The present study was carried out in the mangroves of Soure, Marajó Island, Pará, Brazil, where the great amount of freshwater intake allows the development of transition zones estuary/river defining the geobotanic limits for mangroves in the state of Pará and, hence, for the leaf-removing mangrove crab (Ucididae). For an ecological approach on the interaction between U. cordatus and mangrove forests the following issues were investigated: i) the relationship between the population characteristics of U. cordatus and different transition levels along the gradient of vegetation between mangrove forests and tidal valley forests, ii) the influence of environmental factors of such transition zones on patterns of size and density of U. cordatus population, iii) the impacts of herbivory of U. cordatus on the seedlings production and the possible effects on recruitment process of these seedlings for mangrove forests. Study sites were classified according to the transition level of the tidal várzea forests, and these values correlated with density and size values of U. cordatus population. In each study site the availability of food by litterfall rates, salinity, and indicators of fishing activities on these areas were determined. The impact of U. cordatus on the recruitment of mangrove stands was evaluated by estimating the rate of herbivory and predation on propagules. The results reveal that in the zone of "high transition" local conditions seem to limit U. cordatus stocks, showing that the density of this species population is much lower compared to those recorded in the "low transition" zone. However, areas of high transition offer more favorable conditions for development of the leaf-removing mangrove crab populations, especially with regard to the variability and availability of food and protection from human action in the region. The indicators of fishing activity on U. cordatus revealed that the mangrove forests of the lower transition zone are more subject to overexploitation, mainly for ease of access. The main via of impact on propagules was the consumption rate of 60%, being the rate of export of propagules by tides of only 1%, thus less relevant. U. cordatus may be considered the most important impact agent on the propagules production of these mangrove forests without showing selectivity by size or maturity of seedlings, being important in the regulation of recruitment rates and, hence, in the population dynamics of Rhizophora’s trees in the mangrove forests on the coastal zone of Marajó Island, in the Brazilian Amazon.
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