Rzedowski, Jerzy
September 1991
Acta Botanica Mexicana;sep1991, Issue 15, p47
Academic Journal
This contribution is essentially a complement to the data and concepts exposed in a previous paper, devoted to the topic of diversity and origins of the Mexican phanerogamic flora. A part of the mentioned paper deals with the subject of endemism which, according to evidence and estimations, involves the family level and accounts for ±10% of the genera and ±52% of known species. If an ecologically more natural area (wich expands the territory of Mexico by about 1/3)is taken as reference, these figures rise respectively to ±17% and ±72%. If these figures are compared with existing information for other countries and regions of the world, it can be concluded that the extent of endemism in the flora of Mexico is not as large as that of Australia, Madagascar or South Africa, but surpasses that of Cuba and of the California floristic province and is much larger than those of many other parts of the world. The origin of this significant wealth of endemic organisms must be sought on the one hand in the existence of a fair number of regions that behave as true ecological islands and peninsulas within the territory of Mexico, some of them extending over large portions of the country, and on the other hand, in the events and environmental conditions of the geological past. Particularly, it must be emphasized that during much of the Cenozoic era no terrestrial connection existed with South America and accordingly Mexico bore the form of a peninsula which, much like South Africa, penetrated in form of a wedge toward climatic conditions in sharp contrast with those prevailing on the wider part of the continent. Different types and aspects of endemism in Mexican flora are discussed and it is pointed out that an important proportion of very local and/or rare species can be recognized. The majority of endemics, however, do not belong to this group and in fact many of the most common and characteristic plants of the Mexican landscape belong to taxa of restricted distribution, including a large number of weeds and some cultivars. Gypsophytes stand out among edaphic endemics and this group seems to bear a long evolutionary history. A large additional assemblage of paleoendemics can be distinguished, in part concentrated in areas which acted as refugia during the epochs of changing climates of the Tertiary and the Quaternary. A rough estimation indicates that endemism in the Mexican flora is most accentuated among shrubs' and perennial terrestrial herbs, whereas lianas and aquatic plants show the lowest incidence. Among the larger families, Cactaceae, Rubiaceae and Compositae stand out with about 70% of endemic species, while Orchidaceae and Gramineae only reveal 35% and 25% respectively. A remarkable correlation can be observed between the proportion of endemic genera arid the degree of climatic aridity. At the species level, however, temperate and semi-humid areas are equally privileged in endemics. On the other hand in warm and humid regions endemism is poorly represented. Numbers of localities and regions are indicated, in which a significant concentration of floristic endemism has been detected and in general terms it can be observed that endemic genera are much better represented in the northern half of the country, whereas endemic species are more numerous on the Pacific slopes than on the Atlantic. It is pointed out, however, that in most parts of the country, practically at any point, the flora of terrestrial and not excessively disturbed communities includes a high percentage of endemics.



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