495
The results refl ected that there were 43.8 million
ha of wetlands in the USA in the mid-1950s. This
was about 45 % more than the estimate by the
Circular 39 study (Shaw and Fredine 1956 ). It
was, further, revealed from the NWTS study that
the wetlands in the USA had decreased to 40.1
million ha by the mid-1970s. In addition to the
above, more recent US Fish and Wildlife Service
analyses done during the mid-1970s and mid1980s showed higher estimates for the 1970s and
a continuing slow decline to about 41.8 million ha
during the mid-1980s (Dahl and Johnson 1991 ).
23.1.2 Wetland Losses
Overall, there had been a loss of c 53 % of the
wetlands in the coterminous USA from the 1780s
to 1980s. The wetland area dwindled from about
89.5 million ha to 42.2 million ha. The early evidence of magnitude of wetland loss in the USA
was given by Shaw and Fredine ( 1956 ). They had
shown that there had been a 46 % loss of wetlands in the seven states (which were covered by
Swamp Land Act of 1850) from 1850 to 1950.
Further, Frayer et al. ( 1983 ), through a study of
the wetland losses in the USA, had estimated a
net loss of >3.7 million ha (at 0.4 %/year) from
the 1950s to the 1970s. This had been estimated
as c 185,000-ha annual loss on an average. They
are supposed to have disappeared at rates of 7 and
4.8 % per decade, respectively.
It is pertinent to note here that wetland losses
had continued during the 1980s and 1990s despite
the enactment of strong wetland protection laws
during the mid-1970s in the USA. However, the
rate of losses appeared to be slightly lower. An
overall loss rate of about 0.25 % per year was
reported for the USA from the 1970s to 1980s.
Tiner ( 1998 ) had estimated that wetlands were
still being lost at 47,000 ha/year in the USA.
23.1.3 Wetland Conversions
It could be said that estimates of net wetland
losses by themselves provide an incomplete picture of the dynamics of change. A more complete
picture could possibly show that human activities
had converted millions of hectares of wetlands
from one class to another. Thus, some wetland
classes had increased in area at the expense of
other types. The swamps and the forested riparian wetlands had suffered the greatest loss ( c 1.4
million ha) mostly during the period from 1970s
to 1980s. Estimates revealed that c 800,000 ha
had been converted to agricultural and other land
uses. Large areas were also converted to other
wetland types, namely, 292,000 ha to marshes,
195,000 ha to scrubs and shrubs and 32,000 ha to
non-vegetated wetlands (Dahl and Johnson 1991 ).
23.2 Wetlands of the USA
Two general statements could be put forward
based on a review of several studies which have
been made of wetland trends in the USA. These
are (a) estimates of the area of the wetlands in the
USA, which do vary widely, and (b) most studies
revealed a rapid rate of wetland loss in the USA,
at least before mid-1970s. There are variations in
the number of wetlands in the different inventories prepared. This was mainly because of variations in the purpose of preparation of the
inventory. For example, early wetland censuses
were undertaken by Wright ( 1907 ) and Gray
et al. ( 1924 ) mainly to identify lands suitable for
drainage for agriculture. The later inventories
prepared, for example, by Shaw and Fredine
( 1956 ), were concerned with waterfowl protection. Only those wetland inventories, which were
prepared during the last decade, had considered
all the values of these ecosystems. Further, the
defi nition and the basis of classifi cation of wetlands varied in each study. Also, the methods
available for estimating wetlands changed over
the years or they varied in accuracy. Remote
sensing from aircraft and satellites is an example
of a technique for wetland studies which was not
generally available or used before the 1970s. In
contrast, the early estimates were mainly based
on fragmentary records. It is, further, important
to note here that the borders of geographical or
political units change between censuses in a
number of instances. These lead to gaps or overlaps
23.2 Wetlands of the USA
The results refl ected that there were 43.8 million
ha of wetlands in the USA in the mid-1950s. This
was about 45 % more than the estimate by the
Circular 39 study (Shaw and Fredine 1956 ). It
was, further, revealed from the NWTS study that
the wetlands in the USA had decreased to 40.1
million ha by the mid-1970s. In addition to the
above, more recent US Fish and Wildlife Service
analyses done during the mid-1970s and mid1980s showed higher estimates for the 1970s and
a continuing slow decline to about 41.8 million ha
during the mid-1980s (Dahl and Johnson 1991 ).
23.1.2 Wetland Losses
Overall, there had been a loss of c 53 % of the
wetlands in the coterminous USA from the 1780s
to 1980s. The wetland area dwindled from about
89.5 million ha to 42.2 million ha. The early evidence of magnitude of wetland loss in the USA
was given by Shaw and Fredine ( 1956 ). They had
shown that there had been a 46 % loss of wetlands in the seven states (which were covered by
Swamp Land Act of 1850) from 1850 to 1950.
Further, Frayer et al. ( 1983 ), through a study of
the wetland losses in the USA, had estimated a
net loss of >3.7 million ha (at 0.4 %/year) from
the 1950s to the 1970s. This had been estimated
as c 185,000-ha annual loss on an average. They
are supposed to have disappeared at rates of 7 and
4.8 % per decade, respectively.
It is pertinent to note here that wetland losses
had continued during the 1980s and 1990s despite
the enactment of strong wetland protection laws
during the mid-1970s in the USA. However, the
rate of losses appeared to be slightly lower. An
overall loss rate of about 0.25 % per year was
reported for the USA from the 1970s to 1980s.
Tiner ( 1998 ) had estimated that wetlands were
still being lost at 47,000 ha/year in the USA.
23.1.3 Wetland Conversions
It could be said that estimates of net wetland
losses by themselves provide an incomplete picture of the dynamics of change. A more complete
picture could possibly show that human activities
had converted millions of hectares of wetlands
from one class to another. Thus, some wetland
classes had increased in area at the expense of
other types. The swamps and the forested riparian wetlands had suffered the greatest loss ( c 1.4
million ha) mostly during the period from 1970s
to 1980s. Estimates revealed that c 800,000 ha
had been converted to agricultural and other land
uses. Large areas were also converted to other
wetland types, namely, 292,000 ha to marshes,
195,000 ha to scrubs and shrubs and 32,000 ha to
non-vegetated wetlands (Dahl and Johnson 1991 ).
23.2 Wetlands of the USA
Two general statements could be put forward
based on a review of several studies which have
been made of wetland trends in the USA. These
are (a) estimates of the area of the wetlands in the
USA, which do vary widely, and (b) most studies
revealed a rapid rate of wetland loss in the USA,
at least before mid-1970s. There are variations in
the number of wetlands in the different inventories prepared. This was mainly because of variations in the purpose of preparation of the
inventory. For example, early wetland censuses
were undertaken by Wright ( 1907 ) and Gray
et al. ( 1924 ) mainly to identify lands suitable for
drainage for agriculture. The later inventories
prepared, for example, by Shaw and Fredine
( 1956 ), were concerned with waterfowl protection. Only those wetland inventories, which were
prepared during the last decade, had considered
all the values of these ecosystems. Further, the
defi nition and the basis of classifi cation of wetlands varied in each study. Also, the methods
available for estimating wetlands changed over
the years or they varied in accuracy. Remote
sensing from aircraft and satellites is an example
of a technique for wetland studies which was not
generally available or used before the 1970s. In
contrast, the early estimates were mainly based
on fragmentary records. It is, further, important
to note here that the borders of geographical or
political units change between censuses in a
number of instances. These lead to gaps or overlaps
23.2 Wetlands of the USA