Neotropical moss floras: Species common to North and South America

North and South America share about 675 species that show two basic patterns, namely, those with a continuous range and those with a disjunct distribution. Both may have resulted from step-bystep migration, but the latter, including 118 species, may be due to break up of previous distributions by post-Tertiary tectonic and climatic changes or by long-distance dispersal. The distribution of vascular plants includes many genera common to North and South America (Johnston 1940, Miranda 1959). Some of them have widespread ranges while others exhibit disjunct distributions between both land masses (Good 1964, Rzedowski 1978). While long-range dispersal has been proposed as an explanation (Raven 1963), it is often assumed that the shared taxa resulted from past plant migrations even though certain such patterns as the bicentric distributions are difficult to explain because the herbaceous and woody species seem to represent different periods of floristic interchange (Johnston 1940). Modern studies have proposed origins, times and routes of migration for vascular plants. It is said, for instance, that the high altitude South American flora is largely derived from north temperate sources (Gentry 1982) and that many shared families of flowering plants could be referred to Gondwanan or Laurasian origins (Raven & Axelrod 1974). Gentry (1982), in discussing the floristic richness of the neotropics, pointed out that with the closure of the Central American isthmus many South American taxa moved north to completely dominate the Central American lowlands and that most of this invasion is so recent that there has been little differentiation at the species level. In the upper elevations of Costa Rica the vascular flora shows distinct links with northern Central America and Mexico, and with the Andes; at the generic level, 95% of the flora is shared with the Andean region (Cleef & Chaverri 1992). This seems to reflect dispersal trends that occurred in the area. Hammen and Cleef (1983) proposed that the floristic interchange between North and South America increased little by little since the Miocene through “Mesoamerican” islands and the Antilles; by the