Mangrove Faunal Guilds And Ecosystem Function

Animal species co-occurring in mangrove forests can be separated into guilds characterized by the utilization of available resources (Ray and McCormick 1992). Fauna) guilds described in this section are basically resident species that exploit the habitat with different intensity in space and time, in contrast to the nekton guilds discussed below (Section 13.6). The utilization and exploitation of the mangrove habitat by faunal guilds, both resident and migratory, can contribute to the structure and function of mangrove ecosystems. The loss of faunal guilds, described below, may influence the ecological properties of mangrove ecosystems.

13.4.1 Crabs

Crabs are one of the most important animal groups contributing to the high biodiversity in mangrove ecosystems. Moreover, it is not only their high species diversity but also their functional role that make crabs a fundamental component in the ecological diversity of mangrove ecosystems. Crabs play a central role in the structure and energy flow of these coastal forested wetlands (Michcli et al. 1991) as well as influencing the structure (Warren and Underwood 1986) and chemistry (Smith et al. 1991) of mangrove soils. These roles are accomplished by predation on mangrove seedlings (Smith 1987; Smith et al. 1989), facilitating litter decomposition (Robertson and Daniel 1989), and formation and transfer of detritus to predator food chains (Malley 1978; Jones 1984; Camilleri 1992).

There are about 4500 species of crabs, and they are the largest part of the decapoda. Six of the 30 families of the Brachyura are present in mangrove ecosystems (Mictyridae, Grapsidae, Geocarcinidae, Portunidae, Ocypodidae, Xanthidae), which include an estimated 127 species (Jones 1984). Eighteen of 19 genera occur within the Ocypodidae and comprise at least 80 species. In general, the mangrove crab fauna is dominated by representatives of two families, the Ocypodidae and Grapsidae, and each family by one genus, Uca and Sesarma. respectively. Furthermore, within the Grapsidae the genus Sesarma accounts for over 60 species of crabs predominantly associated with mangroves (Jones 1984). For example, the Indo Malayan region provides the richest zone with 30 species of Sesarma, then cast Africa (9-16), followed by Australasia (8-14) and tropica! America (3-5).

Although the high diversity of crabs and its potential effect on the productivity of mangrove forests has long been recognized (Macnae 1968; Malley 1978), there is little quantitative data on community structure, population dynamics and the ecological interactions between crabs and mangrove litter production. For example, Macnae (1968) correlated the scarcity of leaf litter in Malaysian mangroves with crab consumption, while Malley (1978) and Leb and Sasekumar (1985) provided evidence through gut content analyses that mangrove leaf litter was consumed by Sesarminae crabs, Chiromanthes spp. (Lee 1989). This pattern of litter consumption has also been observed in the majority of the genera Cardisoma, Goniopsis, Ucides and Aratus (Jones 1984). Since crab densities in mangrove forest can be high, crabs may play an important role in leaf litter decomposition and transport to adjacent estuarine waters. Indeed, studies in Malaysia, Jamaica, South Africa, Kenya, India and Puerto Rico show that the crab density may be as high as 63 individuals per m2 (Jones 1984).

The crab community can have significant effects on pathways of energy and carbon flow within the forest, the quantities of organic material available for export from forest, and the cycling of nitrogen to support forest primary production (Robertson 1991; Robertson et al. 1992). Malley (1978) found that the contents of the proventriculus and rectum of the sesarmid crab Chiromanthes onychophorum, a common crab species in mangroves in Malaysia, consisted of more than 95% mangrove leaf material by volume. The first quantitative estimates of litter consumption by crabs was between 22 and 42% of the daily leaf fall (mean 28%) depending on the time of year (Robertson 1986). These rates showed that leaf-burying crabs were a major link between primary and secondary production within mangrove forests in northeastern Australia. Emmerson and McGwynnc (1992) found that leaf litter was the major component in the diet of the crab Sesarma meinerti, a dominate specics in the mangroves of south Africa, and estimated that 44% of Avicennia marina leaf fall was consumed by this species. Leh and Sasekumar (1985) calculated that in Malaysia two sesarmids, Chiromanthes onychophorum and Chiromanthes eumolupe, could remove ~9% of the annual leaf fall from mid-intertidal Rhizophora forests and up to 20 -30% of leaf fall in high intertidal forests. Similarly, Robertson and Daniel (1989) reported that sesarmids removed 71% and 79% of the total annual litter fall from the forest floor in mangrove forest dominated by Ceriops tagal and Bruguiera exaristata, respectively. Yet only 33% was removed in an Avicennia wari«i7-dominated forest.

Leaf processing by crabs can also be responsible for litter turnover rates that are >75 times higher than the rate of microbial decay. Micheli et al.

(1991) found that crab leaf removal (14 g m~2 day ') was much greater than any previous measurement of litterfall in mangrove environments. Lee (1990) estimated that 40% of particulate organic matter produced by the mangrove Kandelia candel and the reed Phragmites communis was consumed by crabs. He emphasized that since tidal inundation in this mangrove forest was infrequent, crab consumption may be enhanced by the long residence times of organic matter on the forest floor. Lee (1989) also observed that crabs from the genus C'hiromanthes were capable of consuming >57% of the litter production by the mangrove Kandelia candel in a tidal shrimp pond. However, Emmerson and McGwynne (1992) stressed that the feeding behavior and feeding rate for each crab species should be known accurately in order to minimize overestimates of litter processing.

Camilleri (1992) demonstrated that crabs, among other invertebrates, break down whole senescent mangrove leaves lying on the mud, thus providing particulate organic matter (POM) for at least 38 species of detriti-vores and forming a primary link in the marine food web inside the mangrove forest at Myora Springs on Stradbroke Island, Australia. He showed that 12 species of leaf shredders manufactured small detrital particles from mangrove leaves that are consumed by at least 38 other species of invertebrates. Therefore, leaf fall from mangrove trees provided food for about 50 species of invertebrates in the mangrove forest. Camilleri

(1992) listed five reasons why species that shred whole leaves into small particles are significant in mangrove ecosystem: (1) they prevent mangrove leaf material from being washed out of the forest; (2) they make POM available as a food source to detritivores which feed on fine POM; (3) tbey regulate the size of POM in the environment; (4) they stimulate the colonization of POM by microfauna and microorganisms making nutrient available to trees; (5) they simplify the structure and chemical composition of detrital particles and that can facilitate degradation by microbial organisms.

In addition to the impact crabs have on organic matter export and decomposition. they may also affect forest structure and species composition along the intertidal zone by consumption of mangrove propagules (Smith 1987, 1988; Smith et at. 1989; Osborne and Smith 1990). Caging experiments in northern Queensland, Australia, showed that Avicennia marina propagules can survive and grow when they are protected from crabs (Smith 1987). Crabs consumed 100% of the post-dispersal propagules of mangroves in Australia mangrove forests, especially of the genus Avicennia. In both Malaysia and Australia graspid crabs composed > 95% of the predators on propagules (Smith 1992). As seed predators, graspid crabs can control where some mangrove species become established in the forest, as shown in southeast Asia, North and Central America, and Australia (Smith et at. 1989). Rased on these studies. Smith (1992) proposed that predation on propagules can influence succession in north Queensland mangrove forests in Australia. Recent studies in Africa (Micheli et al. 1991) also showed that propagule consumption can have an impact on species distribution.

On a global scale, Smith (1992) reviewed current data on propagule consumption by crabs in the New and Old Worlds. He pointed out that in Queensland, Sesarmids remove up to 80% of annual leaf fall (Robertson and Daniel 1989) and 75% of the propagules (Smith 1987) from the forest floor, whereas in Florida and Panama crabs have been indicated as minor consumers of forest primary production (Smith et al. 1989). These differences among continents suggest that the effects of invertebrate biodiversity on ecological function is not consistent globally, since crabs do not play an important role in the structure and function of mangroves in Florida. However, recent studies in Ecuador (Twilley et al. 1993) showed that mangrove crabs, Vcides occidentals, can influence the fate of leaf litter in the Churute Ecological Preserve.

Smith et al. (1991) have proposed that sesarmid crabs represent keystone species since they exert a major influence on mangrove ecosystem functions. For example, they found that sesarmid crabs have an impact on soil ammonium and sulfide levels, and as a consequence on forest productivity and reproductive status. This effect on nutrient cycling has also been reported in high intertidal forests in other mangrove forests in Australia, where between 11 and 64% of nitrogen requirements for forest primary production is recycled through litter processed by crabs (Lee 1989). Along with evidence by others demonstrating the effect of crabs on carbon cycling and forest structure (e.g. Robertson 1986; Robertson and Daniel 1989), Smith et al. (1991) concluded that crabs occupy a keystone position in Australian mangrove forests. Given the lack of extensive data on crab communities (Michelli et al. 1991), it is not clear if indeed crabs are also keystone guilds regulating forest development and productivity in other mangrove ecosystems. Yet current data strongly suggest that crabs play an important role in maintaining a high biodiversity that is linked to significant ecological functions in mangrove ecosystems (Camilleri 1992).

13.4.2 Insects

The ecology of insects in mangrove ecosystems is poorly understood, including inadequate records of their distribution and few studies of their ecological function. One of the more thorough treatments of the subject is by Feller (1993), which includes examples of unpublished records of insects in mangroves of Belize, and summaries of leading hypotheses describing the role of herbivory in mangrove ecosystems. In her own work, Feller tested the importance of soil resource availability on the pattern of herbivory in oligotrophic dwarf mangrove forests on an island in Belize.

The occurrence of insects in mangrove forests may be higher than previously considered (Ruetzler and Feller 1988; Feller 1993). A thorough inventory of insects on small mangrove islands in the Florida keys uncovered 200 species (Simberloff and Wilson 1969). In Belize, the hollow twigs of R. mangle host more than 70 species of insects, including at least 20 species of ants (Lynch, unpublished data, from Feller 1993), while Farnsworth and Ellison (1991, 1993) have identified more than 60 species of folivores feeding on Rhizophora and Avieennia. The research effort in Belize has uncovered many undescribed species of xylophagous insects (half of the 35 species arc new) and shore flies (many of the 50 total species were previously undocumented). Insects habitats are diverse in mangroves, including not only the leaf surface of the canopy, where inventories of species are more common, but also in less obvious sites within twigs, trunks and prop roots of the trees. In general, studies of herbivory of these and other diverse insect guilds have found that they can influence ecological processes such as root branching that enhances tree support (Simberloff et al. 1978), girdling of branches and trunks that causes formation of forest gaps (Feller, 1993, and unpublished material discussed), premature leaf abscission that changes nutrient recycling in the forest canopy (Onuf et al. 1977), and predation on mangrove seedlings (Rabinowitz 1977; Robertson et al. 1990). However, for each study promoting a causal mechanism of herbivory to an ecological function in mangroves, there is a study that contradicts the effect (e.g. Johnstone 1981; Laccrda et al. 1986; Ellison and Farnsworth 1990, 1992). In addition, there are also accounts that insects are a minor component in the ecology of mangroves (Janzen 1974; Huffakcr et al. 1984; Tomlinson 1986). It is not presently clear how changes in the biodiversity of insects can influence the function of mangrove ecosystems; however, it is evident that more ecological studies of these guilds are needed to define their role in particularly oligo-trophic mangrove ecosystems.

Mangrove rookeries (bird nesting sites) are enriched in nitrogen and phosphorous which stimulate the productivity of mangroves by a factor of 1.4. The density and diversity of herbivores is greater on mangrove island rookeries compared with proximal islands that lack nutrient enrichment (Onuf et al. 1977). The increased herbivory by several folivorous caterpillars and scolytid beetles on mangrove rookeries apparently maintained a constant standing crop, despite the increased rate of production. The enhanced growth rate of several herbivores and other fauna on the nutrient-enriched islands suggests that resource utilization may limit population size on the control (unenriched) islands. However, the control sites were of different size and distance from the mainland, confusing the linkage of nutrient enrichment to specific ecological processes. Other studies have found little enhancement of herbivory with increased levels of nutrients in either mangrove soil or leaves (Robertson and Duke 1987; Farnsworth and Ellison 1991; Feller 1993), although responses in herbivory may differ among benign and stressed environments, particularly related to hydroperiod (Feller 1993).

One of the largest scale manipulations of mangrove ecosystems was an experimental study of eight small mangrove islands in Florida Bay specifically to test biogeographic hypotheses of landscape richness (Simberloff and Wilson 1969; Simberloff 1976a). Mangrove islands of different size (264-1263 m2) and distance from source populations (2-432 m) were defaunated by fumigation with methyl bromide, and repeated censuses of animal species were used to determine immigration and extinction processes. Censuses were retaken on islands reduced in area of habitation to test the factors that control equilibration processes on these islands. The studies showed that biodiversity of animals was directly controlled by the size of the island, and the studies allowed for direct determination of extinction rates. Simberloff (1978) also summarized from these studies that community structures in these environments were not deterministic, but were largely controlled by the physical environment together with individual adaptations. While Heatwole and Levins (1972) attempted to relate the results of these mangrove island studies to interactions of trophic structure following disturbance, Simberloff (1976b) concluded that the significance of trophic interactions to the biodiversity of this ecosystem remains unproven, These studies do demonstrate the importance of island size and location (fragmentation relative to source materials) to the resiliency of biodiversity following disturbance in mangrove ecosystems. However, other tests have not demonstrated any relation in levels of herbivory to these dimensions of mangrove islands (Beever et al. 1979; Farnsworth and Ellison 1991). Studies with similar approaches are needed to link these patterns in species richness to specific ecosystem processes of mangroves.

13.4.3 Benthos and epibionts

One of the more comprehensive descriptions of the biodiversity (Alongi and Sasekumar 1992) and ecological function (Alongi et al. 1992; Robertson el al. 1992) of benthic communities is published in a book by Robertson and Alongi (1992) that describes the ecology of mangroves in Australia and surrounding regions. The meio- and macro-faunal diversity of the benthos are better documented than microbial communities (see Section 13.5), as represented by more than 100 species of macrofauna in some mangrove sites such as Thailand (Nateewathana and Tantichodok 1984), northwest Australia (Wells 1983) and southern Africa (Day 1975) (as reported in Alongi and Sasekumar 1992). Microhabitat diversity in the intertidal zone of mangroves is frequently cited as an important factor in the diversity of benthic communities (Figure 13.6), as has been found in temperate zones (.Frith el al. 1976). In addition, the increased habitat diversity afforded by

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