THE during the transition. Proffitt and TravisTHE during the transition. Proffitt and Travis


physiological response of an organism to increasing temperature follows a
sigmoid curve, in which an initial rapid rise in functional processes (e.g.,
respiration, growth rate) slows, plateaus, and then declines as a critical
lethal threshold is reached and then exceeded. Mangrove plants and animals
presumably respond so, but the critical temperatures at which functionality
plateaus and organisms begin to die are uncertain. Rates of leaf photosynthesis
for most species peak at temperatures at or below 30 °C, and leaf CO2
assimilation rates of many species decline, either sharply or gradually, as
temperature increases from 33 to 35 °C. Photosynthesis in exposed leaves is
often depressed due to photo inhibition; mid-day declines of assimilation have
been observed ensuring survival for the photochemical machinery.

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increases alone are likely to result in faster growth, reproduction,
photosynthesis, and respiration, changes in community composition, diversity,
and an expansion of latitudinal limits. Field data indicate that mangroves are
indeed currently expanding into higher latitudes in North America, New Zealand,
Australia, southern Africa, and southern China. This global expansion poleward
is most likely in response to the global rise in sea surface temperatures.


As these
differences are being observed in the sub-tropics & tropics, mangrove
expansion is also being coupled with the changes in precipitation. In an
analysis of mangrove latitudinal changes, Quisthoudt et al. found that
temperature alone does not delimit the latitudinal range of Rhizophora and Avicenna
due partly to large regional differences in monthly temperature change, for
instance, warmest month temperatures are higher at the latitudinal limits in
the northern, than in the southern, hemisphere. While mangrove expansion and
salt marsh contraction are consistent with the poleward increase in temperature
and the reduction in the frequency of extreme cold events, other variables such
as changes in precipitation cannot be ruled out as co-factors.


expansion of mangroves at the expense of salt marshes suggests that a number of
complex ecological interactions are operating during the transition. Proffitt
and Travis propose that this migration may be facilitated by increasing
propagule abundance from greater reproductive rates and greater genetic
variation caused by outcrossing. From field surveys conducted along the
Atlantic and Gulf coasts of Florida, they found that reproductive frequencies
varied significantly, but increased with latitude and more strongly along the
Gulf coast, with a concomitant increase in outcrossing. The migration of
mangroves is self-re-enforcing; more colonizers lead to more propagules and
outcrossing leads to enhanced genetic variation, thus perpetuating and
promoting adaptation to a new environment.


No studies
have yet demonstrated a change in mangrove fauna associated with global
warming, but the results from a few studies of macro- and megafauna from
adjacent habitats have implications for mangrove organisms. An experimental
study has shown that juvenile mullet (Liza vaigiensis) and crescent terapon
(Terapon jarbua) frequenting tropical seagrass beds can be acclimated to higher
water temperatures, approaching the critical limits for marine vertebrates.
Other organisms such as tropical gastropods may respond actively by seeking
cooler sites to survive when temperatures exceed 33 °C. However, tropical
organisms are closer to their upper thermal thresholds than boreal and
temperate organisms, and are thus more vulnerable to rising temperature.


responses to increasing or decreasing precipitation are more straightforward,
but such changes are likely to co-occur with rises in sea level, temperature,
and atmospheric CO2 concentration. Comparing to the arid-zone stands these
mangrove forests in the wet tropics have far greater biomass and productivity,
consist of less dense but taller trees, and tend to inhabit finer sediment
deposits, but there are no clear species richness or diversity patterns between
high and low precipitation areas; low species richness may be attributable to
high variability in annual rainfall. But since mangroves clearly thrive in the wet
environments where they can likely deal with less stressfully with lower
salinity and more available fresh water.



Mangroves continuously responding
to changing sea level. Modern responses have been well-documented during the
past 50 years using a variety of techniques including time series analysis
of photos, remote sensing images, and digital terrain models to estimate the
mangrove growth or contraction, as well as methods to estimate the modern rates
of sediment accumulation.


The modern evidence implies
that mangrove responses to sea level rise correspond roughly to the patterns of
surface elevation change described by McIvor et al. For instance, along the
Pacific coast of Mexico, rise in sea level accompanied by warm waters of El
Niño events have drowned mangroves fringing the shoreline, but has resulted in
a net increase in mangroves driven inland. Local variability plays a key role
in predicting whether or not mangroves of a specific region will survive or
not, as local factors such as geomorphology are important. Also, the mangroves tend
to occupy a range of tidal settings making it hard to offer simple
prognostications. The situation is similar on the Pacific High Islands of
Micronesia where mangrove sedimentation is sufficient to offset elevation
losses in some locations, but not in others; low intertidal mangroves are more commonly
susceptible to the loss of elevation and subsequent flooding than in more
landward zones.


Two coastal settings that
mangroves readily inhabit but where their future is in doubt are river deltas
and low islands. On the low sea level islands of the Pacific, such as in Samoa,
mangroves are migrating landward with rise in sea level, but on many islands,
landward migration is obstructed by coastal development. There are many low
islands in Micronesia and Melanesia where sea-level rise spells local
extinction for mangroves. Worldwide, other low isles are habitats without a
future, as are deltas of a number of large tropical rivers. The exemplar is the
Sundarbans along the Indian and Bangladesh coast, where subsidence and
disappearance of many deltaic islands is ongoing. A time series analysis
(1924–2008) indicates that subsidence, a decline in sediment input from the
Ganges and other rivers due to damming, and rising sea level, have resulted in
a dramatic decline in mangroves on islands in the central and eastern sectors
of the Sundarbans. In the other river deltas, sea level rise, storms, and
cyclones enhanced subsidence and declines in sediment supply, resulting in a
shift of mangroves landwards but with a net contraction. In and proximate to
some river deltas where large, migrating mud banks cyclically accrete and erode
(e.g., the Amazon), the 18.6-year nodal tidal cycle (tidal amplitude affected
by the 18.6-year lunar cycle of ascending and descending nodes of its orbit) is
one of the main drivers of shoreline change.


Experimental studies have
offered some insight into how mangroves respond individually and collectively
to sea level rise. A number of studies show species-specific tolerances to
prolonged water logging. Common mangroves, such as Avicenna marina,
exhibit a high degree of tolerance to water logging, but responses are highly
variable in relation to length and water depth of immersion, salinity,
temperature, and other environmental factors.


Species-specific differences in
flooding resistance may not be the only biological response by mangroves to sea
level rise. A recent review by Yanez-Espinosa and Flores highlights the fact
that mangroves exhibit differences in morphology and anatomy in relation to
environmental change. For a large number of species, leaf anatomy, vascular
vessel densities, diameter, grouping and length, and fibre wall thickness are
affected by variations in salinity and flooding. Vessel density, for instance,
increases in most species from low to high salinity, from high- to low-flooding
level, and from short- to long-flooding period. A few species (e.g., Avicenna
germinans, Laguncularia racemose) have also demonstrated
modification to bark anatomy in response to prolonged flooding, typically
formation of hypertrophied lenticels, adventitious roots, and increased aerenchym
a development in the bark. Over the past 165 years, specific leaf area
of A. marina has correlated positively with atmospheric CO2concentration
and latitude, suggesting thicker, heavier leaves in future because of net
photosynthetic carbon gain.


Mangroves of the future may
very well look and function differently, and be denser in terms of number of
individuals per unit area. But, are mangrove forests keeping pace with current
rates of sea level rise? A statistical analysis of mangrove sedimentation rates
(mm year?1) versus mean sea level rise (mm year?1)
suggests that, on average, accretion rates are keeping pace with sea level rise
(see regression equation and line). However, roughly one-half of the data are
below the 1:1 relationship (red dotted line) which indicates that these
mangroves are not keeping pace; nearly all of these data were derived from
mangroves inhabiting low islands and coastal lagoons in the Caribbean and the
Pacific, and in subsiding river deltas, such as the Mekong and the Sundarbans.
Conversely, the three most rapid accretion rates were from Chinese, Indian, and
Brazilian mangroves inhabiting highly populated and impacted catchments. The
large scatter of data points underscores how mangroves of disparate coastal
settings respond so differently to the same rate of sea-level rise in different
parts of the world. These century-scale accretion rates derived from
radionuclide distributions (e.g., 210Pb) are unlikely to be
reliable indicators of the net result of elevation capital, the accretion rate,
and water level. Thus, although the linear regression analysis is significant,
the reality is that mangroves may respond in complex ways to sea level rise.




There have been a number of
general and local prognostications, especially in regard to sea level rise, but
there have been few attempts at global prediction. There has been only one
smart attempt to predict mangrove distributions under climate change. Using
several mangrove databases for 30 species across 8 genera, Record et al used
the BIOMOD model to make predictions of mangrove species and community
distributions under a range of sea level rise and global climate scenarios up
to the year 2080. The model runs came up with two clear predictions: some
species will continue migrating poleward but experience a decline in available
space; and Central America and the Caribbean will lose more species than other
parts of the world. The latter prediction is in agreement with the work of Polidoro
et al in which extinction risk of threatened species was assessed and the main
geographical area of concern was found to be the Atlantic and Pacific coasts of
Central America.

The recent
climatological forecasts by the Intergovernmental Panel on Climate Change
(IPCC) for until the end of this century predict that globally sea surface
temperatures will rise by 1–3 °C, oceanic pH will decline by 0.07–0.31,
and mean atmospheric CO2concentrations will increase to 441 ppm
(from 391 ppm in 2011). Regional differences will occur for some
parameters such as sea level, which will continue to rise globally at an
average rate between 1.8 and 2.4 mm year?1; precipitation
will increase and decrease in some regions such that arid areas will become
more arid and the wet tropics will become wetter; and salinity will change in
tandem with changes in precipitation. By understanding these climatic
predictions and the known and likely responses of mangroves to changes in
temperature, salinity, sea-level rise, etc.



I offer some

Prediction 1 (Red
lines) : Mangrove forests along the dusty coasts will decline as salinities
rises, freshwater becomes most sparse, and critical temperature thresholds are
reached more frequently (e.g., NW Australia, Pakistan, Arabian Peninsula, both Mexico

Prediction 2 (Orange
lines): Mangrove forests will decline as sediment yield declines, salinity
increases, and sea level rises in tropical river deltas subject to subsidence
intervals (e.g., the Sundarbans; the Mekong, Zaire, Fly Rivers).

Prediction 3 (Purple
lines): Mangrove forests will decline as sea level rises and there is little or
no upland space to colonize (e.g., low islands of Oceania, many Caribbean

Prediction 4 (Blue
lines): Mangroves forests will continue to extend their latitudinal range as
temperature and atmospheric CO2 concentrations rises (New
Zealand, USA, Australia, and China).




Over the
last several years, India has been pursuing a policy of energy conservation and
increased usage of renewable energy technologies. These measures have found
legislative and regulatory backing. Taking this forward, the Central Government
of India and the State Government of West Bengal are currently in the process
of finalising an action plan, Titled ‘West Bengal State Action Plan on Climate
Change’ to address the problem of climate change and its impact on the Sundarbans
as well as take steps for poverty alleviation. Noting the biological importance
of the Sundarbans as well the extreme human dependence on it sustenance, we
believe that neither a human-centric nor a pure conservation approach would
help the long term health of the delta. While the local population recognizes
the economic importance of the forests, extreme poverty has forced many to
adopt certain practices that are harming the Sundarbans, already facing the
onslaught of climate change. While making this entire Sundarbans and forestation
of all reclaimed areas may invite a sharp reaction from the millions of people
inhabiting the area, we believe that the Government of India must take a middle
ground approach to balance out conservation needs of the area along with the human
needs of the people. In this regard, in addition to India continuing to reduce
its emissions and liaise with all governments to collectively reduce emissions
at an accelerated pace, we propose a series of short-term as well as long-term steps
to protect biodiversity of the Sundarbans and also improve the living conditions
of the people living in the area to help reduce the biotic pressure on the
delta and our mitigation strategies are set out below:


Strategies that may be implemented immediately


Zoning Sundarbans according to vulnerability: The Government
of West Bengal should demarcate specific areas of the Sundarbans that are
particularly susceptible to cyclonic storms. Upon completion of such
demarcation, people living in and around the high-risk areas in the Sundarbans
should be strategically relocated to safe inland areas. This is particularly relevant
for those people who live in the critically vulnerable areas of islands that
are submerging or are frequently devastated by cyclonic storms. Additionally, scientists
have noted that increased human and animal interaction has led to man-animal
conflict. Therefore, protection patrols, surveillance of non-forest activities
in the Sundarbans areas and deployment of rapid action forces in case of
conflict situations should be deployed at the earliest. Such measures would go
a long way in protecting the endangered animals in the region and building a relationship
of trust between the people, regulators and forests.


planning and rehabilitation & relocation:

Relocation of the population would be ineffective
if the people are not given alternative job opportunities. Such varied job opportunities
could be included in actively participating in the conservation of Sundarbans
(with individuals being deployed as forest guards or by generating employment
in the tourism industry) or being provided jobs in cities and settlements close
to Sundarbans. The long term sustenance of the Sundarbans is based on the
people owning an economic stake in its conservation. Since the Sundarbans are
located close to Kolkata and other urban settlements, the proximity could be utilised
to promote tourism in the delta, which would bring along with it additional
sources of income.


opportunities that don’t depend on nature:

For successful rehabilitation and relocation, it is
imperative that opportunities be provided that do not depend upon nature.

The State Government will be required to invest in
creation of infrastructural facilities, which while providing employment opportunities
at the short term, will help the Sundarbans be a more accessible area for
dissemination of knowledge. Further, investments are required to provide
education and health to the local people.


efficient disaster management systems: The

State Government must put in place effective early
warnings systems. This must be communicated to the people in real time and the
people must be educated on exactly what to do in such circumstances. The State
Government has to improve its evacuation systems, put in place rapid action
response teams and be more adept and efficient in providing supplies and first aid
to people and animals caught in such disasters. Further, there is a pressing
requirement to set up animal and human flood relief centres.


5)   Protection & distribution of
saline resistant food grains and seeds: Due to the extinction of certain
traditional saline-resistant rice varieties as well as a marked increase in the
salinity in the region, the State Government must protect remaining saline-resistant
food grains and seeds that are saline resistant as well as increase
distribution of such seeds at a subsidised rate to the local population.