Penang’s Gorgeous Mangrove Ecosystems under Threat
By Dr. Shuhaida Shuib (School of Biological Sciences, Universiti Sains Malaysia)January 2022 FEATURE
THE COASTLINE HEADING towards Balik Pulau is fortunately lined with mangroves. This salt-tolerant plant species described as halophytes, thrives in Penang's intertidal environment, where the temperature and salinity gradient fluctuate.
Mangroves are among nature's most productive ecosystems. They function as carbon sinks, capturing the element in the form of surrounding trees and belowground biomass, and act as natural bulwarks against the devastations brought by tsunamis, tidal erosions and tropical storms.
Furthermore, the mangrove ecosystem provides, among other benefits, timber and charcoal, food, medicinal compounds and abundant fishing grounds.
Their ecological importance can also be viewed in the context of habitat complexity, defined as the presence of physical structures in an environment. The mangroves' complex, interwoven root systems, especially the Avicennia species, have pneumatophores or breathing roots.
The surface of the pneumatophores allows for gas exchange, a useful feature considering the often waterlogged mangrove sediment. Stilt roots growing out from the Rhizophora mangrove tree trunk provide support in the soft mud. Various-shaped tree trunks, different leaf morphologies and branches laden with fruits or flowers are also facets of this habitat complexity.
Some fish and invertebrate species use mangrove forests as nursery grounds and as their home throughout their life cycle. Macrofauna such as gastropods (snails) and crabs are among the common groups of invertebrates found here, and these make use of the mangroves' structural complexities by colonising the surfaces of pneumatophores, the underside of the leaves, in between macroalgae patches, and even the trees' trunks and other roots.
Crabs from the families Grapsidae and Ocypodidae are found in great abundance and diversity in the mangroves of Balik Pulau. They are small in size, often found living on or within the sediment. In a study conducted by Abdul Rahim (2018, unpublished data), approximately 16 crab species were discovered in the Balik Pulau mangroves, from Kuala Sungai Pinang and Pulau Betong. These are Sesarmidae, Varunidae, Dotilidae, Camptandriidae and Ocypodidae.
The most common crab species are Sesarmid crabs from the genus Perisesarma. The remarkable adaptative skills exhibited by mangrove crabs allow for the dominance and spread of this macrofaunal group across various intertidal zones, and in some cases, in disturbed mangrove areas as well. Any clearing of mangrove forests, with its cascade of negative impacts, invariably upsets the symbiotic relationship between habitat complexity and mangrove crabs.
The shelter function of this habitat complexity moderates the effects of severe environmental conditions and limits predation. The mangroves' canopy shades the crabs from heat during the day, and shields them from their natural enemies. Some crab species have also been known to hide among pneumatophore roots (where they also feed off the epiphytic algae that grows on them).
Mangrove Crabs as Bioindicators
There is a tendency, when assessing the health and status of mangroves, to focus only on its commercial macrofauna. Mangrove crabs are unpopular commercially, yet their activities profoundly influence productivity and the nutrient cycle in mangroves, food webs and sediment biogeochemistry.
Bioturbation occurs when crabs dig up sediment to create multiple burrows and extensive tunnel systems underneath the mangrove forest floor. The burrow constructions shed light on the species' ability to strategise to find the most suitable substrate and location for burrow placements. A closer inspection also reveals that the crabs prefer to build burrows near mangrove roots to limit collapse.
The bioturbation process disturbs the sediment by aerating and mixing its layers for oxygen penetration; this facilitates changes in mangrove sediment chemistry and microbial activity. Other important components of nutrient cycling in mangroves are its leaf litter and detrital matter. Sesarmid crabs primarily feed on mangrove leaves in varying stages of decomposition. Browned leaves called senescent leaves are preferred over freshly fallen ones since these have a higher nitrogen content. The latter are still collected, but in order to improve the palatability and nutritional content, they are left to age in the burrows before consumption. Faecal material from the crabs also becomes more available to other organisms.
Several species of mangrove crabs have also been observed to supplement their diet with algae, fungus and animal matter. Certain bigger-sized crabs feed on smaller-sized ones.
As bioindicators, these crabs are useful in monitoring mangrove health, made more pressing now with rampant deforestation activities for urbanisation and aquaculture farming. Houses in some areas are today even built with no buffer zones and stand immediately adjacent to mangrove forests.
More and more rubbish and plastic trash have started to collect on the mangroves' shores. Hermit crabs have been found seeking shelter among the plastic bags strewn haphazardly along the length of the coastline, with bigger-sized crabs trapped in the tangle of fishing nets. It is inevitable that maintenance of the mangrove's ecosystem functions will suffer from the decrease in crab activity and diversity.
The less tolerant species are likely to altogether disappear, and with it, their role in contributing directly to mangrove productivity. The more tolerant species may have a fighting chance, although in the long term, their dominance may negatively impact the overall interactions among other species living in the ecosystem.
To minimise the impact of diversity loss, conservation efforts such as replanting have been introduced to restore damaged mangrove forests, and to serve as a multidimensional device in addressing mangrove destruction and decline in species diversity, as well as socialising increased awareness and cooperation among the public, decision-makers and parties of interest.
But this by no means justifies the view that "what is cut down can be replanted". It is imperative to understand that the ecosystem processes and the carrying capacity of a natural mangrove are different from fragmented mangroves that are replanted. What was lost may be slow to recover or be irreversibly damaged. Restoring an ecosystem to a semblance of its original state may be an impossible feat.
Diversity loss is already happening, especially considering that most conservation initiatives focus on charismatic fauna or species with commercial economic value. A more extensive approach needs to be applied. Understanding the relationships between mangroves and their macrofauna and other organisms should be included in the planning for the sustainable use and future of Penang's mangroves.
Dr. Shuhaida Shuib (School of Biological Sciences, Universiti Sains Malaysia)
is a marine ecologist at Universiti Sains Malaysia. She studies the diversity of macrofaunal organisms inhabiting intertidal environments, and is currently monitoring the impacts of disturbance on mangrove crabs and gastropods on Penang Island.