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.

1986年11月至1987年11月间，研究团队选取昆士兰州北部英厄姆（Ingham）附近的LA样地，以及汤斯维尔（Townsville）附近的LB、LC两处样地，开展红树林凋落物动态（mangrove litter dynamics）研究。
LA样地的红树林为混合林，优势种为白骨壤（Avicennia marina）；LB样地同样为混合林，优势种为海漆（Excoecaria agallocha）、直瓣木榄（Bruguiera exaristata）和榄李（Lumnitzera racemosa）；LC样地则为以角果木（Ceriops tagal）为主的近乎纯林。
三处样地的海拔均高于海图基准面（chart datum）3米。据估算，在为期12个月的研究期间，LA样地的潮汐淹没（tidal inundation）天数为66天，LB与LC样地则分别为82天。该区域年平均气温约为25℃，降水具有季节性特征，12月至次年3月为降水高峰期。
针对每类林型，研究人员在林冠下布设6个尺寸为1.25m×1.25m的凋落物收集器（litter catchers），以监测凋落物输入（litter fall），监测对象包括叶片、花、芽、繁殖体以及托叶。在每个凋落物收集器附近，划定一块1.25m×1.25m的地面样方（ground plot）。
待大潮（spring tides）淹没林分数日后，开展凋落物输入量测定，同时清理地面样方内的凋落物，将其带回实验室后置于80℃下烘干1周，以此计算凋落物现存量（standing stock）。
每隔1至4周（且始终在下一次潮汐淹没林分前），收集所有凋落物收集器与地面样方内的凋落物，经分类、烘干后称重。每个收集器与其对应地面样方的凋落物重量差值，即为食叶蟹类移除的凋落物量。
采用陷阱诱捕法（pit traps）评估各林型的蟹类群落组成。在大潮期间，研究人员连续3天布设3组各含9个陷阱的网格样带。同时估算了蟹类洞穴密度（burrow densities）。
为证实蟹类是林地表层凋落物移除的主要动因，研究团队于1986年11月（潮汐间歇期）在各林分内随机开展叶片绑缚试验。将一段1米长的尼龙绳系于叶片叶柄（petiole），另一端固定于地面的木桩上。实验共分为3组，每组20片叶片，放置2周后，统计掉入蟹洞的叶片数量，以及仍留在地面的绳结与叶片（无论叶片是否完整）的数量。
通过将各林型的凋落物平均瞬时现存量，与此前通过凋落物袋法（litter-bag studies）得到的年凋落物周转（litter turnover）次数相乘，估算出由微生物分解导致的年度凋落物通量。
通过将一年中各时段的潮汐淹没天数，与对应时段的平均总凋落物输入速率相乘，估算出所有林型的新鲜掉落（漂浮）凋落物的潮汐输出（tidal export）量。将各林型的该数值按全年累加，得到年度总潮汐输出量。
1987年10月至12月期间，研究团队在另外3处物种组成相似的样地开展了短期凋落物输入与蟹类取叶量测定，以验证上述林型的凋落物处理模式是否与蟹类物种组成无关。
这三处样地分别为：欣钦布鲁克岛传教士湾的白骨壤林（SA样地）与木榄（Bruguiera gymnorrhiza）林（SB样地），以及汤斯维尔东南部霍顿河沿岸的角果木林（SC样地）。研究人员未在该三处样地布设蟹类陷阱，仅通过野外观察评估蟹类物种的相对重要性。
本研究旨在明确不同高潮间带（high intertidal zone）红树林的凋落物周转年度收支，并评估红树林蟹类在凋落物处理过程中的贡献。