The keystone role of leaf-removing crabs in mangrove forests of North Brazil

Principle factors which influence mangrove leaf litter turnover, in particular the role of leaf-removing crabs, were evaluated in a riverine mangrove site near Braganca (Para, North Brazil). Our special interest was focussed on the role of the leaf-removing crab Ucides cordatus. Leaf litter fluxes between the mangrove forest and the adjacent estuary were investigated by estimating the biomass and fate of leaf litter material and propagules. Vegetation is dominated by Rhizophora mangle, with Avicennia germinans trees, both up to 25 m high, found intermittently. During 1997, Rhizophora trees produced around 1.40 g DW m(-2) d(-1) of leave fall and 0.75 g DW m(-2) d(-1) of propagules. Leaf decomposition rates on the ground were about 0.06 g DW m(-2) d(-1), irrespective of species, habitat or site exposure. This amount accounts for < 3% of total leaf fall. Average leaf litter biomass present on the ground was 0.01 g DW m(-2) d(-1). When the mangrove forest was flooded (on average 10 days per month) the quantity of leaf litter and propagules washed out with the spring tide was 10 and 17 times greater than during neap tide. Nevertheless, tidal export and decomposition together made up less than 39 percent of annual leaf litter fall. The bulk of the remaining amount is apparently removed by Ucides. Each crab consumed about 1.30 g DW of leaf litter material and propagules per day. Since the average density of these crabs was 1.38 crabs m(-2), it is proposed that Ucides is a keystone species in Bragantinian mangroves.