Organisms in tropical forests are being subjected to massive disruptions in the form of wholesale exchanges of species among regions, introduction of alien predators and pathogens, overharvesting, habitat destruction, pollution, and, in the future, climate changc. Changes in land use in the tropics are creating extensive areas of agricultural land, pasture and early successional patches at the expense of late successional and mature forest communities. Accompanying these changes are major reductions in the sizes of populations, and extinctions of species that depend upon the habitats that are being destroyed. Rates of forest destruction and subsequent species loss are higher in tropical regions today than elsewhere on Earth (Sader and Joyce 1988; Whitmore and Sayer 1992; Wilson 1992; FAO 1993). The ecosystem-level conscqucnces of these changes are not well known and, unfortunately, loss of species is irretrievable.
Relative constancy of temperature characterizes tropical regions, but total annual rainfall and the length and severity of dry seasons varies strikingly with topographic position and latitude. Seasonality of rainfall exerts a strong influence on temporal patterns in primary and secondary production (Janzen and Schoener 1968; Opler et al. 1976; Lieberman 1983; Leighton and Leighton 1983; Bullock and Solis-Magallanes 1990; Loiselle 1991), and on temporal variations in rates of decomposition (Birch 1958; Jordan 1985; Leigh et al. 1990). Species richness in most taxa of macroorganisms is positively correlated with annual rainfall (Gentry 1988) and inversely correlated with the length of the dry season, both variables being strongly correlated in tropical regions.
Moist lowland tropicai forests are characterized by both high richness of species in many taxa and complex biotic interactions and linkages. Most tropical plants are animal-pollinated (Bawa 1979, 1990; Bawa and Beach
Functional Rales of Biodiversity: A Global Perspective «x--.
Edited by H.A. Mooney, J.H. Cushman, E. Medina. O.E. Sa)a and E.-D. Schulze (ww l*»,/ © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd unf.p
1981; Baker et al. 1983; Bawa and Hadley 1990). They are fed upon by a wide variety of animals, ranging from highly specialized to generalized species (Dirzo 1987), and they also depend upon animals for dispersal of their seeds (Levey et al. 1993; Estrada and Fleming 1986). Many biologists have assumed that tropical animals are, on average, more specialized in their diets than their temperate counterparts (Janzen 1973, 1980; Gilbert and Smiley 1978; Beaver 1979), but there are insufficient data on the diets of most tropica] organisms to either support or reject this view (Marquis and Braker 1992). Similarly, ecologists cannot yet distinguish between the competing hypotheses that "tropical ecosystems are species-rich because they are stable," that "tropical ecosystems are stable because they are species-rich", or that "tropical ecosystems are species-rich because they are unstable" (MacArthur 1972; Brown 1981; Karr and Freemark 1983; Lugo 1988a).
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