Mangroved forests of South America
Stabroek News, March 25, 2008
http://www.stabroeknews.com/?p=360VEGETATION AND SPECIES COMPOSITION
Mangrove forests in South America are found on the Atlantic and
Pacific coasts and in the bays and estuaries of eight countries. They
are distributed from the Praia do Sonho in the south (State of Santa
Catarina, Brazil) to the city of Sechura, along the Piura River
(Peru), where only monospecific forests are found (Avicennia
germinans).
A wide range of mangrove tree heights may be found in the different
countries of this region and even within the same country. While their
canopy cover rarely exceeds 20 m in height, tall trees of 45 to 50 m
may be found, for example in the 'Manglares Cayapas-Mataje' Ecologic
Reserve (Ramsar site) or in the states of Amapá, Pará, and Maranhão
(Brazil), where there are specimens of Avicennia spp. with a diameter
of about 1 m and trees of Rhizophora harrisonii reaching 40-45 m. In
these areas, mangroves extend up to 40 km inland and they remain
relatively unthreatened due to the inaccessibility and low human
population density. Well-developed mangrove communities are also found
in the delta of the Amazon River, where trees can reach 40 m in
height, sustained by high rainfall and by the river system itself. Due
to the extremely high inputs of freshwater here, mangroves have to
compete with local freshwater hardwoods, lianas and palms, which
restricts mangrove distribution in the area. The Pacific coast of
Colombia and the Orinoco delta of the Bolivarian Republic of Venezuela
have significant mangrove forests as well, with trees up to 30 m in
height, while 20-25-m mangrove trees have been reported in Guyana.
As is the case in North and Central America, mangrove forest structure
and species composition in South America differ from one coast to the
other. Colombia is the only country with both Caribbean and Pacific
coasts. More than 70 percent of its mangroves are found on the Pacific
coast, where they form well-structured, tall forests.
The lower rainfall and smaller tidal fluctuations of the Caribbean
coast limit forest development, and only small, narrow fringes of
mangroves are found (sometimes in the form of dwarf trees as in the
Guajira department). Notwithstanding these limitations, some
well-developed mangrove forests also grow along this coast, for
example in the Magdalena river estuary.
The mangrove tree species diversity of this region is low - only ten
native species (Table 13) - the lowest worldwide. Three species
(Avicennia germinans, Laguncularia racemosa and Rhizophora mangle) are
very common and are found in all eight countries; others, such as
Avicennia bicolor and Pelliciera rhizophorae, are found with less
frequency or their presence is uncertain.
MANGROVE RESOURCES: STATUS AND TRENDS 1980-2005
Mangroves in South America currently cover slightly less than 2
million hectares, down from some 2.2 million hectares in 1980.
About half the mangrove area of the region is found in Brazil - which
also has the third largest mangrove area in the world (Table 14 and
Figure 3, page 12). More than 90 percent is found in five countries:
Brazil, Colombia, the Bolivarian Republic of Venezuela, Ecuador and
Suriname (Figure 13). Guyana, French Guiana and Peru share the
remaining 140 000 hectares. The relatively small mangrove area in
these countries might be explained by the relatively small land area
(Guyana) or by the narrow and in some zones arid or rugged coasts of
Peru and French Guiana, which are not well suited to the development
of mangroves.
During the 1980s and 1990s, mangrove lands were often considered
unproductive and unhealthy. This attitude and the resultant activities
caused a loss of some 250 000 ha of mangroves in the region (11
percent of the 1980 extent) (Figure 14). This loss is low in
comparison with other regions, but can be significant at the national
and local levels, where rural populations often depend on mangroves
for subsistence and for commercial fishing.
In absolute terms, the countries that experienced the highest losses
of mangroves were Colombia, Ecuador, Brazil and the Bolivarian
Republic of Venezuela, all of them with more than 30 000 ha of
mangroves lost since the 1980s. In Colombia the extent of mangroves
has continued to decline over the past 25 years, even though the
annual change rate has decreased from -1.12 percent (1980-1990), to
-0.86 percent (1990-2000) and finally to -0.58 percent in the last
five years. Extensive conversion for shrimp ponds, development of
urban, industrial and tourist infrastructures and reclamation of land
for agricultural crops and pasture led to the loss of 90 000 ha since
the 1980s. Awareness of the importance of this coastal ecosystem is
now slowly increasing and some efforts in reforestation and/or
afforestation have been undertaken (2000) as a joint initiative by the
Ministry of the Environment (Ministerio del Ambiente), the Corporación
Nacional de Investigación y Fomento Forestal (CONIF), ITTO and local
communities.
One success story comes from Ecuador, where the significant losses of
the 1980s and 1990s (some 40 000 ha) are slowly being recovered and
the mangrove area now seems to be stable. The main cause of loss of
mangroves was the unsustainable development of shrimp ponds, which
started in 1966 as a small-scale activity. However, high international
demand converted shrimp into a major trade item, and Ecuador rapidly
became one of the largest producers in South America. The industry
brought significant gains to the country and to local populations,
becoming one of the economic activities with the most growth
nationally and in the region. The outbreak in Ecuador of the
white-spot syndrome virus (WSSV) had a tremendous impact on the
shrimp-farming industry. This impact, together with the activities of
several environmental organizations, the consequent abandoning of the
ponds, the ban on mangrove cutting implemented in 1994 and the
increase in field controls, favoured the natural recolonization of
mangroves in several places. The combination of these factors led to a
slight increase in the extent of mangroves from 2000 to 2005.
Brazil lost at least 50 000 ha of mangroves over the last 25 years,
mainly along the southern coast. The country still possesses
significant and relatively pristine mangrove forests, because most
mangrove areas are located in the less urbanized northern states.
No recent quantitative national data are available (other than the
1991 figures reported in Table 14). However, the relatively low
population pressure on the northern states and government efforts to
protect and rehabilitate damaged forests seem to have contributed to a
major reduction in the loss of mangrove area, which is estimated at
zero in the period 2000-2005. New and updated inventories are needed
to provide more detailed estimates for this country. It is hoped that
a recently approved Global Environment Facility (GEF) project
(Conservation and Sustainable Use of the Biodiversity of the Globally
Important Brazilian Mangrove) will contribute to filling current
information gaps.
Among the most significant causes of past losses and conversions are
the intensive use for timber production, urban development
(southeastern states) and to a lesser extent for fuelwood. Mariculture
(mainly shrimp farming) began in Brazil in the 1970s and it is now
widely practised - primarily along the coast of the northeastern
states (e.g. Ceará, Rio Grande do Norte) - and still represents a
significant threat to mangroves. The impact of this activity is a
national concern, mostly because of the consequent eutrophication of
the waters caused by the high emission of nitrogen and phosphorus and
the widespread illegal cutting of mangroves. In 2003 shrimp farms
covered approximately 13 000 ha, many of these adjacent to mangrove
forests, which has generally resulted in eutrophication of mangrove
creeks and tidal channels.
Intensive use of mangrove forests as a source of fuelwood, development
of urban and tourist infrastructures and conversion of land use to
agriculture appear to be the main causes of mangrove area changes in
the Bolivarian Republic of Venezuela, where more than 35 000 ha
disappeared in the past 25 years. Some additional losses and
degradation have been caused by oil spills. In this country, as well,
the government is now making efforts to protect these coastal
ecosystems.
In relative terms, the country that experienced the largest loss was
Peru (annual change rate 1980-2005: -2.42 percent), followed by
Ecuador (-1.19 percent), Colombia (-0.91 percent) and the Bolivarian
Republic of Venezuela (-0.60 percent). Mangroves in Peru have
traditionally been used for the production of fuelwood, charcoal and
poles. However, the main cause of their loss is uncontrolled clearing
for conversion of the land into shrimp ponds. This activity began in
the 1970s and soon became a major source of employment for many
farmers. For example, the industry led to the expansion of the city of
Tumbes, which has nearly doubled its population since the 1960s. The
logging of mangroves is now banned and this, together with the
increasing protection and control of the major mangrove forest in the
country (Santuario Nacional Los Manglares de Tumbes), may have led to
the reduction of the annual rate of loss, which appears to be close to
zero in the last five years (2000-2005).
French Guiana is the only country in the region that did not
experience major changes in mangrove area over the past two decades.
In this country, other forest types are used as the principal source
of timber/fuelwood, and no serious threats seem to pressure the
mangroves. Unfortunately, however, the area estimates for this country
had to be based on qualitative information, because no reliable data
were available since 1980 (Table 14). New inventories are needed in
order to have a clearer picture of the current extent of mangroves.
In Guyana, as well, updated inventories would contribute greatly to a
better estimate of the extent of the mangroves.
MAIN USES AND THREATS
Mangrove forests in South America have traditionally been used for a
wide range of purposes, and the local population still often rely on
them for their livelihood.
The collection of wood forest products is a common practice. Timber
and fuelwood are harvested from mangroves in Brazil, Colombia,
Ecuador, Guyana, Peru and the Bolivarian Republic of Venezuela.
Production of charcoal from these forests seems to be less widespread,
but it is a traditional practice in Colombia, Ecuador, Guyana and
Peru. Extraction of tannin from the bark of mangrove trees (especially
of Rhizophora mangle) was previously a common activity in Brazil,
Colombia, Ecuador and Guyana, and contributed substantially to
supplying national industries.
Rural communities often rely on mangroves as a source of food and
additional income through the collection of non-wood forest products.
Molluscs, crabs and other crustaceans are collected for local use or
for national consumption. In Ecuador, for example, 2.0-2.5 million
molluscs are collected every month from mangrove forests (Spalding et
al., 1997). In Guyana, Avicennia germinans seeds are used as food.
Mangrove forests also indirectly influence the local economy by aiding
commercial and small-scale fishing activities, which in the Bolivarian
Republic of Venezuela, for example, have an important role in the
national economy.
Mangrove forests have been converted to other land uses, such as the
production of salt (Ecuador) or reclaimed for agriculture (especially
in Colombia, Ecuador and Guyana) or for urban and tourism development
(e.g. in Colombia and the Bolivarian Republic of Venezuela). In
addition, as was described in the previous section, the conversion of
mangrove lands for shrimp ponds was a major cause of mangrove
destruction in past years, and even though this activity has been
reduced, it still represents a significant threat.
Awareness of the importance of these coastal ecosystems is growing,
but limited understanding of their services and benefits and lack of
harmonization and implementation of existing laws still represent
threats to mangroves. These forests continue to be under serious
threat also from increasing urban and industrial pollution (e.g.
Peru), the flow of pesticides to coastal zones (e.g. Suriname) and oil
spills (particularly in the Bolivarian Republic of Venezuela). The
planning and construction of dams or changes in river flows upstream
are also current threats. These decrease the freshwater input,
consequently increasing salinity and reducing nutrients and sediments
transported to the coast.
Mangroves have a significant role in erosion and accretion along the
coastline. An interesting case comes from French Guiana. The entire
coast of this country is affected by the load dispersal system
associated with the mouth of the Amazon River (located some 500 km
east of the country), from which some 1 billion tonnes of sediment are
poured into the ocean every year. It is estimated that 10 percent
reaches the coasts as suspended load (Rudant, 1994). The mangroves
assist in trapping these sediments.
MANGROVE CONSERVATION AND MANAGEMENT
Awareness of the importance of these forests is slowly but steadily
increasing, and mangroves are under some form of protection in most
countries of South America.
However, illegal cutting and use is unfortunately continuing. Legal
protection of mangroves has a long history in Brazil, where the
earliest record dates from 1760.
The King of Portugal, concerned about the loss of potential sources of
tannin, issued an edict to restrict the cutting of mangroves for
fuelwood unless their bark was also used. More recently, the
government has published laws to conserve mangroves and to establish
12 new protected areas. In addition, projects for the conservation and
sustainable use of mangroves in protected areas are being prepared,
which could help reduce pressure on these forests.
Afforestation and reforestation activities have been undertaken (e.g.
in Colombia and Guyana). All South American countries with the
exception of Guyana have at least one Ramsar mangrove site, indicating
added political will to protect these habitats and their environmental
richness. However, more efforts could be undertaken at the national
and regional levels to implement appropriate strategies and
effectively protect these ecosystems. In Ecuador, after extensive
mangrove clearing during the 1970s, the government declared mangroves
protected forests in 1986 and prohibited cutting of mangrove trees in
1994. Forest protection policies and legislation exist in Guyana, as
well, although not dealing directly with mangroves. In Suriname
mangroves are protected together with other swamp forests in
multiple-use management areas, and in Peru, where mangrove clearing is
now prohibited, most mangroves have been protected since 1980 in the
Santuario Nacional Los Manglares de Tumbes, which was declared a
Ramsar site in 1997.
