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.

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 H 2 S content: H 2 S 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. (B) Physical Factors

(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 BruguieraKandeliaAegiceras

Different kinds of droppers BruguieraKandeliaAegiceras Short & fat

Different kinds of droppers BruguieraKandeliaAegiceras Short & fatLong & tapering

Different kinds of droppers BruguieraKandeliaAegiceras Short & fatLong & taperingSmall, curved

Different kinds of droppers BruguieraKandeliaAegiceras Short & fatLong & taperingSmall, curved Red

Different kinds of droppers BruguieraKandeliaAegiceras Short & fatLong & taperingSmall, curved RedGreen

Different kinds of droppers BruguieraKandeliaAegiceras Short & fatLong & taperingSmall, curved RedGreen

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 Middle shore Lower shore Estuarine snail Estuarine snailEstuarine snail Periwinkle Mud snailStarfish Mudskipper MudskipperMudskipper 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.

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.

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). e. Adaptations to absorb and conserve water

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.

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. f. Adaptations for germination and dispersal of seeds

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) algaemangrove plants oyster starfish zooplanktons barnacles small fish large fish periwinkle whelk

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.