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Geographical distribution
Ipomoea is a tropical genus and this is reflected in its distribution in the Americas with few species found north or south of 30 degrees latitude. The main exception lies in the Eastern United States where several species, I. coccinea, I. lacunosa, I. sagittata and I. pandurata, extend north to at least 35 degrees, I. repanda as far as 43°N in Ontario, Canada. The complete absence of Ipomoea from California in the west apart from a few introduced ornamentals (as well as in central Chile) suggests that it cannot tolerate a Mediterranean climate with arid summers and cool wet winters.
Within the neotropics Ipomoea is widely distributed but is noticeably less diverse in the equatorial region with relatively few species in Amazonia, Ecuador (Austin 1982a) or Colombia (Bernal et al. 2015). Although a partial explanation lies in the low diversity of Ipomoea species generally in rain forest, it does not account for the lack of species diversity in the dry forests of the Caribbean coasts or of the inter-Andean valleys such as the Colombian Magdalena. Species diversity rises as one moves away from the Equator and the countries with the greatest diversity of species lie mostly within the 15 to 30 degrees of latitude, notably Mexico and Brazil, both large countries with extensive subtropical dry forest. Some smaller countries in these latitudes, such as Paraguay (Wood et al. 2017c), Bolivia (Wood et al. 2015) or Cuba (Wood and Scotland 2017c) are proportionately as rich.
Most species of Ipomoea are relatively localized in their distribution often being found in a single region or country. However, there is a large set of species (I. alba, I. batatas, I. cairica, I. carnea subsp. fistulosa, I. corymbosa, I. hederifolia, I. indica, I. muricata, I. nil, I. purpurea, I. quamoclit and I. tricolor) that occur around cultivation or in disturbed places near settlements throughout the tropics and are found in almost every country of the Americas with a tropical climate. To this group should be added some other pantropical species that are also widespread but absent from many countries including I. acanthocarpa, I. aquatica, I. asarifolia, I. fimbriosepala, I. mauritiana, I. setifera and I. triloba. All these pantropical species occur sporadically, occasionally abundantly, in different neotropical countries but there is little geographical patterning to their distribution. A similar pattern can be observed in the palaeotropics. Of the 26 species recorded for the Flora of the Mascarenes (Bosser and Heine 2000 all but one also occur in the Americas. Equally all but two species recorded from Hawaii are also present on the American continent.
Of species never found in the Old World, Ipomoea aristolochiifolia is probably the most widespread, being found from Argentina north to Mexico, although it is absent from the Caribbean islands. Other very widespread species include I. philomega, I. batatoides, I. ramosissima and I. regnellii but, apart from I. ramosissima, none extends into Argentina and all peter out as they enter Mexico. Two species, I. dumetorum and I. clavata extend along the Andean Chain from Argentina or Bolivia north to Mexico but are absent elsewhere. More frequent are species that extend from the United States or Mexico southwards to northern South America. These include I. capillacea, I. cholulensis and I. lindenii that are restricted to the mountain chains and I. minutiflora, I. meyeri, I. trifida and I. tiliacea which are common in the Caribbean (except I. minutiflora) and Central America extending into northern South America, in the case of I. tiliacea south along the eastern edge of Brazil almost to Uruguay. Of some interest are two upland species, I. plummerae and I. pubescens, common around the 20–30° latitude in both hemispheres but largely absent from intermediate equatorial regions.
Ipomoea plummerae and I. pubescens are not the only species with disjunct distributions. Ipomoea crinicalyx and I. amnicola are also amphitropical in distribution but there is suspicion that the latter has been introduced into the northern hemisphere. Several annual species like I. parasitica, I. heptaphylla, I. longeramosa and I. neurocephala are very scattered in their distribution, being known from many countries but, with the exception of I. longeramosa in NE Brazil, from only one or few collections in each case. The occurrence of the South American I. subrevoluta on the Isla de Juventud (Pinos) in Cuba and also on Trinidad is remarkable but it perhaps arrived as a result of the movement of migratory water birds. Ipomoea thurberi also has a curious distribution with isolated populations in Guatemala and Nicaragua which are disjunct from each other as well as from the main population in northern Mexico and Arizona. In South America remarkable disjunctions are noted for species found on isolated granite domes around the Amazon. Ipomoea chiquitensis and I. graniticola are known from a few locations separated by many thousands of kilometres (Wood et al. 2017c). The apparent disjunctions in the distribution of two species found in the Amazon basin, I. amazonica and I. velutinifolia can be explained by inadequate collecting in areas separating known locations. The most inexplicable disjunction, however, is that of I. eremnobrocha known from the Cerro Campana in Panama and from a number of locations in NE Brazil.
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