaminatum, P. straminicaule, P. tysonii, P. ubigantupense, P. utleyanum, and P. zhuanum.