Plant-Animal Interactions In Tropical Mangrove

Tropical mangrove forest ecosystems rely strongly on trophic interactions between plants and animals in controlling population, community and ecosystem level processes (Robertson 1988). Positive plant-animal interactions include litter processing, remineralization of detritus within forests and the removal of particulates from the forests to other near-shore habitats (Cannicci et al. 2008). There are also negative plant-animal interactions including herbivorous insect attacks on trees both directly and indirectly (Cannicci et al. 2008). Direct insect attacks are the removing of leaves from the trees and boring holes in the bark/trunks, and indirect attacks are the destruction of seeds preventing the propagation of the trees. A particularly influential group of species in tropical mangrove forests are Sesarmid crabs (Sesarmidae) (Robertson 1991). These crabs are strong post-dispersal predators of tree seeds and propagules, and also are a major consumer of leaf litter. Sesarmid crabs have both a positive and negative interaction with the plant species in mangroves; these crabs can affect the species distribution patterns both positively and negatively (Robertson 1988).

Sesarmid crabs affect the tree species distribution patterns within the mangrove system. The classic view of mangrove within-habitat zonation patterns of tree species focuses only on factors such as competition for light, tidal inundation and porewater (groundwater) salinities, with no consideration for animal influence (Watson 1928; Maenae 1968). Smith (1987a,b,c) challenged this view and found that Sesarmid crabs are important post-dispersal predators on mangrove seeds/propagules. These crabs affect the plant species distribution across the intertidal zone equally, only limited by the presence or absence of species in each zone of the system (Smith 1987a). There is strong variation in the strength of predation rates between plant species, with the highest predation pressures on those species with seeds that contained high sugar value and low tannin/fiber concentrations (Robertson 1991). The variation of seed predation rates on each species is also affected by the dominance of conspecific adult tree individuals (Smith 1987a). Predation rates were found to be highest where conspecific trees are rare or absent. A tree species may be present in some but not all zones in the ecosystem (Johnstone 1983). Seed/propagule predation by Sesarmid crabs is strongest in the areas where this tree species is not usually present (Johnstone 1983). This differential predation across the intertidal zone is a strong influence on the mangrove species distribution patterns (Janzen 1970; Clark & Clark 1984).

Mangrove system function is heavily affected by litter consumption rates, and the main food item of Sesarmid crabs in mangroves is leaf litter (Malley 1978). This makes Sesarmid crabs a major factor on litter processing. The crabs consume the leaves on the surface as well as carrying them down into their burrows and consuming them there, which can result in the scarcity of leaves on mangrove forest floors (Maenae 1968). The removal of leaves from the surface and burying them in burrows allows for microbial decomposition to occur. It has been found that these crabs have a significant quantitative impact on the amount of litter processed and removed from the system (Robertson 1986; Robertson and Daniel 1989a). This in turn affects the system-level processes of decomposition and remineralization of the detritus/litter, and tidal removal of particulates to near-shore systems (Robertson 1991). Low and mid intertidal zones are affected less by the crabs than are high intertidal zones in terms of litter processing (Robertson 1988, 1989). Low and mid intertidal zones have about 25-28% litter removal by crabs, and about 71% by tidal export (Robertson 1991). High intertidal zones conversely have about 79% litter removal by crabs and about 25% by tidal export (Robertson 1991). The litter processing by crabs is important in affecting system function of mangroves in that it affects the amount of available nitrogen in the system (Robertson 1991). Nitrogen is required for primary production in these forests, and is contained in the leaf litter after the trees have shed them. If all the leaves were removed by tidal export, the nitrogen contained in those leaves would be removed from the system and the system would eventually be essentially depleted of available nitrogen and primary production would cease. The crabs decompose the leaf litter and recycle the nitrogen back into the system, continuing primary production. The available nitrogen controls the growth and productivity of the mangrove ecosystem.

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Sesarmid crabs affect tree species distribution and system function of mangrove forest ecosystems as an adult, land species. Mangrove seed predation and litter consumption and burial control which species can exist and where in the system they can grow, and also control how much the system can grow and continue to be productive. It has also been found that in their larval form Sesarmid crabs dominate the zooplankton in the waterways of the mangrove system, which serve as a main food source for fish that are using the mangroves as nurseries (Robertson 1988). This shows that Sesarmid crabs also play a key role in the “nursery ground” function of mangroves (Robertson 1988). There is a strong trophic connection between mangrove leaf litter, Sesarmid crabs, and juvenile fish (Blaber and Milton 1990). If the amount of litter is decreased, such as by removing high intertidal forests, a decrease in crab populations and in turn fish populations and productivity of the system can result. If the organic content in the water is increased, the larval form of the crabs can be killed off threatening the crab populations and again both the fish populations and system productivity would decrease (Wolanski and Ridd 1986). Sesarmid crabs and tidal inundation work in tandem to process, recycle, and remove leaf litter and particulate matter.

Sesarmid crabs are dependent on the productivity of the mangrove system; the primary production of seeds and leaves is necessary for the crabs to survive. The tree species are dependent on the crabs to recycle nitrogen into the system and allow the trees to grow and be productive. The crabs and the trees are codependent on each other, and when one is affected or limited the other is limited as well. Sesarmid crabs are a major factor influencing mangrove system function and structure. They are integral both on land and in the water, affecting both plants and other animals in the system.

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