Convention on biological diversity
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Tropical Forest BiomesOverview of the functioning of the ecosystemOne of the most distinctive characteristics of tropical forest is their biological richness, particularly in the number of species per unit of area (UNEP, 1995). In addition, for still unknown reasons, natural forests are relatively resistant to invasion by alien species. Invaders tend to be restricted to disturbed areas (Rejmanek, 1998; Whitmore, 1991). The complexity and diversity of tropical forests, especially humid forests, can be matched only by the underwater diversity associated with some coral reefs (Longman and Jenik, 1974). However, understanding the functioning of tropical forests is not a simple task because of the large spatial scale of the forests, the long time scale of their evolution and the high diversity of organisms in the forest fauna and flora. Climate and soils are the main factors controlling the distribution and the composition of tropical forests. A third factor is the interactions of their biotic components and with human activities (Longman and Jenik, 1974). Effect of species and genetic diversity on forest ecosystem processesProductive capacity, decomposition and nutrient cyclingIn tropical moist forests, the number of species within a functional group greatly exceeds the number of key ecological processes, and even highly fragmented and disturbed forests have more species than the minimum to yield full primary productivity (UNEP, 1995). Thus, in tropical forests, biomass production under relatively constant conditions is insensitive to species richness (Orians et al., 1996). However, some authors suggest that species richness may influence the rate at which biomass accumulates after a disturbance (Denslow, 1995). There is no inter-annual accumulation of litter in tropical soil, as the decomposition processes are too rapid (Barrow, 1991), thus soil degradation occurs very rapidly following the removal of the forest cover. Soil degradation is further enhanced in humid regions through runoff caused by heavy rains. The case of mangrovesThe intertidal forested wetlands, known as mangroves, support a vast amount of biodiversity in the tropical estuarine ecosystems. Mangroves also play an important role in maintaining water quality and shoreline stability by controlling sediment distribution in estuarine waters. Although there are relatively few species of mangrove trees (see Chapter I, paragraph 86), mangrove ecosystems are unique because they include structural niches and refugia for numerous animal species (UNEP, 1995). For example, crabs, mainly represented by two families - Grapsidae (63 mangrove species) and Ocypodidea (over 80 species) - are very abundant in mangroves systems and are considered to be keystone species. They positively influence tree productivity and reproduction, presumably by aerating the soil through their burrowing activities (Smith et al., 1991). Role of mobile species in forest biodiversityMobile species, such as pollinators and seed dispersal agents, have an important role in tropical forests. Many plants depend on a small suite of frugivores, such as bats ands birds, for dispersing their seeds, and loss of those species can have major influences on the long term population dynamics of many tree species (Orians et al., 1996). In turn, loss of tree species may affect pollinators and dispersal agent diversity. Functional properties over long temporal scalesThe functioning of a tropical ecosystem depends on the formation and maintenance of the structure in the forest, which is the result of photosynthesis and biogeochemical cycling over a long time frame (many decades or even centuries) (Orians et al., 1996). Certain life forms, such as the one represented by palms, lianas and epiphytic bromeliads, can be considered as “structural keytone species” because their removal would influence the rate of recovery of the forest after perturbations (Denslow, 1996). Undisturbed tropical forests are resistant to invasion by exotic species. The mechanism of this resistance is unknown, however (Rejmanek, 1998). Impact of human activities and their consequences on the delivering of good and servicesBiodiversity and loss of habitatsHuman impact on tropical forest biodiversity is mainly through land use change by converting forest areas to pastures, croplands, and plantations (UNEP, 1995) or by agricultural use and subsequent degradation and erosion. Arid and savannah forests are also threatened by desertification, which is exacerbated by overgrazing. The extent to which human activities are leading to the extinction of species in tropical forest is poorly documented, but current habitat and species loss in moist tropical forests are believed to be elevated and more important in tropical regions than anywhere in the world (Whitmore and Sayer, 1992). Such losses are expected to influence functional properties of tropical ecosystem (Orians et al., 1996). Forest fragmentation, deforestation, reduction of habitat size and edge effects all have important effects on biotic linkages and on ecosystem functioning. Many tropical plants are animal-pollinated (Bawa and Hadley, 1990) and depend on animals for the dispersal of their seeds (Estrada and Flemming, 1986). As a result of forest fragmentation, the loss of pollinators or other functional guilds, such as seed-dispersing birds and bats, may affect the plant reproductive biology, forest structure and dynamics; and it may also affect the long term population of many tree species (UNEP, 1995; Howe and Smallwood, 1982; Terborgh, 1986), all having an impact on potential goods and services to humans. Forest management in the tropics often consists of selective harvesting of commercial trees. Such trees can often occur at low densities. As in the case of selective harvesting in temperate forests, building of logging roads in tropical forests has an important negative impact on overall ecosystem functioning of the forests as the roads disrupt streams, lead to soil erosion, provide access to humans and open gaps in the canopy. Furthermore, logging roads in primary tropical forests often lead to poaching, uncontrolled settlements and illegal deforestation. However, in terms of providing goods and services, studies have demonstrated that secondary forests can be managed to provide many of the products that small-farmer households traditionally obtained from primary forests, while providing some of the environmental benefits of primary forests (Chapin, 1998 [or Chapin et al., as in the list of references?). Mangrove ecosystems are also under threat due to human activities such as land use change. Vast areas of mangroves are converted to other uses. For example, Thailand and Indonesia have lost 50% of their original mangrove ecosystems, Philippines 80% and Malaysia 32% (UNEP, 1995). Indirect causes of biodiversity loss include human alteration of upland watershed causing changes in fresh waters pathways or pollutions (UNEP, 1995). Climate change is also expected to have a direct dramatic effect on mangroves through the sea level rise. Carbon poolTropical forests still represent the second largest pool of terrestrial carbon after the boreal forests (UNEP, 2000). However, from 1980 to 1998, net emissions of carbon dioxide into the atmosphere have occurred, mainly from land use change in the tropics (UNEP, 2000). It is interesting to observe that this net tropical emission is partly balanced by a net carbon uptake by the vegetation in middle and high latitudes, as a result of land use practices and natural regrowth (UNEP, 2000). While carbon fluxes might be weighed together in this way, the loss of biodiversity from tropical forest cannot, of course, be offset by any gains in other biomes. Water distribution and qualityWatershed deforestation, inappropriate agriculture practices, and soil erosion have dramatic effects on water quality of streams and rivers. In addition, silt particles, which are carried to coastal zone, may cause death to coral reefs (UNEP, 1995). |