134
historical overexploitation of parrotfish and mangrove deforestation synergistically reduced herbivory and secondary
production (Mumby et al. 2004). This highlights once again
that research into the effect of exploitation on connectivity
between tropical ecosystems is limited, and that there is a
need for seascape-wide and cross-disciplinary research in
the tropics.
A Warmer Ocean
Anthropogenic activities have had demonstrated localized
impacts in tropical marine ecosystems, however,
anthropogenic- induced stress in the form of global climate
change is having impacts on these ecosystems on a global
scale. Climate change occurs in several forms, but one of the
most studied is the increase of atmospheric and oceanic temperatures. These temperature elevations can be attributed to
the post-industrialization increase in atmospheric carbon
dioxide (CO 2 ), a greenhouse gas, which is responsible for
trapping the Earth’s outgoing radiation within the atmosphere, and consequently allowing the planet to warm. By
the end of the century, global surface temperature is estimated to have increased by 4 °C (IPCC 2013). The ocean has
been heating up by absorbing 90% of incoming solar radiation since 1971 (Riser et al. 2016). This temperature increase
is not the only impact of global warming, a number of indirect impacts are also expected including; the melting of glaciers and ice sheets resulting in sea level rise, increased
precipitation resulting in elevated terrestrial runoff, and
increased frequency and intensity of storms (Knutson et al.
2010; Trenberth 2011; Godoy and De Lacerda 2015).
At present, the distribution of some mangroves and seagrass species is confined by minimum and maximum air and
sea temperatures (Short et al. 2007; Bjork et al. 2008; Ward
et al. 2016). For the mangroves and seagrass systems not living towards the edge of their tolerance limits, an increase in
temperature could initially result in positive responses
(Alongi 2015). However, once their tolerance limits are surpassed the consequences are severe. A decrease in productivity and growth leads to a shift in community composition,
favoring those better adapted to cope with the elevated temperatures which could ultimately lead to the disappearance
of mangrove and seagrasses species with low thermal tolerances (Pernetta 1993; Short et al. 2011). Furthermore, temperature increase has been shown to cause changes in
reproduction patterns and altered metabolism (Short and
Neckles 1999; Gilman et al. 2008; Yeragi and Yeragi 2014;
Arunparasath and Gomathinayagam 2015). These examples
highlight the direct consequences of elevated atmospheric
and oceanic temperatures, however, some of the most threatening impacts come from the collateral impacts of climate
change.
If sea level rises due to glacial melting, it will not only
result in changes to flooding duration and frequency, but also
of salinity. Although mangroves are sensitive to such changes
(Friess et al. 2012; Ward et al. 2016), they also exhibit exceptional resilience through their ability to actively modify their
environment and migrate both inland and seawards (Fig. 6)
(Krauss et al. 2014; Ward et al. 2016). Roots of mangrove
trees trap sediment, allowing it to settle in the surrounding
area. However, the ability of mangrove forests to respond to
sea level rise will depend on sediment type and, importantly,
the rate of sediment accretion (Ward et al. 2016). If sedimentation rates remain higher than the rate of sea level rise, then
mangrove forests will respond by raising the seafloor and
progressively moving inland (Alongi 2008; Godoy and De
Lacerda 2015; Lovelock et al. 2015). By contrast, if sea level
rise exceeds the sedimentation rate, then the forest will
drown (Godoy and De Lacerda 2015; Ward et al. 2016).
The exceptional migratory capabilities of mangroves have
enabled some forests to, contrary to what one might expect,
have a positive response to climate change scenarios. Due to
changes in global temperature regimes, mangroves are
expanding their inland and poleward limits. The decreases in
cold and arid conditions are enabling mangrove expansion
into new territories (Godoy and De Lacerda 2015). Whilst
the polar shift of mangrove forests essentially represents a
re-distribution but potential survival of the ecosystem as a
whole, inland migration may have severe impacts on other
ecosystems, such as seagrass meadows, begging the question: will the survival of mangrove vegetation come at the
cost of other ecosystems, or will these ecosystems also adapt
by migrating?
Although also having distributional limits defined by sea
and air temperatures, seagrass meadows can be seen as more
sensitive than mangroves due to the fact they are exposed to
elevated temperatures during the day, and can be subject to
desiccation during low tides and consequently high ultraviolet and photosynthetically active radiation (Dawson and
Dennison 1996; Durako and Kunzelman 2002; Campbell
et al. 2007). Whilst direct impacts of temperature increase on
seagrasses are similar to mangroves, changes in the carbon
balance have been observed as an additional consequence.
Carbon balances, particularly in the substrate, are affected by
the increase in photosynthesis, which initially has benefits,
but also disadvantages as the increased labile carbon (a
product of photosynthesis), is transferred to the substrate,
and in turn may alter microbial communities important for
the maintenance of soil nutrients (Cotner et al. 2004; Koch
et al. 2007). Thus, increasing water temperature in seagrass
meadows directly affects nutrient composition not only of
the sediment, but also the water column.
However, similarly to mangroves, the collateral impacts
of global warming will also have substantial impacts on seagrass meadows. Light, nutrients, and turbidity, which influH. S. Earp et al.
historical overexploitation of parrotfish and mangrove deforestation synergistically reduced herbivory and secondary
production (Mumby et al. 2004). This highlights once again
that research into the effect of exploitation on connectivity
between tropical ecosystems is limited, and that there is a
need for seascape-wide and cross-disciplinary research in
the tropics.
A Warmer Ocean
Anthropogenic activities have had demonstrated localized
impacts in tropical marine ecosystems, however,
anthropogenic- induced stress in the form of global climate
change is having impacts on these ecosystems on a global
scale. Climate change occurs in several forms, but one of the
most studied is the increase of atmospheric and oceanic temperatures. These temperature elevations can be attributed to
the post-industrialization increase in atmospheric carbon
dioxide (CO 2 ), a greenhouse gas, which is responsible for
trapping the Earth’s outgoing radiation within the atmosphere, and consequently allowing the planet to warm. By
the end of the century, global surface temperature is estimated to have increased by 4 °C (IPCC 2013). The ocean has
been heating up by absorbing 90% of incoming solar radiation since 1971 (Riser et al. 2016). This temperature increase
is not the only impact of global warming, a number of indirect impacts are also expected including; the melting of glaciers and ice sheets resulting in sea level rise, increased
precipitation resulting in elevated terrestrial runoff, and
increased frequency and intensity of storms (Knutson et al.
2010; Trenberth 2011; Godoy and De Lacerda 2015).
At present, the distribution of some mangroves and seagrass species is confined by minimum and maximum air and
sea temperatures (Short et al. 2007; Bjork et al. 2008; Ward
et al. 2016). For the mangroves and seagrass systems not living towards the edge of their tolerance limits, an increase in
temperature could initially result in positive responses
(Alongi 2015). However, once their tolerance limits are surpassed the consequences are severe. A decrease in productivity and growth leads to a shift in community composition,
favoring those better adapted to cope with the elevated temperatures which could ultimately lead to the disappearance
of mangrove and seagrasses species with low thermal tolerances (Pernetta 1993; Short et al. 2011). Furthermore, temperature increase has been shown to cause changes in
reproduction patterns and altered metabolism (Short and
Neckles 1999; Gilman et al. 2008; Yeragi and Yeragi 2014;
Arunparasath and Gomathinayagam 2015). These examples
highlight the direct consequences of elevated atmospheric
and oceanic temperatures, however, some of the most threatening impacts come from the collateral impacts of climate
change.
If sea level rises due to glacial melting, it will not only
result in changes to flooding duration and frequency, but also
of salinity. Although mangroves are sensitive to such changes
(Friess et al. 2012; Ward et al. 2016), they also exhibit exceptional resilience through their ability to actively modify their
environment and migrate both inland and seawards (Fig. 6)
(Krauss et al. 2014; Ward et al. 2016). Roots of mangrove
trees trap sediment, allowing it to settle in the surrounding
area. However, the ability of mangrove forests to respond to
sea level rise will depend on sediment type and, importantly,
the rate of sediment accretion (Ward et al. 2016). If sedimentation rates remain higher than the rate of sea level rise, then
mangrove forests will respond by raising the seafloor and
progressively moving inland (Alongi 2008; Godoy and De
Lacerda 2015; Lovelock et al. 2015). By contrast, if sea level
rise exceeds the sedimentation rate, then the forest will
drown (Godoy and De Lacerda 2015; Ward et al. 2016).
The exceptional migratory capabilities of mangroves have
enabled some forests to, contrary to what one might expect,
have a positive response to climate change scenarios. Due to
changes in global temperature regimes, mangroves are
expanding their inland and poleward limits. The decreases in
cold and arid conditions are enabling mangrove expansion
into new territories (Godoy and De Lacerda 2015). Whilst
the polar shift of mangrove forests essentially represents a
re-distribution but potential survival of the ecosystem as a
whole, inland migration may have severe impacts on other
ecosystems, such as seagrass meadows, begging the question: will the survival of mangrove vegetation come at the
cost of other ecosystems, or will these ecosystems also adapt
by migrating?
Although also having distributional limits defined by sea
and air temperatures, seagrass meadows can be seen as more
sensitive than mangroves due to the fact they are exposed to
elevated temperatures during the day, and can be subject to
desiccation during low tides and consequently high ultraviolet and photosynthetically active radiation (Dawson and
Dennison 1996; Durako and Kunzelman 2002; Campbell
et al. 2007). Whilst direct impacts of temperature increase on
seagrasses are similar to mangroves, changes in the carbon
balance have been observed as an additional consequence.
Carbon balances, particularly in the substrate, are affected by
the increase in photosynthesis, which initially has benefits,
but also disadvantages as the increased labile carbon (a
product of photosynthesis), is transferred to the substrate,
and in turn may alter microbial communities important for
the maintenance of soil nutrients (Cotner et al. 2004; Koch
et al. 2007). Thus, increasing water temperature in seagrass
meadows directly affects nutrient composition not only of
the sediment, but also the water column.
However, similarly to mangroves, the collateral impacts
of global warming will also have substantial impacts on seagrass meadows. Light, nutrients, and turbidity, which influH. S. Earp et al.