5.8.1. State of Wetland Services
18.104.22.168. Habitat and Biodiversity
Global estimates for the area of wetlands vary according to the definition
of wetland used. Spiers (1999) reports global estimates for natural freshwater
wetlands of 5.7 million km2. The estimate by Matthews and Fung (1987)
is 5.3 million km2, which represents approximately 4% of the Earth's
land surface, although a more recent estimate by Lappalainen (1996) is somewhat
larger, at 6.4 million km2. This compares well with Finlayson and
Davidson's (1999) estimate of about 7 million km2, including
1.3 million km2 of rice paddy.
Maltby and Proctor (1996) estimate that peatlands cover about 4 million km2
(±4%), constituting about 75% of wetlands. More than 90% of peatlands
are in temperate, boreal, and subarctic regions. The total area of tropical
peatlands is estimated to be 0.37-0.46 million km2 (i.e., approximately
10 % of the global resource), but the full extent is uncertain (Immirzi et al.,
Many wetlands have irregular wetting and drying cycles, driven by climate.
To date, little attention has been given to the impact of climate change on
these less regular cycles of wetlands in semi-arid and arid regions (Sahagian
and Melack, 1998). Changes in the area of these wetlands can be immense but
could be monitored by using area-based parametersfor example, functional
parameters and wetland extent expressed in terms of ha-days (Sahagian and Melack,
Species that form wetland plant communities are adapted to varying degrees
to life in a flooded environment. These phenomena show large spatial variability,
and different species show varying degrees of susceptibility to them, so it
is not surprising that wetland vegetation exhibits such a high degree of variation
in species composition (Crawford, 1983). Peatland plant communities have been
observed to change over long periods of time, reflecting the peat accumulation
process and leading to gradually drier conditions. This inherent changeability
of wetland communities results largely from their occurrence in environments
where a single extremely variable habitat factorwater supplyis predominant
(Tallis, 1983). Consequently, land use and climate change impacts on these ecosystems
can be expected to be mediated through changes in the hydrological regime.
Primary production in wetland communities is highly variable (Bradbury and
Grace, 1983; Lugo et al., 1988). Generally, wetland communitieswhich are
dominated by trees, sedges, and grasseshave higher production rates than
those characterized by shrubs and mosses. Organic matter produced in many wetlands
is accumulated partially (2-16%Päivänen and Vasander, 1994)
as peat. A necessary antecedent condition for peat formation and accumulation
is an excess of water stored on the mineral soil or sediment surface. This arises
in humid climatic regions where precipitation exceeds evaporation (Ivanova,
1981; Clymo, 1984) or in more arid climatic regions where lateral inputs of
water via surface runoff and/or groundwater seepage are sufficient to exceed
evaporative demand (Glaser et al., 1996).
Tropical peatlands play an important role in maintenance of biological diversity
by providing a habitat for many tree, mammal, bird, fish, and reptile species
(Prentice and Parish, 1992; Rieley and Ahmad-Shah, 1996; Page et al., 1997);
some of these species may be endemic or endangered. In common with other peatlands,
tropical systems have a significant scientific value that goes beyond their
plant and animal communities.
Within the peat also lies a repository of paleoenvironmental and paleogeochemical
information that is extremely important in understanding past climatic conditions.
These paleorecords are used to estimate rates of peat formation or degradation,
former vegetation, climatic conditions, and depositional environments (Morley,
1981; Cecil et al., 1993; Moore and Shearer, 1997).