131
for poles, firewood, and more recently wood-chips (for conversion to rayon), whilst Xylocarpus is more suitable for furniture/carving, and Aviennia is considered too soft to have
any real harvesting value (Ewel et al. 1998). As a result of
expanding human populations in recent decades, anthropogenic activities and our exploitation of these ecosystems has
resulted in large-scale degradation and destruction of mangroves forests (Kathiresan and Bingham 2001; Spalding
et al. 2010). Monospecific stands are exceptionally threatened by harvesting (Ewel et al. 1998), and when coupled
with forest clearance to make way for development, vast
areas of mangrove are being removed. It is estimated that one
quarter of original mangrove cover (>200,000 km
2
), has been
lost due to human activities, at a rate of 0.66–2% per annum
(Duke et al. 2007; Spalding et al. 2010). This exceeds the
loss rates reported for other threatened ecosystems (Stone
2007; Kathiresan 2008). For instance, of coral reefs, 10%
have already been lost (Wilkinson 1992) and rainforests are
lost at a rate of 0.8% per annum (Valiela et al. 2001).
Consequently, mangroves are considered critically endangered or approaching extinction in 26 of the 120 countries in
which they exist (Kathiresan 2008). Clearance and fragmentation of mangroves is of global concern due to its impact on
ecosystem services like coastal protection, sediment trapping, nutrient cycling, and loss of habitats for commercially
important species.
Mangroves provide coastal protection by mitigating the
impact of tidal surges and waves caused by hurricanes and
tsunamis (Duke et al. 2007). Estimates show that per kilometer of mangrove width, surges reduce in height by 5–50 cm,
and surface wind waves reduce by up to 75% (McIvor et al.
2012). During the super cyclone, which hit Orissa (India) in
1999, mangroves significantly reduced the number of deaths
and damage to property (Badola and Hussain 2005).
Evidence from the Indian Ocean tsunami in December 2004
showed that villages in India with mangrove buffers were
damaged to a lesser extent compared to nearby villages with
no mangroves (Kathiresan and Rajendran 2005; Vermaat and
Thampanya 2006). The degree of protection provided by
mangroves is attributed to several factors: forest width and
slope, tree and root density, and tree height (Alongi 2002).
Yet in many regions, clear-cutting and felling of mangroves
significantly reduces the forest width as well as tree and root
densities, and consequently lessenes the buffering capacity
of mangrove ecosystems to the threats posed by hurricanes
and tsunamis (Ellison 1994; Kathiresan and Bingham 2001;
Spalding et al. 2010). This buffering capacity is cited as one
of the most severely undervalued ecosystem services provided by mangroves (Barbier et al. 2011). More recently,
studies have shown that the value of this service is augmented at sites where other foreshore ecosystems (i.e., seagrasses and coral reefs) are present. Guannel et al. (2016)
concluded that mangroves in combination with a second
foreshore ecosystem attenuate significantly more wave
energy compared to any one ecosystem alone.
Sediment trapping and nutrient cycling pathways further
connect mangroves to adjacent ecosystems (Ewel et al.
1998). Riverine transport and terrestrial runoff are important
pathways to coastal environments and provide loads rich in
sediments, nutrients, organic matter, and at times, pollutants,
to coastal environments (Ramos et al. 2004). These terrestrially derived components are caught and slowed by the complex aerial root structure of mangroves, and become
immobilized and sequestered within mangrove systems
before they reach the clear, nutrient-limited waters of often
adjacent seagrass and coral reefs (Morell and Corredor 1993;
Valiela and Cole 2002). On Pohnpei (Federal States of
Micronesia), reduction of forest width to make way for a
road, led to the death of the remaining downstream mangroves which could not withstand the increased sediment
loads that buried lenticels on pneumatophores, prop roots
and young stems (Ewel et al. 1998). In regions where seagrass beds and coral reefs neighbor mangroves, loss and degradation of the mangrove forest due to harvesting activities
can be seen to reduce sediment and nutrient trapping capacities, thus increasing the risk of sedimentation and eutrophication (see section “A Nutritious Ocean” for a review) in
neighboring ecosystems. Despite several mentions of the
important role mangroves play in protecting adjacent systems from sedimentation (Morell and Corredor 1993; Valiela
and Cole 2002; Schaffelke et al. 2005), limited case studies
exist showing the impact of mangrove harvesting on sedimentation of adjacent ecosystems.
In terms of carbon, mangroves have a dual capacity as
both a sink of atmospheric CO 2 , and a source of oceanic carbon (Singh et al. 2005; Duke et al. 2007). Their high levels of
productivity, which reached 26.70 t ha
−1
year
−1
for Rizophora
apiculata in Thailand (Christensen 1978), shows that their
role in atmospheric CO 2 assimilation to build biomass is of
considerable importance (Spalding et al. 2010). However, it
is hypothesized that net primary production of mangroves
may be in excess of the carbon utilized in the system, consequently an estimated 20–30% is ‘outwelled’ to adjacent ecosystems (Bouillon et al. 2008; Granek et al. 2009),
corresponding well to the 50% organic matter export estimate proposed by Dittmar et al. (2006). Although accurate
quantification of the mangrove carbon budget remains limited (Bouillon et al. 2008; Alongi 2009), research has shown
that clearing of mangroves could result in carbon emissions
of up to 112–392 Mg ha
-1
(Donato et al. 2011). These emissions would significantly influence global CO 2 concentrations, which in turn drive climate change (see sections “A
Warmer Ocean” and “A Sour Ocean” for reviews). Although
their impact on carbon export to the coastal ocean remains
unknown (Donato et al. 2011), what is known is that alterations to these fluxes could impact habitats and food resources
For a World Without Boundaries: Connectivity Between Marine Tropical Ecosystems in Times of Change
for poles, firewood, and more recently wood-chips (for conversion to rayon), whilst Xylocarpus is more suitable for furniture/carving, and Aviennia is considered too soft to have
any real harvesting value (Ewel et al. 1998). As a result of
expanding human populations in recent decades, anthropogenic activities and our exploitation of these ecosystems has
resulted in large-scale degradation and destruction of mangroves forests (Kathiresan and Bingham 2001; Spalding
et al. 2010). Monospecific stands are exceptionally threatened by harvesting (Ewel et al. 1998), and when coupled
with forest clearance to make way for development, vast
areas of mangrove are being removed. It is estimated that one
quarter of original mangrove cover (>200,000 km
2
), has been
lost due to human activities, at a rate of 0.66–2% per annum
(Duke et al. 2007; Spalding et al. 2010). This exceeds the
loss rates reported for other threatened ecosystems (Stone
2007; Kathiresan 2008). For instance, of coral reefs, 10%
have already been lost (Wilkinson 1992) and rainforests are
lost at a rate of 0.8% per annum (Valiela et al. 2001).
Consequently, mangroves are considered critically endangered or approaching extinction in 26 of the 120 countries in
which they exist (Kathiresan 2008). Clearance and fragmentation of mangroves is of global concern due to its impact on
ecosystem services like coastal protection, sediment trapping, nutrient cycling, and loss of habitats for commercially
important species.
Mangroves provide coastal protection by mitigating the
impact of tidal surges and waves caused by hurricanes and
tsunamis (Duke et al. 2007). Estimates show that per kilometer of mangrove width, surges reduce in height by 5–50 cm,
and surface wind waves reduce by up to 75% (McIvor et al.
2012). During the super cyclone, which hit Orissa (India) in
1999, mangroves significantly reduced the number of deaths
and damage to property (Badola and Hussain 2005).
Evidence from the Indian Ocean tsunami in December 2004
showed that villages in India with mangrove buffers were
damaged to a lesser extent compared to nearby villages with
no mangroves (Kathiresan and Rajendran 2005; Vermaat and
Thampanya 2006). The degree of protection provided by
mangroves is attributed to several factors: forest width and
slope, tree and root density, and tree height (Alongi 2002).
Yet in many regions, clear-cutting and felling of mangroves
significantly reduces the forest width as well as tree and root
densities, and consequently lessenes the buffering capacity
of mangrove ecosystems to the threats posed by hurricanes
and tsunamis (Ellison 1994; Kathiresan and Bingham 2001;
Spalding et al. 2010). This buffering capacity is cited as one
of the most severely undervalued ecosystem services provided by mangroves (Barbier et al. 2011). More recently,
studies have shown that the value of this service is augmented at sites where other foreshore ecosystems (i.e., seagrasses and coral reefs) are present. Guannel et al. (2016)
concluded that mangroves in combination with a second
foreshore ecosystem attenuate significantly more wave
energy compared to any one ecosystem alone.
Sediment trapping and nutrient cycling pathways further
connect mangroves to adjacent ecosystems (Ewel et al.
1998). Riverine transport and terrestrial runoff are important
pathways to coastal environments and provide loads rich in
sediments, nutrients, organic matter, and at times, pollutants,
to coastal environments (Ramos et al. 2004). These terrestrially derived components are caught and slowed by the complex aerial root structure of mangroves, and become
immobilized and sequestered within mangrove systems
before they reach the clear, nutrient-limited waters of often
adjacent seagrass and coral reefs (Morell and Corredor 1993;
Valiela and Cole 2002). On Pohnpei (Federal States of
Micronesia), reduction of forest width to make way for a
road, led to the death of the remaining downstream mangroves which could not withstand the increased sediment
loads that buried lenticels on pneumatophores, prop roots
and young stems (Ewel et al. 1998). In regions where seagrass beds and coral reefs neighbor mangroves, loss and degradation of the mangrove forest due to harvesting activities
can be seen to reduce sediment and nutrient trapping capacities, thus increasing the risk of sedimentation and eutrophication (see section “A Nutritious Ocean” for a review) in
neighboring ecosystems. Despite several mentions of the
important role mangroves play in protecting adjacent systems from sedimentation (Morell and Corredor 1993; Valiela
and Cole 2002; Schaffelke et al. 2005), limited case studies
exist showing the impact of mangrove harvesting on sedimentation of adjacent ecosystems.
In terms of carbon, mangroves have a dual capacity as
both a sink of atmospheric CO 2 , and a source of oceanic carbon (Singh et al. 2005; Duke et al. 2007). Their high levels of
productivity, which reached 26.70 t ha
−1
year
−1
for Rizophora
apiculata in Thailand (Christensen 1978), shows that their
role in atmospheric CO 2 assimilation to build biomass is of
considerable importance (Spalding et al. 2010). However, it
is hypothesized that net primary production of mangroves
may be in excess of the carbon utilized in the system, consequently an estimated 20–30% is ‘outwelled’ to adjacent ecosystems (Bouillon et al. 2008; Granek et al. 2009),
corresponding well to the 50% organic matter export estimate proposed by Dittmar et al. (2006). Although accurate
quantification of the mangrove carbon budget remains limited (Bouillon et al. 2008; Alongi 2009), research has shown
that clearing of mangroves could result in carbon emissions
of up to 112–392 Mg ha
-1
(Donato et al. 2011). These emissions would significantly influence global CO 2 concentrations, which in turn drive climate change (see sections “A
Warmer Ocean” and “A Sour Ocean” for reviews). Although
their impact on carbon export to the coastal ocean remains
unknown (Donato et al. 2011), what is known is that alterations to these fluxes could impact habitats and food resources
For a World Without Boundaries: Connectivity Between Marine Tropical Ecosystems in Times of Change