INTRODUCTION
Background
Generally, mangroves refer to highly adapted plants found in tropical intertidal forest communities. The term “mangrove” could have been derived from the Malay word “manggi-manggi”, for a mangrove tree Avicennia and the Arabic “el gurm”, for the same, as “mang-gurm”. Mangroves can be used to refer to a species, plant, forest or community.
Sungei Buloh Wetlands Reserve (SBWR) is a 87-hectare park officially opened on the 6th of December 1993. It includes Pulau Buloh and is situated in North-west of Singapore, between Kranji Reservoir and Sarimbun Reservoir. On 1 Jan 2002, 130 ha of Sungei Buloh was officially gazetted as a nature reserve. In the same year, it was recognized as a site of international importance for migratory birds.
The SBWR is home to a total of 248 native and naturalised vascular plant species (15 ferns, 1 gymnosperm, 233 angiosperms). Currently, majority of the flora are early successionals, native and exotic weeds. However, the original flora was very different and is mainly made up of coastal forest and mangrove species. The difference in species and introduction of numerous weeds show that the Man’s interference with the environment has had an impact on the integrity of the mangrove.
The most common trees in SBWR are the Avicennia alba, Bruguiera cylindrical,Rhizophora apiculata and Sonneratia alba. In the undergrowth, there are numerous shrubs of Acanthus ebracteatus and A. volubili. The more prominent climbers are the Finlaysonia obovata, and Derris trifoliata. Even endangered species, like the Cassine virburnifolia, are found here.
The SBWR also has very diverse fauna. Archer fish surface during high tides, while monitor lizards and migratory birds are sighted during the northeast monsoon. Purple and Grey herons live on the east of the mangrove where they nest and feed chicks at different times of the year. Otters from Malaysia have also been sighted frequently.
Adaptations
Combating Salinity
Mangrove trees are able to tolerate more salt in their tissues than other plants. Some plants like the Rhizophora, Bruguiera and Sonneratia have more effective ultra-filtration at the root level to exclude more salt. Salt is stored in old leaves, which are later shed.
The Avicennia and Sea Holly can tolerate high levels of salt in their tissues. Their sap can be up to one-tenth as salty as seawater. They then secrete the excess salt through special cells on their leaves.
Roots
Mangrove roots provide support in unstable soils so as to withstand currents and storms. Besides, they also help the plant to breathe to avoid suffocation in the oxygen poor mud. Mangrove plants develop aerial or air-breathing roots that take in air above ground.
Aerial roots have tiny pores on their surface to take in air. All aerial roots also contain large air spaces, which transport air and provide a reservoir of air during high tide. Roots for absorbing nutrients are tiny and emerge near the muddy surface.
The Sonneratia also produces such roots, but they are cone-shaped instead. The Bruguiera sends out knee roots that emerge from the ground then loop back in. The Avicennia has pencil-like roots to allow the absorption of atmospheric oxygen through specialized root cells known as lenticels. The Rhizophora has roots that branch from trunks like stilts for extra support and air absorption.
Most mangrove trees have narrow vessels that are densely and evenly distributed throughout the wood. Thus, they are able to withstand damage to the bark and outer trunk.
Seed Dispersal
Many seedlings have floatation devices to enable it to be dispersed a distance away from the parent plant. This is to ensure less competition between the parent plant and the seedling.
For some plants, the seed within the fruit starts to germinate while it is still on the mother tree, and the mother tree channels nutrients to the growing seedling. For other plants like the Avicennia, the growing seed breaks through the fruit wall only after the fruit falls off, allowing the seed to drop away more quickly in water of the right warmth and salinity. In others, the growing seedling breaks through the fruit wall to form a stem and sometimes even roots. The whole seedling is then called a propagule or a potential plant. In some trees, the seedlings only fall at high tide.
When the propagule falls, it floats horizontally initially and drifts with the tide. It can survive for long periods at sea. The tip is water absorbent while the top end is water repellent. After some weeks, the tip gradually absorbs water and the seedling floats vertically and starts to sprout its first leaf from the top, and roots from the bottom. When it hits land, it grows more roots and sprouts more leaves. It has a long stem to receive more sunlight and oxygen as seedlings are often completely submerged at high tide.
Loss of Water
Mangrove trees have to spend energy to get rid of salinity. Thus, mangroves have many water conserving features of desert plants. To minimise water loss through evaporation, plants may have thick waxy leaves or hairy leaves to trap an insulating layer of air near the leaf. They may also store water in succulent leaves.
Camouflaging
Animals living in the mangrove often have the ability to blend into the surroundings so that they are not easily preyed on. By camouflaging, animals can also prey on other organisms more easily. For example, the changeable lizard is able to change its colour to match that of the surroundings. The Yellow Bittern is also well camouflaged against the vegetation, and can be seen fluttering off immediately when disturbed.
Abiotic Factors
Soil
Mangrove soils are made up of marine alluvium, transported as sediment and deposited by rivers and the sea. Soils are made up of sand, silt and clay.
There are basically two types of topsoil sandy and clayey types. The lighter-coloured sandy topsoil is porous and facilitates water percolation and aeration during low tide. The darker-coloured clayey topsoil is less well aerated.
Subsoils, the soils below the surface, are typically waterlogged, have little aeration, and contain a lot of organic matter decomposing at a very slow rate.
Soil condition is one of the contributing factors of zonation among animals and plants. For example, the Rhizophora copes better with soft hurnus-rich mud while the Bruguiera favours stiff clay containing little organic matter.
PH Level
Dissolved calcium of shells and offshore coral make brackish waters alkaline. Mangrove soils are neutral to slightly acidic due to the sulphur-reducing bacteria, and the presence of acidic clays.
Oxygen
The amount of dissolved oxygen in mangrove waters is generally lower than that of the open sea. This low oxygen content further reduced in areas of organic pollution. The oxygen in the soil between sediments is used up by the decay and respiration of bacteria.
The oxygen content in the topsoil is replenished by the circulation of tidal water. In subsoils, the organic content and fine particle size of mud result in anoxic conditions, tolerated only by anaerobic bacteria that break down organic material without oxygen.
Nutrients
Nutrients are produced with imports from rivers and the sea. Rain regularly flushes out detritus from rivers to the mangrove, and the sea brings in dissolved and suspended organic matter as well as microscopic organisms that are consumed by filter feeders during high tide. The receding sea drains through soil, which acts as a sieve, leaving a layer of microscopic organisms deposited on the surface. These are grazed by the emerging terrestrial fauna during the low tide.
Winds and currents
The Northeast monsoon (Dec-Mar) is characterised by strong winds and heavy rainfall. The first intermonsoon period (Apr) has little wind. During the Southwest monsoon (May-Sep), as Singapore is blocked by the island of Sumatra, less precipitation is received in Singapore. The second intermonsoon (Oct-Nov) is much like the first, may bring heavy rain sometimes. Sea currents are generated mainly by the monsoon winds, and follow their lead.
Light, Temperature and Humidity
Mudflats are exposed to sunlight during diurnal low tides and become very hot and highly reflective, whereas the forest canopy shades the mangrove floor, keeping it cool. In 1997, the haze over Singapore caused by forest fires in Indonesia reduced light intensity considerably, thus lowering ambient temperatures in the mangroves.
Tides
In Singapore, high tides and low tides alternate twice a day. Singapore experiences predominantly mixed semi-diurnal tides. When the moon and sun align during a full or new moon every two weeks, the resultant spring tides are a high and low water spring tides. Mangrove forests grow between the mid-tide level and the highest high water spring tide.
Salinity
Common salt is the main dissolved solid in seawater, with the average salinity at 35 ppt. The degree of salinity may be categorised into oligohaline waters of low salinity (0.5-5 ppt), mesohaline waters of intermediate salinity (5-18 ppt) and polyhaline waters of high salinity (18-30 ppt). Specific readings of salinity within a mangrove may range from 0.5-35 ppt. This is due to the variation of tides.
Inside the mangroves, the influence of freshwater runoff from the land becomes significant, particularly during monsoons. Small streams in the mangrove are oligohaline and some are even freshwater. In the narrow mangroves, the effect of freshwater inflow is considerable.
Aim
We aim to find out whether the presence of visitors has had an impact on the integrity of the mangrove through a transect survey. We would also like to identify the relationship between flora or fauna distribution and abundance to existing abiotic factors. From our findings, we intend to suggest appropriate recommendations with regards to the status of public access to the boardwalk area. We would also like to propose strategies on how to balance visitorship and preservation of the mangrove, especially the part that contains the most diversity of species.
Hypothesis
We believe that with less human intervention with the mangroves, more mangrove species will be preserved and protected. Endangered species will be able to ensure its continuity for a longer period of time, and thriving species would be able to survive in better conditions. In this case, we are assuming that litter is the main source of contamination of mangrove swamps and that other factors, like diseases and release of toxic waste into neighbouring water bodies, are unlikely to occur.
We also believe that with the changes in abiotic factors, such as the humidity level, temperature, time, light intensity and type of soil, we would be able to see some form of transition of organisms from sea to land. We feel that various species thrive better in different abiotic conditions. Hence, as we move from sea to land, the diversity of mangrove species will change.
METHODOLOGY
For our experiment, we felt that the belt transect was highly ineffective for this case. The boardwalk covers a big area, ranging from the back mangrove to the land. If we were to carry out a belt transect, we would only be studying a specified area of the mangrove. In this case, it would almost be impossible to see the transition of mangrove species from the sea to the land. Hence, we ruled out the belt transect.
Consequently, we decided to carry out continuous line transects at every 5 metre interval. This was because the organisms are fairly sparsely distributed. In this method, we would not overlook any species along the line and thus provides more detailed information. Furthermore, this technique is much more efficient considering the limited amount of time we had to conduct the survey. Besides, this method allows us to see the transition of mangrove species and the change in abiotic factors from the sea to the land. Although the boardwalk is curved, the line transect could still be implemented as the purpose of this experiment is to see the transition of species from sea to land.
We took our first reading at Platform B and carried on taking readings at every 5 metre interval until we reached the ending point.
We used the data logger to measure the relative humidity, temperature and light of the area. First, we connected the humidity, temperature and light sensor to the Multi Log Pro. Then, we held the sensors away from us for 60 seconds. Next, we pressed the “Enter” button on the Multi Log Pro and recorded the data. We repeated these steps at every interval. We were careful not to drop any part of the data logger into the swamp and did not place the sensors directly under sunlight for it will affect the readings taken.
At every interval, we counted the number of species of Rizophora, Avicennia, Brugeria, Sea Holly, Mangrove Fern, Sea Hibiscus and crabs. Most importantly, we calculated the number of articles of rubbish at each interval. We did so by, first, counting the number of species to our left up to 5 metres away. Next, we counted the number of species to our right until the 5-metre mark. Besides, we recorded the type of soil at each interval and the time we reached each interval.
All these information were then tabulated into a table and graphically represented. The results were tabulated using the DACFOR scale, whereby the percentage abundance of a species at each interval is catergorised as dominant, abundant, common, frequent, occasional or rare.
RESULTS
Interval
22 * * *
21 * * *
20 * * * * * *
19 * * * * *
18 * * * * *
17 * * * * *
16 * * * *
15 * * * * *
14 * * * *
13 * * *
12 * * * * *
11 * * * *
10 * * * *
9 * * * * * *
8 * * * * *
7 * * * * *
6 * * * *
5 * * *
4 * * *
3 * * * * *
2 * * *
1 * * *
Rizophora Avicennia Crabs Brugeria Sea Sea Mangrove Rubbish
Holly Hibiscus Fern
Interval Time (am) Type of soil Interval Time (am) Type of soil
1 10.25 under water 12 10.54 under water
2 10.28 under water 13 10.57 water logged
3 10.30 under water 14 11.00 water logged
4 10.33 under water 15 11.03 water logged
5 10.35 under water 16 11.07 dry (clayish)
6 10.37 under water 17 11.10 dry (clayish)
7 10.42 under water 18 11.12 water logged
8 10.45 under water 19 11.15 water logged
9 10.47 under water 20 11.18 water logged
10 10.50 under water 21 11.20 dry (back mangrove)
11 10.52 under water 22 11.23 dry (back mangrove)
DISCUSSION
Flora
The Rizophora and Avicennia species are mostly sighted from intervals 1 to 12, meaning that they are situated in areas where the soil is underwater, the light intensity is low and relative humidity is around 60%. These plant species is most suited to live in areas nearer to the sea and under much shade.
The Brugeria species is very sparsely distributed and can be found in between intervals 12 to 14. This indicates that the Bruggeria thrives better in areas in between the land and the sea. This species also survives in locations where the light intensity is from 5 to 10 kilowatts and the relative humidity is around 65%.
The Sea Holly species is densely populated in intervals 7 to 12 and 15 to 20. This suggests that the Sea Holly survives better nearer to the land, where there is higher light intensity and higher relative humidity, and where the temperature is around 30C.
The Sea Hibiscus and the Mangrove Fern are most commonly found in intervals 14 to 22. This means that these species is more suitable to live in an environment with higher light intensity and relative humidity and temperature.
Fauna
Crabs are more commonly found near the sea, as there is more percentage abundance of crabs from intervals 1 to 12. This could probably be due to the fact the crabs’ source of food is located nearer to the sea. Crabs survive better when the light intensity is low and relative humidity is around 60%. This means crabs tend to live under shade, also seen by the empty crab holes they previously burrowed.
Rubbish
Rubbish is sighted at every interval. It is most commonly found in intervals 10 to 15, 20 and 21. There are fewer articles nearer to the sea as the SBWR has already put a barrier to minimize the litter from neighbouring water bodies entering the reserve. As for the intervals listed above, the articles are most likely generated by visitors to the reserve. The percentage abundance of rubbish in interval 21 is the highest compared to other intervals. This interval is also very near to the land, further suggesting that visitors could have dumped their unwanted things there before carrying on their journey. As for intervals 10 to 15, visitors could have dropped their handheld articles through the holes of the boardwalk’s platform, making it impossible to retrieve. Thus, these articles get carried away by the tide and end up at these intervals.
We also discovered a crab surviving on a styrofoam box, which is covered by a plastic bag. This scenario clearly suggests how litter affects the ecosystem. The crab may accidentally consume bits of styrofoam, which cannot be digested or decomposed. Other organisms may then eat the crab, swallowing the bits of styrofoam in the process too. When more of such cases appear, the amount of styrofoam in the system builds up so much that the organism eventually dies. This may lead to extinction of particular species. Hence, even though the rubbish may not be retrievable, it must be cleared in one way or another so that ecological balance is maintained.
RECOMMENDATIONS
We believe that further experiments should be done to re-evaluate the impact of human intervention against the mangroves. For example, the SBWR can limit the number of visitors each day so as to track how much litter these visitors generate. After a period of time, the SBWR can take a walk around the boardwalk to see any significant changes in the swamps and organisms. Also, the experiment that we have done should be repeated over a period of time so that more accurate results can be collated on the abiotic factors affecting the growth of various mangrove species in an area.
To conserve the reserve, we first have to ensure that the mangrove does not get littered. A rule can be imposed by the SBWR that all visitors have to dispose of all consumed drinks and food before going on the boardwalk. Other articles will have to be put in the backpack before proceeding.
Besides, as visitors proceed further into the reserve where the gaps in between planks of the boardwalk widen, we suggest that nets be put under the planks in case anything falls through the gaps. At least any litter can be retrieved easily by removing the net. This solution may not be as effective when there is high tide, as the tide may still wash away the articles in the net, but it can still be implemented temporarily. A better solution would the reconstruction of a new boardwalk where the planks are built closer to each other, closing up gaps. While the reconstruction is taking place, the temporary solution can be executed to minimize the rubbish on the reserve.
Furthermore, the SBWR can liaise with more schools to send students to the SBWR to clear up the mangroves. Students will in turn obtain their Community Involvement Programme (CIP) hours and perhaps a certificate showing their efforts in helping to maintain the cleanliness of the reserve. The SBWR should also address this situation to the Ministry of Environment in Singapore such that the management would be able to help with the liaison. Moreover, the management would be able to call out to all Singaporeans for volunteers to clean up the mangroves.
We highly encourage the SBWR to temporarily close down the back mangrove where there is more diversity of species. This allows time for people to clear up the litter around that area, and also ensures that species in that area is protected for a period of time. In this way, any endangered species in the back mangrove will be ensured of their continuity and thriving species will be ensured of better living conditions. In the meantime, visitors will still be allowed to visit the SBWR, but they will only access areas that are open. Following that, the back mangrove should be reopened in phases, while other areas should be closed. The SBWR should consider taking up this suggestion by constantly blocking access to various areas of the boardwalk until they feel that the diversity of organisms in the reserve has been conserved adequately. Additionally, viewing points can be created for visitors to observe areas that have been closed from a far distance. This solution will not significantly affect the number of visitors to the boardwalk. In the first place, not many tourists have the SBWR on their itenary. Besides, those who intend to visit SBWR will still be able to see the organisms in the mangroves.
