Download ECOLOGICAL STUDY OF LIFE IN A MANGROVE HABITAT

ECOLOGICAL STUDY
OF LIFE IN A
MANGROVE HABITAT
(A) INTRODUCTION: MANGROVE HABITAT
The mangroves is a purely tropical phenomenon
which marks the transition between the sea and the
land, or more accurately an estuary and the land,
since mangrove plants only grow in areas of
reduced salinity.
Typically, mangrove are backed by tropical rain forest,
but in HK the estuary areas have from early times
been the preferred sites for human settlement and so
here they usually pass into cultivated paddy fields,
fish ponds, and rural communities.
In HK, the mangroves are rather small and
not very extensive because here they are
near to the northern limit of their range,
and they have been subjected to
considerable human interference.
Mangroves can be found along the coast of
Deep Bay and along the coast of Tolo
Harbour (along Ting Kok Road).
(B) Physical Factors
1. Gradient of slope
2. Nature of Soil
3. Tides
4. Salinity
5. Rainfall
6. Temperature
(C) Biotic Components
(D) Adaptations of living organisms in mangrove
habitat
(E) Interrelationship among organisms
(F) Succession of plants
(B) Physical Factors
1. Gradient of slope
Extremely gentle (from less than 1 in 200 on the
seaward side to about 1 in 20 on the landward
side).
Thus, great areas are affected by the tides.
2. Nature of Soil
Soil of mud flat composed of fine silt particles
loosely packed together. Hence soil is
structureless.
The silt particles are easily washed away by water
currents. Consequently, there is the problem of
anchorage for plants and sedentary animals.
pH: 7-8.
(B) Physical Factors
3. Tides
The alternate submergence and exposure of the
muddy soil by the tides have the following
consequences:
a. A waterlogged soil with bad soil aeration.
- the plants are deficient in oxygen supply.
- high H2S content: H2S is produced by the
sulphur bacteria under the anaerobic condition.
It reacts with the minerals of the soil, producing
the grey colour of the soil.
b. A zonation pattern of animals and plants.
(B) Physical Factors
4. Salinity
Though the salinity is lower than that of normal sea
water, the salinity of the sea water still poses
osmotic problems to the organisms there.
As the habitat is in an esturarine area, the
freshwater brought down by the river will make
the salinity lower nearer to the landward side.
Thus, a gradient of salinity occurs from the
landward side to the seaward side and this is also
responsible for the zonation pattern seen.
(B) Physical Factors
5. Rainfall
The rain provides freshwater to the habitat.
A heavy rain will cause drastic changes in the
salinity of the sea water and the soil,
especially in the upper regions.
Only plants that can tolerate such sudden
fluctuations can survive in the upper shore.
(B) Physical Factors
6. Temperature
The low winter temperature is responsible for the
small size of the mangrove plants and the absence
of some species.
The temperature fluctuation is greater at the
landward side as this area is exposed for a longer
period of time.
When temperature is high:
- soil temperature is high, and only those organisms that
can a high temperature can survive.
- amount of dissolved oxygen in soil water decreases.
- water evaporates from soil faster. This may lead to
desiccation and increase in salinity.
(C) Biotic Components
i. Plants:
The plants are mostly woody shrubs,
belonging to several different families, and
ranging 1-2 in in height.
Some have a spreading system of prop roots
which furnish many physical niches for
animal colonization.
The plants show Zonation and Succession.
The plants more lower down (Avicennia) are the pioneer plants.
of animals
Zonation
of plants
Different kinds of droppers
Bruguiera
Kandelia
Aegiceras
Different kinds of droppers
Bruguiera
Short & fat
Kandelia
Aegiceras
Different kinds of droppers
Bruguiera
Kandelia
Short & fat
Long & tapering
Aegiceras
Different kinds of droppers
Bruguiera
Kandelia
Aegiceras
Short & fat
Long & tapering Small, curved
Different kinds of droppers
Bruguiera
Kandelia
Short & fat
Long & tapering Small, curved
Red
Aegiceras
Different kinds of droppers
Bruguiera
Kandelia
Aegiceras
Short & fat
Long & tapering Small, curved
Red
Green
Different kinds of droppers
Bruguiera
Kandelia
Aegiceras
Short & fat
Long & tapering Small, curved
Red
Green
Green
ii. Animals:
The leaves, stems and roots of the mangrove plants
are colonized by different animals, a number of
which characteristically occur in this habitat alone.
On the hard tree trunks, a pattern of zonation similar
to that seen on rocky shores is apparent, largely
because of the similarity of the substrates.
The succession of plant species also brings with it a
succession of animals species.
Some live on the mud surface or just beneath the
surface. Few burrow to any depth because of the
anaerobic nature of the substrate.
Summary of location of animals
Air – some birds
Leaves – large mud snail, periwinkle
Trunks/stems – Acorn barnacle, mangrove mussel, oyster
Leaf Litters – Flatworm, sea slater
Mud Surface
Upper shore
Estuarine snail
Periwinkle
Mudskipper
Middle shore
Lower shore
Estuarine snail
Estuarine snail
Mud snail
Starfish
Mudskipper
Mudskipper
Large mangrove clam Nerita spp.(snail)
Cockle
Inside mud
peanut worm, fiddler crab, sesarmid crab, soldier crab,
mudskipper, etc
(D) Adaptations of living organisms in Mangrove habitat
a. Adaptations to loose nature of soil
1.Shallow but extensive root system for anchorage (most
plants)
2.Prop Roots (Bruguiera, Kandelia)
(D) Adaptations of living organisms in Mangrove habitat
a. Adaptations to loose nature of soil
1.Shallow but extensive root system for anchorage (most
plants)
2.Prop Roots (Bruguiera, Kandelia)
3.Cable root - long, horizontal and shallow roots (10-20 cm
below surface) extending over a large area (Avicennia).
Both prop roots and cable roots anchor the plants against
the wash of the waves.
4.Some animals are permanently attached to tree trunks
e.g. barnacles by cement glands, mussels by byssus
threads.
5. Some animals make burrows in the mud flat
e.g. polychaeta worms, crabs etc.
b. Adaptations to bad soil aeration
1. No tap root system. Root system is shallow and superficial.
2. Pneumatophores:
erect roots, protruding out of the soil surface and branching
out from cable roots.
There are numerous lenticels for gaseous exchange.
Chlorophyll is also present. e.g. Avicennia, Aegiceras.
3. Knee joints:
bendings of cable roots protruding out of mud surface.
lenticels are present for gaseous exchange (Bruguiera).
4. Aerenchyma: A spongy tissue with many inter-connected
air spaces in cable roots and pneumatophores for
transportation of air to underground parts of the root
system, e.g. in Aegiceras.
c. Adaptations to periodic exposure of organisms
due to daily inundation by the tides
5. Building burrows with inlets and outlets by burrowers
Water can circulate through these burrows, providing
more oxygen, e.g. polychaeta worms.
6. Possession of haemoglobin to increase ability to extract
oxygen from the near-anaerobic environment. e.g.
polychaeta worms.
Note: bad soil aeration does not affect green algae and
animals living above soil.
These animals only respire under water and therefore
face problems when exposed to air during low tide.
c. Adaptations to periodic exposure of organisms due to
daily inundation by the tides
1. Trapping water inside body to prevent desiccation
e.g. barnacles possess valves,
oysters’ 2 halves can close up.
2. Behavioural adaptation
i. clams, barnacles can undergo a period of aestivation
during low tide by trapping water in their gill chamber.
ii. many annelid worms burrow into mud to avoid being
dried up.
3. Mudskippers which are small fishes
living in very shallow water or just above
the water level possess bony gill rakers
which extends into the gill filaments to
prevent their collapsing if exposed to air.
In addition, the large buccal cavity traps
water which baths the gills.
Therefore, they can still function when the
animal is above water.
d. Adaptations to high salinity of sea water and
fluctuating salinity of soil
1. Mangrove plants have a slightly greater physiological
tolerance to a higher salinity than land plants.
E.g. Avicennia is capable of growing in soil with salinity of
up to 90 o/oo (therefore can grow and colonize even the
seaward edge of the mud-flat) while Bruguiera can grow
in salinity of up to 25 o/oo (can only colonize the
landward margin of the mudflat).
2. Special salt glands to excrete excess salt onto the surface
of leaves. e.g. Kandelia, Avicennia, Excoecaria.
3. Salt can be stored in aging leaves and got fall off
together with the old leaves.
4. Body fluids of animals are isotonic to sea water.
e. Adaptations to absorb and conserve water
The sea water poses problems for the plants to
obtain water for their use.
Since water is difficult to obtain, the mangrove
plants have developed special measures to
conserve water.
Thus, many of them show xerophytic adaptations.
1. All mangrove plants develop a very low internal
water potential which enables them to absorb
and transport water from the salty environment.
e. Adaptations to absorb and conserve water
2.The root system of all mangrove plants arc
widespread, shallow to enable them to take rapid
advantage of the freshwater provided by the
summer rains.
3. Xerophytic adaptations
- leaves covered with thick cuticle (e.g. Bruguiera,
Kandelia, Aegiceras).
- well protected stomata.
- thick fleshy leaves and they are reduced in area
to store water and reduce water loss by
transpiration (e.g. Bruguiera, Kandelia,
Aegiceras).
f. Adaptations for germination and
dispersal of seeds
When the seeds of the fruits germinates, they
face to face 2 main problems:
1) difficulty in absorbing water for
germination and growth.
2) anchorage.
f. Adaptations for germination and
dispersal of seeds
The mangrove plants solve these problems-by having
viviparous germination:- (Kandelia, Bruguiera and
Aegiceras).
The seedlings of these plants develop while they are still
attached to the parent plant.
They continue to absorb water and nutrients from the parent
plant.
When they are released, they are already well-developed, in
the form of a dropper containing a lot of water, food,
chlorophyll and equipped with a fleshy and buoyant
hypocotyl for dispersal by water.
In Kandelia and Bruguiera, the radicle has a pointed end so
that when these large droppers drop onto the mud flat ,
they are anchored in an upright position.
The plumule then gives rise to a new plant.
g. Adaptation to tile lower temperature HK
Distribution of mangrove plants is limited by low
temperature.
These plants are found only in tropical areas.
The size of the mangrove trees in HK rarely
exceeds 10 feet in height because of the lower
winter temperature.
In other areas, it may reach 100 feet.
h. Adaptations concerned with feeding in animals
1. Filter feeding mechanism in oysters, barnacles,
worms.
2. Periscopic eyes of the mudskippers.
This enables the animal to see its prey or enemy
when the rest of the body is submerged in the mud
or water. Its body colour is camouflaged against its
background.
i. Adaptation for attachment and
movement
In the mudskipper, the pectorial fins are
strong and used for jumping on mud.
While pelvic fins are modified to become a
sucker for attachment.
(E) INTERRELATIONSHIP AMONG
ORGANISMS
1. Trophic relationships (Draw a possible food web here)
large fish
small fish
starfish
whelk
barnacles
periwinkle
oyster
zooplanktons
algae
mangrove plants
2. Interactions (Give examples)
Predation:
whelk  large fish
Grazing:
algae  periwinkle
Filter feeding:
barnacle
intraspecific competition for food:
small fish
interspecific competition for food:
algae fed by both zooplanktons and periwinkle
interspecific competition for space, attachment sites:
oyster and barnacle
(F) Succession of plants
This can easily be observed in a Mangrove
habitat.
The pioneer plants (Av) are more tolerant to
the adverse environmental conditions.
Their growth makes the conditions better for
the other plants (K, Br, Ae, Ex).
These plants in turn make time soil better for
plants like Lum.
Finally, the back of beach plant can colonize
the area.
Succession of plants begin from the shore:
Lumnitzera
 Exoecaria
 Aegiceras
 Bruguiera
 Kandelia
 Avicennia (pioneer plant)
The different stages of succession can always be
seen as one goes from the seaward side to the
landward side, as the reclaming process
(succession of mangrove plants) slowly occurs
towards the sea.