Mangroves and Pollinating Bats?

Mangrove forests are comprised of unique plant species that
form the critical interface between terrestrial, estuarine, and near-shore
marine ecosystems in tropical and subtropical regions. The term mangrove is
generally used to describe the assemblage of trees and shrubs that grow in
saline coastal habitats, normally found in the tropics and sub-tropics.
Mangrove plants are not land builders but once established, play a key role in
stabilization of newly accreted sediment. Mangrove tree species have developed
a wide range of features, including specialized stomatal, leaf, seed,
and root structures, as adaptations for living in a saline environment with high
incidence of inundation. Most mangrove species are viviparous; seeds
germinate while still attached to the tree. Buoyant seeds develop into mature propagules,
which then drop and disperse by water. Of note is the diversity in root
structures of the mangrove tree species, designed to facilitate anchorage,
nutrient uptake, and gaseous exchange. Some species, such as Heritiera fomes,
Sonneratia apetala, and Avicennia officinalis, have pneumataphores;
upward projecting root branches that enable the gaseous exchange required for
respiratory metabolism. Other species, such as Rhizophora mucronata,
have “stilt” or “knee” roots for gaseous exchange, while additional features
such as the “tap” roots (buttresses) of Heritiera fomes and the
horizontal spread of roots in general, add to overall stability. Some plants
also excrete excess salt through leaves.

The importance of mangroves for humans and a variety of
coastal organisms has been well documented. They protect inland human communities
from damage caused by coastal erosion and storms, provide critical habitat for
a variety of terrestrial, estuarine and marine species, and serve as both a source
and sink for nutrients and sediments for other inshore marine habitats
including seagrass beds and coral reefs. Mangrove species that form
dense and often monospecific stands are considered “foundation
species” that control population and ecosystem dynamics, including fluxes
of energy and nutrients, hydrology, food webs, and biodiversity. Mangroves have
been widely reviewed as supporting numerous ecosystem services including flood
protection, nutrient and organic matter processing, sediment control, and
fisheries. Mangrove forests are the economic foundations of many tropical
coastal regions providing at least US$1.6 billion per year in “ecosystem
services” worldwide. It is estimated that almost 80% of global fish catches
are directly or indirectly dependent on mangroves. Mangroves sequester up to
25.5 million tonnes of carbon per year (where
is this statistic from?) and provide more than 10% of essential organic
carbon to the global oceans. Although the economic value of mangroves can be
difficult to quantify, the relatively small number of mangrove species
worldwide collectively provide a wealth of services and goods while occupying
only 0.12% of the world’s total land area. Mangrove ecosystems are the most biologically productive
ecosystem on earth. They are the keystone habitats by the sea and ecologically
most diverse across the tropical belt. Briefly speaking mangroves provide six essential ecological
services: (1) land maturation, (2) protection of human habitation from
cyclones, (3) oxygen production, (4) waste recycling, (5) food supply, and (6)
carbon cycling.

Despite its enormous biodiversity significance, mangroves
are the most vulnerable to anthropogenic persecution due to varying degree of
misconception surrounding its values to human and other animals in tropical ecoregions.
With almost half (44%) of the world’s population living within 150 km of a
coastline, heavily populated coastal zones have spurred the widespread clearing
of mangroves for coastal development, aquaculture, or resource use. At least
40% of the animal species that are restricted to mangrove habitat and have
previously been assessed under IUCN Categories and Criteria are at elevated
risk of extinction due to extensive habitat loss. It is estimated that 26% of
mangrove forests worldwide are degraded due to over-exploitation for fuelwood
and timber production. Similarly, clearing of mangroves for shrimp culture
contributes ~38% of global mangrove loss, with other aquaculture accounting for
another 14%. In India alone, over 40% of mangrove area on the western coast has
been converted to agriculture and urban development. However the overlapping of
marine and terrestrial resources in mangroves creates tenure ambiguities that
complicate management and may induce conflict between competing interests.
Globally, between 20% and 35% of mangrove area has been lost since
approximately 1980, and mangrove areas are disappearing at the rate of
approximately 1% per year with other estimates as high as 2–8% per year. These
rates may be as high as or higher than rates of losses of upland tropical wet
forests and current exploitation rates are expected to continue unless mangrove
forests are protected as a valuable resource. Given
their accelerating rate of loss, mangrove forests may at least functionally
disappear in as little as 100 years. The loss of individual mangrove species is
also of great concern, especially as even pristine mangrove areas are
species-poor compared with other tropical plant ecosystems. However, there is
very little known about the effects of either widespread or localized mangrove
area loss on individual mangrove species or populations. Additionally, the
identification and implementation of conservation priorities for mangroves has
largely been conducted in the absence of comprehensive species-specific
information, as species-specific data have not been collated or synthesized.
Species information including the presence of threatened species is important
for refining conservation priorities, such as the designation of critical
habitat, no-take zones, or marine protected areas, or to inform policies that
regulate resource extraction or coastal development. Below we have provided
graphical images of native distributions of mangrove species’ richness and
proportion of IUCN Red List of Threatened (Critically Endangered, Endangered,
and Vulnerable) mangrove species respectively.

The loss of individual mangroves species and associated
ecosystem services has direct economic consequences for human livelihoods,
especially in regions with low mangrove species diversity and low ecosystem
resilience to species loss. In the Gulf of California, for example, where there
are only 4 mangrove species present (Avicennia germinans, Rhizophora samoensis, Laguncularia racemosa, Conocarpus erectus), it is estimated that one linear
kilometer of the species R.
samoensis, listed as Near Threatened, provides up to 1 ha of
essential marine habitat and provides a median annual value of US$37,000 in the
fish and blue crab fisheries. Nutrients
and carbon from mangrove forests provide essential support to other near shore
marine ecosystems such as coral reefs and seagrass areas, and enrich
coastal food webs and fishery production. Avicennia species are dominant in inland or basin
mangrove forests in many parts of the world. However, 3 of 8 (38%) species in
this genus are in threatened or Near Threatened categories. Loss of these
species and the mangrove forests they dominate will have far reaching
consequences for water quality and other near shore ecosystems in coastal
communities around the globe. For example, water purification services provided
by these mangrove species in the Muthurajawela Marsh, Sri Lanka are valued at
more than $US 1.8 million per year. Riverine
or freshwater-preferring species, such as the Endangered Heritiera fomes and Heritiera globosa,
buffer coastal rivers and freshwater communities from sedimentation, erosion
and excess nutrients. Heritiera
globosa is a very rare species confined to western Borneo, while Heritiera fomes is more
widespread in south Asia, but has experienced significant declines in many
parts of its range. Localized or regional loss of these coastal or fringe
mangrove species reduces protection for coastal areas from storms, erosion,
tidal waves, and floods with the level of protection also dependent on the
quality of remaining habitat. Two of 4 (50%) fringe mangrove species present in
Southeastern Asia (Sonneratia
griffithii, Aegiceras
floridum) are listed in threatened or Near Threatened categories.
In other areas, such as Brazil, the central Pacific islands, or West Africa,
fringe mangrove forests are often comprised of only one or two species. Even
though these species are globally listed as Least Concern, local and regional
loss of mangroves in these areas will have devastating impacts for coastal
communities. The loss of species may indeed be of greatest economic concern in
rural, high-poverty areas where subsistence communities rely on mangrove areas
for fishing and for direct harvesting of mangroves for fuel, construction or
other economic products. Finally it is
important to note that the amount of mangrove area in some countries is
increasing due to reforestation and restoration efforts. Although regeneration
of degraded mangrove areas is thought to be a viable option in some areas,
successful regeneration is generally only achieved by the planting of monocultures
of fast-growing species, such as Rhizophora
or Avicenna species. Many rare and slow growing species are not
replaced and many species cannot be easily replanted with success. In sum,
mangrove areas may be able to be rehabilitated in some regions, but species and
ecosystems cannot be effectively restored.

Megachiroptera is one of the two suborders of mammalian
order Chiroptera that is bats. Of the rich diversity of vertebrate
fauna, bats are unique in being the only group of mammals that, like birds,
have sustained flight. One of the 26 mammalian orders, the Chiroptera includes
1117 species of

bats world over in rather two unequal suborders – the Megachiroptera
(consisting 186 species of Old

World fruit bats or flying fox in one family) and the Microchiroptera
(consisting 931 species in 17 families). Megachiroptera also
colloquially known as Megabats of the order Chiroptera comprises
fruit bats also known as flying foxes. The Megabat, contrary to its
name, is not always large: the smallest species is 6cm long and thus smaller
than some microchiroptera or microbats. 
The largest reach 40cm in length and attain a wingspan of 1.7m, weighing
in at up to 1.6kg. Most fruit bats have large eyes allowing them to orient
themselves visually in twilight and inside caves and forests. Their sense of
smell is excellent. In contrast to the microbats, the fruit bats do not
use echolocation apart from one or two species. Megachiroptera are frugivorous or
nectarivorous i.e., they eat fruits or lick nectar from flowers. Often the
fruits are crushed and only the juices consumed. The teeth are adapted to bite
through hard fruit skins like mangrove fruits. Large fruit bats must land to
eat fruit, while the smaller species are able to hover with flapping wings in
front of a flower or fruit. Frugivorous bats aid the distribution of plants
(and therefore, forests) by carrying the fruits with them and spitting the
seeds or eliminating them elsewhere. Nectarivores actually pollinate
visited plants. They bear long tongues that are inserted deep into the flower;
pollen passed to the bat is then transported to the next blossom visited,
thereby pollinating it. This relationship between plants and bats is a form of mutualism
known as chiropterophily.

Megachiroptera comprising flying foxes or fruit bats are
animals of extraordinary ecological and economic importance throughout forests
of the Old World tropics. Nearly 200 species play an essential role as mangrove
pollinators and seed dispersers, yet they are frequently misunderstood
intensely persecuted and exceptionally vulnerable to extinction Their role in
the propagation of numerous important mangrove plants remains virtually
uninvestigated. However, my review of already available literature demonstrates
that at least half of the total mangrove plant species (there are roughly 58
species of mangrove trees and plants that are identified and named so far) rely
to varying degrees on large populations of flying foxes for propagation. These
plants, in addition to their many ecological contributing produce some 448
economically valuable products. The fact that flying foxes are increasingly threatened and that
few baseline data exist on population trends is cause for concern. Many appear
to be in severe decline, and several species are already extinct. Old World phytophagous
bats (Megachiroptera: Pteropodidae) number 186 species of which 79% are
Asian and 21% African. Early bats were perhaps initially attracted to mangrove
flowers and fruit by the insects found around them, later finding the plants
themselves nutritious. Species of Megachiroptera today feed upon floral
resources, fruit and leaves from a total of at least 188 plant genera in 64
families. They may effect both pollination and seed-dispersal, and both bat-flower
and bat-fruit syndromes are commonly recognized. Individual species are
generally catholic in their feeding, favored food varying with locality and
season. Depending upon roosting habits and season, megabats may travel
considerable distances each night to feed and may undertake seasonal migrations.
Their feeding in essentially valuable mangrove species is now seriously
threatened by habitat destruction for shrimp aquaculture farming in many South
and South East Asian countries. The single Megachiropteran family, the
Pteropodidae, ranges from Africa, the eastern Mediterranean, Madagascar and the
Indian Ocean islands in the west, across mainland southern Asia, throughout the
islands of the western Pacific from the Ryukyu Archipelago and Ogasawara-shato
in the north, to coastal eastern Australia, New Caledonia and the Loyalty
Islands in the south, and east to Fiji, Tonga, Samoa and the

Cook Islands. There are over 42 genera containing of over
186 species in Megachiroptera. The largest and best known genus, Pteropus,
with 57 species, is primarily an island taxon, with 55 species (96.5%) having
some or all of their distribution on islands that mainly mangrove dominated
coastal islands. In this genus levels of endemism are extremely high, with 35
species (61.4%) confined to single islands or small island groups. Only nine
species are found in continental areas (five in Asia and four in Australia),
and only

two (P. lylei and P. poliocephalus) are restricted to
continents. Below are two images, one shows the Megachiroptera
distribution and other one shows mangrove distribution. Combined two images
together, one can appreciate how closely the Megachiroptera habitats are
closely fall into mangrove habitats across the globe.

There are over 50 true mangrove species with over 100
associated species so far documented and named. The most richest diversity of
mangroves is in Indo West Pacific biogeograpic realm (see
image below) where species diversity is as high as 40. Notably
Indo-Malayan islands pose the highest mangrove diversity where endemism if
islandic Megachiroptera is high and a key contributor of the mangrove
pollination. Despite the lack of ecological research linking the significant
correlation of mangrove pollination by the species of fruit bats or flying
foxes of the Megachiroptera guild, it is evident that mangrove and fruit bats
developed an interesting symbiotic relationship that posses far reaching
conservation implications for both the mangroves and the flying foxes. Below is
a graphical demonstration of the proportional
distribution of the mangrove species in each genus and its pollinating share by
Megachiroptera of fruit bats.

Mangrove habitat loss has been cited by a number of authors
as a major factor contributing to declines in fruit bat populations. Although
information on habitat requirements is limited for some species, it is evident
that there is considerable ecological variation within the family.
Deforestation, widespread in almost all tropical mangroves of the world, has
had several identifiable consequences for fruit bat populations. Many species,
particularly those inhabiting mangrove swamps (e.g. Pteropus vampyrus in
Malaysia and Indonesia) and lowland forest, have lost critical roosting areas.
Mangrove swamps are being

destroyed by the shrimp aquaculture, woodchipping industry,
firewood, and coastal development, and lowland forest is felled for agriculture
and timber. For example all the 8 species of mangrove genus Sonneratia
are pollinated by fruit bats and Sonneratia being commercially and
socio-economically valuable mangrove tree being cleared out with dramatic
population decline of the fruit bats in various mangrove ecosystems in South
and South East Asia.

Mohammed Ashraf, Wildlife Biologist, Graduate of Anglia
Ruskin University, Cambridge, England