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Figure 11-2. Vertical aerial view of salt marsh of the Plum Island estuary, Massachusetts, United States. This area
was settled in the 1640s and ditched for improved hay production, which continued until the 1960s. The distinctive
wavy pattern, known as cow lick, is typical of Spartina patens (compare with previous i gure). Blimp aerial
photograph by S.W. Aber, J.S. Aber, and V. Valentine (visible in upper left corner).
is maintained through the dynamic and complex
cycling of nutrients and energy transfers between
biotic and abiotic components within the system.
Reliably quantifying global estimates of
wetland habitats is difi cult due to the lack of
systematically collected data from many parts
of the world (Scott and Jones 1995; Finlayson
et al. 1999; Zedler and Kercher 2005). Recent
ongoing efforts have attempted to i ll this data
gap through inventorying satellite imagery
(Rebelo, Finlayson and Nagabhatla 2009).
However, much work remains to be done. Past
studies estimated the total expanse of wetlands
globally to be somewhere in the region of 12.8
million km 2 based on a variety of international
inventories (Finlayson et al. 1999). This makes up
close to 12 percent of the Earth's total land area.
However, other estimates suggested totals as low
as 5.3 million km 2 (Matthews and Fung 1987).
While such wide ranges leave much uncertainty
as to the exact extent, at present the international
Ramsar Convention on Wetlands provides the
most comprehensive repository of information
on wetland habitats globally (Ramsar 2010c).
The Convention on Wetlands was signed
in Ramsar, Iran in 1971 and currently has
160 Contracting Parties worldwide. Contracting
countries to the Ramsar Convention are obliged
to provide detailed information on wetlands
listed as part of the wetlands conservation treaty
and these data are made available through the
Ramsar Sites Information Service (RSIS). Sys-
tematically gathered site-specii c geographic
and biophysical data along with management
and conservation issues pertaining to each site
are provided and regularly updated. While the
RSIS may not list all wetland habitats found
within individual countries, it goes a long way
toward providing a comprehensive database on
at least the more important and well-recognized
wetland sites.
For the United States, a recent comprehensive
study conducted using satellite imagery and
digital photography indicates 43.6 million total
hectares of wetlands across the conterminous
states (Dahl 2006). Ninety-i ve percent of this
area comprises fresh-water wetlands while the
rest is represented by marine or coastal habitats.
Between 1998 and 2004, the study found an
increase of approximately 12,950 hectares of
wetlands annually for the United States, and
these net gains were attributed to wetland res-
toration and enhancement activities as a result
of regulatory requirements.
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