nests, roosts and resting places. Such nutrient "hot spots" may be important for the regeneration of trees and other processes. Termite mounds are an important example (Nye 1955; Cox and Gakahu 1985; Oliveira-Filho 1992).
Water distribution and quality Many tropical rivers and streams flowing through inhabited areas are polluted. Pollution alters the species richness and trophic dynamics of rivers, as found in lead- and mercury-polluted segments of the Orinoco River caused by gold mining (Pfeiffer and De Lacerda 1988), but there are no theoretical or empirical reasons to believe that resistance to degradation of water quality is related to loss of species. Similarly, the quantities of water flowing, and the temporal pattern of flows, is strongly influenced by vegetative cover, particularly the loss of forests, but the influence of plant species richness per se on patterns of water discharge from tropica! forests is probably also small.
Atmospheric properties and feedbacks Tropical forests are currently a net source of atmospheric carbon dioxide, due to the reduction in the total acreage of forests and to subsequent burning. The sequestering of carbon by growing tropical forests is apparently poorly correlated with plant specics richness. Tropical plantations can and do accumulate carbon at rates similar to, or greater than, those of natural species-rich forests of the same age (Cuevas et at. 1991; Ewcl et at 1991; Lugo 1992).
Landscape and waterscape structure Deforestation is dramatically altering tropical forest landscapes and waterscapes. Such fragmentation is evidently leading to loss of species, but how loss of species may, in turn, influence the structure of tropical landscapes or waterscapes is unclear. No existing theories predict such relationships.
Animal movements The transfer of most energy across tropical landscapes is the result of movement of animals. Tropical regions are invaded each year by many thousands of migrant birds that breed at high latitudes but winter in the tropics. Migrants may outnumber residents during part of the year in some tropical habitats. These migrants may compete with themselves and with residents for food (Keast and Morton ¡980; Greenberg 1986), and they are potential agents of disease transmission, although little is known about the diseases of tropical birds or whether migrants are sources of infections in resident species.
Many species of birds, butterflies and moths migrate seasonally within the tropics, either elevationally (especially nectarjvorous and frugivorous species) or from dry to wet forests during dry seasons (Stiles 1988; Loiselle 1991). These migrants are also potential movers of pathogens and they carry large numbers of propagules across the landscape. The importance of migration corridors and suitable areas in which to live throughout the year are known to be important for the viability of populations of within-tropics migratory species, but the consequences of the potential loss of those species for the functioning of tropical forests are yet to be investigated.
In many ecosystems, bats and some species of birds assemble in large colonies during the breeding season. These colonies concentrate large quantities of nutrients in small areas, but colonies of birds in tropical forests are typically very small, and they probably have little effect on the concentration of nutrients. Notable exceptions are bat roosts in caves and the large colonies of oilbirds (Steatornis caripensis) that nest in caves along the Andean chain in South American. These birds carry large quantities of fruits, particularly of palms and lauraceous species, into their breeding caves, where the regurgitated seeds may accumulate to depths of several meters on the cave floors (Snow 1962), but these nutrients are, for the most part, unavailable to growing plants.
Animals may have a variety of effects on nutrient processing in tropical forests. Among these soil mixing and promotion of aggregate soil structure by earthworms and burrowing mammals, redistribution and concentration of canopy tissues in the soil, release of methane by wood-eating insects having methane-producing gut symbionts, and production of readily decomposed frass and feces. In Neotropical forests, leaf-cutting ants concentrate large quantities of nutrients in and around their large subterranean nests (Haines 1975). Most of theses processes are influenced by many species of animals, but because typically one or a few species of leaf-cutting ants dominate a particular forest, nutrient-concentrating processes may be highly sensitive to the loss of a single species.
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