such as high rates of nutrient-use and water-use efficiency;
and (4) positive and negative feedbacks that provide malleability to help dampen variations in recovery. Their simple architecture can lead to rapid recovery or rehabilitation
post-disturbance, as there is redundancy of keystone
species.
Fauna and trophic structure
Trees and bacteria constitute the bulk of forest biomass,
but many other organisms originating from adjacent terrestrial and marine environments are found in mangroves
(Macnae, 1968). Birds, bats, monkeys, tigers, insects, fish,
amphibians, reptiles, and a rich fauna of estuarine and
marine plankton and benthic invertebrates spend all or part
of their life cycle in the forest canopy, soils, and tidal
waterways (Kathiresen and Bingham, 2001; Nagelkerken
et al., 2008). These populations and communities overlap,
as mangroves are ecotones having a high level of connectivity with both terrestrial and marine ecosystems (Feller
et al., 2010).
The most functionally conspicuous organisms in mangroves are sesarmid and grapsid crabs, being keystone
engineers in many forests (Cannicci et al., 2008;
Nagelkerken et al., 2008). The significance of crabs as
biological drivers of forest structure and function was recognized in the late 1970s and early 1980s (Kathiresen and
Bingham, 2001). Sesarmid crabs are very important in
shaping mangrove structure and functioning, especially
in Old World mangroves, while ocypodid crabs play the
same role in New World mangroves. Crabs process
a large proportion of algal and leaf biomass and are ecosystem engineers in their ability to bioturbate and aerate
mangrove deposits and, along with other predators, consume and bury large quantities of propagules
(Kristensen, 2008). A number of models have been proposed to explain the mode of crab control via propagule
predation on forest structure: (1) the dominance-predation
model, which states that there is an inverse relationship
between predation rates of different species in relation to
its dominance in the forest; (2) the canopy-gap-mediated
model, which postulates that predation can be more
intense under closed canopies than in relatively large gaps;
(3) the flooding regime model, which suggests that the
level of propagule predation is inversely related to inundation time (the more time the forest floor is flooded, the less
time crabs have to prey on propagules); and (4) the spatiotemporal biocomplexity model, which hypothesizes that
while low water or high water conditions are a key driver
in initial plant establishment or failure, other factors
related to the dry or wet conditions (e.g., salinity, drought,
soil texture) come into play, so forest patch structure may
be the result of differences in environmental drivers.
Regardless of causative mechanism(s), the net result of
propagule predation is reduced completion among
saplings.
Mangrove structure and function are shaped by
a variety of other organisms. Gastropods, for example,
do so by consuming large volumes of mangrove material,
such as litter, algae, and wood, and wood-boring isopods
help to facilitate fungal decomposition of wood. Similarly,
herbivorous insects such as ants, moths, and caterpillars
bore into wood as well as consume flowers, fruits, seeds,
and leaves. Ant-plant interactions can play a key role in
regulating vegetation structure and function (Cannicci
et al., 2008).
Above the substratum, mangrove roots are often overgrown by epibionts such as tunicates, sponges, algae,
and bivalves, while the forest floor and canopy – visited
by birds, insects, bats, rats, and even monkeys – are
fauna-rich. Both the canopy and epibiotic communities
are very diverse with close associations between tree and
animals; some of the interactions are highly complex
(Ellison and Farnsworth, 2001). The functional significance of many of the organisms, especially the vertebrates, is largely unknown. Root epibionts are known to
be highly diverse and an attractant for a wide assortment
of invertebrates, as well as having an important role in
nitrogen transformation processes (Ellison et al., 1996).
Mangrove plankton and nekton are, like their benthic
and canopy-living counterparts, key players in the flow
of materials and energy in mangrove ecosystems. In both
soils and tidal waters, a large proportion of organic matter
and energy flow is funneled through a highly diverse,
actively growing, “microbial loop or hub” consisting of
Archaea, bacteria, protists, and viruses and subsequently
transferred to higher consumers such as zooplankton and
fish; metabolic by-products such as respired CO 2 and
wastes are similarly transferred and integrated into various
biogeochemical cycles that help to sustain life (Figure 3).
Mangrove microbes are highly abundant and productive,
fuelled by new and recycled DOM and inorganic nutrients. Indeed, the first link in the “microbial loop or hub”
is the uptake of exudates from phytoplankton cells and cell
contents released during “sloppy feeding” by microzooplankton, so there is a strong link between microbial and
phytoplankton productivity. Trophic relationships within
and between microbial assemblages are virtually
unknown, but presumably intense, as protists such as
amoebae and flagellates are voracious consumers of bacteria and are known to graze heavily on bacterioplankton
(Lee and Bong, 2007). In contrast, phytoplankton communities in mangrove waters are thought to be species-poor
due to inhibitory effects of high concentrations of soluble
tannins and other polyphenolics. Phytoplankton abundance and productivity range widely in mangroves, usually in relation to light availability and flushing rates of
waterways; stagnant or polluted waters are common
throughout Asia, for example, with high rates of primary
production in these virtual nutrient “soups.”
Zooplankton communities are the crucial link between
microbes (to which some of the tiniest zooplankters
belong), penaeid shrimps, and zooplanktivorous fish.
The main factor controlling zooplankton abundance and
species composition is the seasonal change in salinity,
with the onset of the monsoon season the prime stimulus
398
MANGROVES
and (4) positive and negative feedbacks that provide malleability to help dampen variations in recovery. Their simple architecture can lead to rapid recovery or rehabilitation
post-disturbance, as there is redundancy of keystone
species.
Fauna and trophic structure
Trees and bacteria constitute the bulk of forest biomass,
but many other organisms originating from adjacent terrestrial and marine environments are found in mangroves
(Macnae, 1968). Birds, bats, monkeys, tigers, insects, fish,
amphibians, reptiles, and a rich fauna of estuarine and
marine plankton and benthic invertebrates spend all or part
of their life cycle in the forest canopy, soils, and tidal
waterways (Kathiresen and Bingham, 2001; Nagelkerken
et al., 2008). These populations and communities overlap,
as mangroves are ecotones having a high level of connectivity with both terrestrial and marine ecosystems (Feller
et al., 2010).
The most functionally conspicuous organisms in mangroves are sesarmid and grapsid crabs, being keystone
engineers in many forests (Cannicci et al., 2008;
Nagelkerken et al., 2008). The significance of crabs as
biological drivers of forest structure and function was recognized in the late 1970s and early 1980s (Kathiresen and
Bingham, 2001). Sesarmid crabs are very important in
shaping mangrove structure and functioning, especially
in Old World mangroves, while ocypodid crabs play the
same role in New World mangroves. Crabs process
a large proportion of algal and leaf biomass and are ecosystem engineers in their ability to bioturbate and aerate
mangrove deposits and, along with other predators, consume and bury large quantities of propagules
(Kristensen, 2008). A number of models have been proposed to explain the mode of crab control via propagule
predation on forest structure: (1) the dominance-predation
model, which states that there is an inverse relationship
between predation rates of different species in relation to
its dominance in the forest; (2) the canopy-gap-mediated
model, which postulates that predation can be more
intense under closed canopies than in relatively large gaps;
(3) the flooding regime model, which suggests that the
level of propagule predation is inversely related to inundation time (the more time the forest floor is flooded, the less
time crabs have to prey on propagules); and (4) the spatiotemporal biocomplexity model, which hypothesizes that
while low water or high water conditions are a key driver
in initial plant establishment or failure, other factors
related to the dry or wet conditions (e.g., salinity, drought,
soil texture) come into play, so forest patch structure may
be the result of differences in environmental drivers.
Regardless of causative mechanism(s), the net result of
propagule predation is reduced completion among
saplings.
Mangrove structure and function are shaped by
a variety of other organisms. Gastropods, for example,
do so by consuming large volumes of mangrove material,
such as litter, algae, and wood, and wood-boring isopods
help to facilitate fungal decomposition of wood. Similarly,
herbivorous insects such as ants, moths, and caterpillars
bore into wood as well as consume flowers, fruits, seeds,
and leaves. Ant-plant interactions can play a key role in
regulating vegetation structure and function (Cannicci
et al., 2008).
Above the substratum, mangrove roots are often overgrown by epibionts such as tunicates, sponges, algae,
and bivalves, while the forest floor and canopy – visited
by birds, insects, bats, rats, and even monkeys – are
fauna-rich. Both the canopy and epibiotic communities
are very diverse with close associations between tree and
animals; some of the interactions are highly complex
(Ellison and Farnsworth, 2001). The functional significance of many of the organisms, especially the vertebrates, is largely unknown. Root epibionts are known to
be highly diverse and an attractant for a wide assortment
of invertebrates, as well as having an important role in
nitrogen transformation processes (Ellison et al., 1996).
Mangrove plankton and nekton are, like their benthic
and canopy-living counterparts, key players in the flow
of materials and energy in mangrove ecosystems. In both
soils and tidal waters, a large proportion of organic matter
and energy flow is funneled through a highly diverse,
actively growing, “microbial loop or hub” consisting of
Archaea, bacteria, protists, and viruses and subsequently
transferred to higher consumers such as zooplankton and
fish; metabolic by-products such as respired CO 2 and
wastes are similarly transferred and integrated into various
biogeochemical cycles that help to sustain life (Figure 3).
Mangrove microbes are highly abundant and productive,
fuelled by new and recycled DOM and inorganic nutrients. Indeed, the first link in the “microbial loop or hub”
is the uptake of exudates from phytoplankton cells and cell
contents released during “sloppy feeding” by microzooplankton, so there is a strong link between microbial and
phytoplankton productivity. Trophic relationships within
and between microbial assemblages are virtually
unknown, but presumably intense, as protists such as
amoebae and flagellates are voracious consumers of bacteria and are known to graze heavily on bacterioplankton
(Lee and Bong, 2007). In contrast, phytoplankton communities in mangrove waters are thought to be species-poor
due to inhibitory effects of high concentrations of soluble
tannins and other polyphenolics. Phytoplankton abundance and productivity range widely in mangroves, usually in relation to light availability and flushing rates of
waterways; stagnant or polluted waters are common
throughout Asia, for example, with high rates of primary
production in these virtual nutrient “soups.”
Zooplankton communities are the crucial link between
microbes (to which some of the tiniest zooplankters
belong), penaeid shrimps, and zooplanktivorous fish.
The main factor controlling zooplankton abundance and
species composition is the seasonal change in salinity,
with the onset of the monsoon season the prime stimulus
398
MANGROVES