Manual de Plantas de Costa Rica 193 
The more recent (late Tertiary and Quater- 
nary) geological periods in southern Central 
America have been marked by fluctuations in 
sea level, topography, temperature, and pre- 
cipitation, with cooler, drier intervals alternat- 
ing with relatively warm, wet intervals (e.g., 
the present one). The result has been a contin- 
uously changing mosaic of closed, wet forests, 
and more open, drier forests or savanna-like 
vegetation. Savannas, never abundant in Cen- 
tral America, appear to be a fairly recent de- 
velopment, provoked or at least augmented by 
human intervention. 
The foregoing summary of the history of 
vegetation in southern Central America was 
abstracted mainly from Burnham & Graham 
(1999), with some reference to Rich & Rich 
(1983). These works should be consulted for 
additional information and sources. 
Melocactus curvispinus 
Endemism and diversity 
Mesoamerica is considered one of nine zones in the world with exceptionally high en- 
demism, harboring 5000 endemic species, equivalent to 1.7% of the global estimate of 
300,000 species of vascular plants (Myers et al., 2000). Costa Rica contributes ap- 
proximately 1000 endemic species, an important 20% of this regional diversity. 
The knowledge of taxonomic diversity for any country depends (especially in the 
case of large, complex groups) on the existence of specialists, in the country and in the 
world, who work with specimens of their group from the country. According to Jorgen- 
sen & Leén- Yanez (1999), this factor explains the apparent differences between Ecua- 
dor and Peru in terms of total species known, e.g., in the families Orchidaceae (3013 
spp. in Ecuador and 1587 spp. in Peru) and Asteraceae (863 spp. and 1432 spp., re- 
spectively). Another way to make a similar point is to say that the known plant species 
diversity of any country depends on whether or not it has a recently published flora 
and/or an active flora project. Similarly, the degree of endemism for any continental 
country or region is influenced by the amount of effort put into botanical exploration 
and the continued taxonomic revision of its flora. This may be part of the reason that 
endemism of seed plant species is apparently so low in countries such as Nicaragua 
(1.3%; Stevens, 2001) and Belize (1.2%; Balick et al., 2000) compared to Costa Rica 
(11-12%, according to our unpublished data gleaned from Manual manuscripts and 
