3. Food Web Ecology of Soft Shores: A Brief Introduction
Coastal soft-bottom marine habitats such as mangrove forests, tidal flats and shallow sub-tidal sediments play a critical role in providing essential ecosystem services to coastal communities.
One of those services is to support coastal fisheries by acting as nursery grounds and feeding habitats for commercially important species. The fish supported by the productivity provided by their habitats, in turn, supply food to humans and wildlife species of high conservation values such as horseshoe crabs, birds and dolphins. The ability of these habitats to fulfil this supporting role is underpinned not only by their generally high productivity, but also their collective function as connected habitats that allow fish (hereafter including other coastal organisms such as crustaceans) to progress through various larval and juvenile stages in the life cycle, successively utilising the chain of connected habitats.
With consumption of locally generated foods, these life history stages perform a process of ‘trophic relay’ that results in effective transfer of energy from the shallow habitats progressively to support offshore fish production. This trophic connectivity is a critical component of the ecological connectivity that underpin the function and services of coastal marine habitats.
Lee SY, Primavera JH, Dahdouh-Guebas F, McKee K, Bosire JO, Cannicci S, Diele K, Fromard, F, Koedam N, Marchand C, Medelssohn I, Mukherfee N, Record S 2014. Ecological role and services of tropical mangrove ecosystems: a reassessment. Global Ecology and Biogeography 23:726-743
Food web dynamics, namely, the pattern of food production and their transfer among organisms, is a key function underpinning all ecosystems. The magnitude, direction and efficiency of energy transfer not only defines key functional elements of the ecosystems but also determines ecosystem properties such as stability and capacity for goods and services (e.g. fishery production) to mankind. As such, understanding the food web structure and dynamics are clearly vital in the conservation and management of our ecosystems.
Conventional approaches to conservation have focused on the survival of individual species, but it is now widely recognised that species survival must depend upon functional ecosystems. To this end, the International Union for the Conservation of Nature (IUCN), for example, has recently promoted a new major initiative called the Red List of Ecosystems to assess the vulnerability of ecosystems around the world.
The typical food web of Hong Kong's soft shores:
A mangrove is a shrub or a tree that grows in the coastal saline or brackish water in the tropics and subtropics. Mangroves occur in the intertidal region and have prominent root systems and physiology to withstand the daily rising and receding tides. Both fresh and fallen mangrove leaves and litter have been regarded as vital food sources to many organisms in coastal waters, especially tropical estuaries.
A mangrove associate is a plant that grows at the landward fringe of the mangrove forest and is seldom immersed by tides. Like the mangroves, the fresh and fallen parts of the mangrove associates are critical food sources to other organisms in the habitat.
The saltmarsh is a coastal wetland that is regularly flooded by saline or brackish water. It is built on waterlogged mud and peat, and is dominated by salt-tolerant herbaceous plants. Both the plants and the peat make up the large depository of organic matter that drives the habitat’s food web.
Seagrasses are flowering plants that inhabit the shallow depths of saline or brackish water. They are different from seaweed in that they have roots, stems, leaves, and produce flowers, fruits, and seeds. Few animals consume seagrass directly (such as the dugong and some marine turtles), but the dead and decaying parts of seagrasses are important food source to the marine invertebrates – clams, for instance.
Microphytobenthos (MPB) refers to the assemblage of photosynthetic, unicellular microalgae and cyanobacteria living on or near the surface layer of sediments. Diatoms make up a large part of the MPB. The MPB is a significant food source to many benthic organisms, such as crabs and gastropods.
Macroalgae are large photosynthetic, multicellular algae that occur in the marine environment. They have a plant-like appearance, but lack root and vascular systems, and do not produce flowers, fruits, and seeds. Macroalgae are the primary food source to herbivorous marine animals.
Phytoplankton is a collective term for all photosynthetic microalgae that are suspended in large water bodies. They drift along with the current due to limited mobility. Phytoplankton form the basis of the marine food web, as they are a major food source to many small organisms, including zooplankton and juvenile fishes.
Grazers are herbivorous organisms that feed on low-lying vegetation and algae. They are the key level of organisms that connect the primary producers and organisms further up the food chain.
Deposit feeders consume fragmented dead and decaying plant and animal matter, as well as the MPB by ingesting bulk sediment. In this way, they contribute to the process of decomposition and nutrient cycling. Deposit feeders are detritivores.
Filter feeders consume live and dead suspended organic particles in the water column by passing the water through specialized filtering structure. The suspended organic particles are such as detritus, animal waste, and plankton.
Predators are carnivorous animals that kill and consume other organisms. Predators play an important role in maintaining the balance of the ecosystem by controlling prey populations. Most of the seafood we consume are predatory species, e.g. shrimps, crabs and fishes.
Decomposers are organisms that chemically breakdown dead and decaying plant and animal matters and absorb nutrients from there. These organic matters are broken down into inorganic materials and released back into the environment. They play a vital role in the food web as recyclers.
Scavengers consume dead organisms caused by any factors other than predation. Similar to deposit feeders, they facilitate nutrient cycling but through consuming dead body.