Processing of mangrove litter by crabs in north Queensland

A site near Ingham (LA) and two sites near Townsville (LB and LC) in north Queensland, were chosen for a study of mangrove litter dynamics, between November 1986 and November 1987. The mangrove forest at site LA was a mixed forest dominated by Avicennia marina, while LB was a mixed forest dominated by Excoecaria agallocha, Bruguiera exaristata and Lumnitzera racemosa. Site LC was a nearly pure stand of Ceriops tagal.All three sites were 3m above chart datum and it was estimated that, during the twelve month study, tidal inundation of the sites occurred on 66 days at site LA and on 82 days at sites LB and LC. The mean annual air temperature in this region is approximately 25°C and rainfall is seasonal with highest falls in the period December to March. Litter fall (leaves, flowers, buds, propagules and stipules), was monitored by deploying six (1.25 m x 1.25 m) litter catchers under the forest canopy, per forest type. Close to each litter catcher a ground plot (1.25 m x 1.25 m) was marked out. Several days after spring tides had inundated each forest, litter fall was measured and at the same time the ground plot was cleared of litter, which was returned to the laboratory, dried (80° C for 1 week) and used to calculate the standing stock of litter. After 1 to 4 weeks, and always before the next tidal inundation of the forest, all litter was collected from the catchers and from the ground plot and the litter was sorted, dried and weighed. The difference between the weight of litter in each catcher and from the associated ground plot was the amount removed by leaf-eating crabs.Pit traps were used to assess crab community composition in each forest type. Three grids of nine traps were deployed over three days during a spring tide event. Estimates of burrow densities were also made.To demonstrate that crabs were responsible for removing litter from the forest floor, leaves were tethered randomly within each forest in November 1986, in a period between tidal inundations. A 1 m length of nylon twine was tied to the petiole of each leaf and the twine attached to a peg on the ground. Leaves were laid out in three groups of twenty and left for two weeks, after which the number of leaves down crab holes and the number of tethers and leaves which remained above ground (whether leaves remained intact or not) were counted. The annual flux of litter due to microbial decay was estimated by multiplying the mean instantaneous standing stock of litter in each forest type by the number of turnovers of litter per year, obtained from previous litter-bag studies of decomposition.Tidal export of freshly-fallen (floating) litter from all forest types was estimated by multiplying the number of days of tidal inundation occurring during each period of the year by the mean total litter fall rate at that time. These values were summed over the year for each forest type.In the period October-December 1987 short-term measurements of litter fall and leaf removal by crabs were performed once at each of three other sites, of similar species composition, to determine if the pattern of litter processing in each of the above forest types was independent of crab species composition. Avicennia (Site SA) and Bruguiera gymnorrhiza forests (Site SB) were chosen in Missionary Bay, Hinchinbrook Island, and a Ceriops forest (Site SC) was chosen on the Haughton River south-east of Townsville. Crabs were not trapped at these sites and the relative importance of species was assessed by observation.
 This study was undertaken to determine an annual budget for litter turnover in different high intertidal zone mangrove forests and to assess the contribution of mangrove crabs to litter processing.

Organization: Australian Ocean Data Network