Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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Animals that occur in both mangrove and neighboring terrestrial forests and swamps are within the second category, and this fauna includes mostly insects, birds, and mammals. These include water rats, bandicoots, bush pigs, wallabies, sugar gliders, possums, bats, and birds such as the Magpie Goose, cassowary, and Brush Turkey (Appendix 8.3). One of the most complex ecological relationships in this category is that between Philidris ants and epiphytic myrmecophytes, Hydnophytum moseleyanum, in mangrove forests in northern New Guinea (Maeyama and Matsumoto 2000). Both Philidris ants and the epiphyte are common throughout the forests of New Guinea, also occupying mangrove trees. The relationship is mutualistic. The epiphyte gets sustenance by absorbing nutrients from the detritus stored inside tree cavities by the ants, and the ants obtain honeydew secreted by scale insects attracted to the shoot tips of the host mangrove tree.
The third category, strictly estuarine and marine organisms, enter mangrove creeks and waterways on the rising tide and depart as tides recede. Fish are the predominant animals in this category. As Haines (1983) describes for the Purari delta, few fish species are confined to any one zone but many species are confined to a range of zones with species of the same or closely related genera replacing each other along salinity gradients. Some species are wide ranging, such as the Archer Fish (Toxotes chatareus). The Saltwater Crocodile (Crocodilus porosus) lives only in the seaward limits within waterways, while the Freshwater Crocodile (Crocodilus novaeguineae) occurs in river waters upstream.
Some marine invertebrates, such as the penaeid prawns (Table 5.4.6) and the Giant Freshwater Prawn (Macrobrachium rosenbergii), use the mangroves as nursery grounds during their early life stages. The most abundant and widespread species of commercial importance is the Banana Prawn (Penaeus merguiensis). This species is more abundant in regions of high salinity but it exhibits the life cycle typical of all penaeids. During the first stage, planktonic post-larvae settle in estuaries where they feed and grow until adolescence. An oceanic stage begins when the prawns emigrate from the estuaries to coastal waters where, after a period of growth to adulthood, they move into deeper waters to spawn. Penaeus merguiensis, Metapenaeus demani, and Metapenaeus eborancensis spawn in waters 10–20 meters deep, whereas other species such as Metapenaeus ensis and Penaeus semisulcatus spawn in waters up to 60 m deep. The larvae then move shoreward via currents, initially settling in the headwaters of mangrove-lined creeks and waterways. Juveniles and adults congregate in these river mouths before migrating out to sea. The cycle is continuous throughout the year.
Food Web Dynamics
Although little if any information exists on the trophic ecology of New Guinea mangroves, it is assumed that the dynamics of food webs in Papuan mangroves are similar to those in other tropical mangroves. Mangrove links with coastal fisheries have received a lot of attention, but the food webs of mangroves are mostly detritus-based, with most trophic activity focused on interactions among fauna either directly or indirectly through consumption of tree material such as leaves, flowers, propagules, wood, bark, and roots (Robertson, Alongi, and Boto 1992).
Direct grazing on mangrove tissue, mainly by insects and arboreal crabs, generally constitutes a small proportion of energy flow. More recently, evidence of the trophic importance of algal food resources in mangrove ecosystems has emerged, demonstrating that a number of key faunal groups depend on phytoplankton, benthic microalgae, or macroalgae growing on above-ground roots and other tree parts, for food. From a nutritional perspective, algae are a better food than detritus derived from mangrove trees because of easier digestion and relatively higher nitrogen content.
Various species of mammals, insects, and birds permanently or temporarily reside in some mangrove canopies. The feeding ecology of mangrove-associated birds is fairly well understood. Bird communities can be spatially and trophically complex and include up to eight feeding guilds: granivores, frugivores, piscivores, aerial hawkers, and hovering, gleaning, fly catching, and bark-foraging insectivores (Kathiresan and Bingham 2001).
On and beneath the forest floor, crabs are generally the keystone group driving food webs. Sesarmid crabs (Grapsidae) are the most conspicuous organisms, but fiddler crabs (Ocypodidae) are also abundant, being highly efficient consumers of benthic microalgae. Recent work throughout the world has shown that large proportions of leaf and other litter deposited on the forest floor is consumed or hidden underground by crabs (Kathiresan and Bingham 2001). This pathway has profound effects on energy and carbon flow within mangrove forests, as the quantities of material available for export from forests are reduced, and the cycling of nutrients to support forest primary production is enhanced. Material that is consumed or hidden by crabs underground must eventually be decomposed by microbial communities in the sediments.
The major pathway of trophic dynamics in mangrove sediments is via detritus/ algae to microbe to crab. This is, of course, an overly simplistic depiction of fairly complex interrelations among bacteria, fungi, protozoa, nematodes and other worms, algae, detritus, crabs, and other invertebrates (Figure 5.4.7). In mangrove waters, large swimming organisms, such as fish and prawns, are at the apex of a fairly complex food web in which ‘‘the microbial loop’’ forms a crucial part (Figure 5.4.7). The ‘‘microbial loop’’ is fuelled by the dissolved exudates of phytoplankton, especially from those algal cells broken up by ‘‘sloppy feeding’’ zooplankton. The rates of microbial activity in mangrove waters are thus tightly linked to rates of phytoplankton production.
In New Guinea waters, rates of primary production vary greatly depending on the extent to which suspended particulate loads and tides affect turbidity and the availability of light. In the Fly delta, phytoplankton production is highly variable, with rates depending greatly on water clarity (Robertson et al. 1993; Robertson, Dixon, and Alongi 1998). Inside the delta where waters are most turbid, rates are low, ranging from 22 to 95 mg C per m2 per day. At the delta mouth where waters are deeper and less turbid, rates were considerably higher, ranging from 188 to 693 mg C per m2 per day. Rates of bacterial production mirror those of the phytoplankton, suggesting a close trophic link. Zooplankton biomass can be highly variable, weakly correlating with phytoplankton biomass but most often associated with large pieces of mangrove debris floating down river. A similar trophic connection exists in the Purari delta, where phytoplankton production is low in turbid waters but microbial activity is high (Pearl and Kellar 1980). In Indonesian man-grove waters, rates of phytoplankton production are most often light-limited (Soemodihardjo 1987).
Figure 5.4.7. A conceptual model of food webs within mangrove forests and in adjacent waterways, dominated by trees, crabs, and ‘‘the microbial loop.’’
A complex consortium of microbes is responsible for colonizing and decomposing organic particles, including algal cells, and being the food for many larger planktonic organisms, such as larval invertebrates. Unfortunately, actual rates of trophic transfer from microbes to zooplankton are unknown for Papuan and Papua New Guinea waters. Larger animals such as birds and crocodiles, although highly conspicuous, generally do not play a major role of mangrove energy flow.
In the Indo-West Pacific region, most mangrove forests occur in estuaries or as dense forests with intersecting tidal waterways in relatively protected embayments, and have a high proportion of forest to open water. Within such habitats, man-grove vegetation is likely to be the dominant contributor to food webs. Work using stable isotopes confirms that many consumers in mangrove habitats have an isotope signal close to that of mangrove tissue (e.g., Rodelli et al. 1984). In more open mangrove habitats, such as fringing mangroves with open canopies, algae appear to be more important as a food source (Bouillon et al. 2002).
Mangrove waterways are often dominated by zooplankton and fish, with densities usually greater than in adjacent habitats. It is generally believed that the higher numbers of organisms in mangroves compared with adjacent habitats is a reflection of greater availability of food, as well as the increased availability of refugia from large predators. In one of the more detailed surveys of fish and their feeding relationships, Haines (1983) found that the fish fauna of