Mangrove Crab Ucides cordatus Removal Does Not Affect Sediment Parameters and Stipule Production in a One Year Experiment in Northern Brazil

Abstract

Mangrove crabs influence ecosystem processes through bioturbation and/or litter feeding. In Brazilian mangroves, the abundant and commercially important crab Ucides cordatus is the main faunal modifier of microtopography establishing up to 2 m deep burrows. They process more than 70% of the leaf litter and propagule production, thus promoting microbial degradation of detritus and benefiting microbe-feeding fiddler crabs. The accelerated nutrient turn-over and increased sediment oxygenation mediated by U. cordatus may enhance mangrove tree growth. Such positive feed-back loop was tested in North Brazil through a one year crab removal experiment simulating increased harvesting rates in a mature Rhizophora mangle forest. Investigated response parameters were sediment salinity, organic matter content, CO2 efflux rates of the surface sediment, and reduction potential. We also determined stipule fall of the mangrove tree R. mangle as a proxy for tree growth. Three treatments were applied to twelve experimental plots (13 m × 13 m each): crab removal, disturbance control and control. Within one year, the number of U. cordatus burrows inside the four removal plots decreased on average to 52% of the initial number. Despite this distinct reduction in burrow density of this large bioturbator, none of the measured parameters differed between treatments. Instead, most parameters were clearly influenced by seasonal changes in precipitation. Hence, in the studied R. mangle forest, abiotic factors seem to be more important drivers of ecosystem processes than factors mediated by U. cordatus, at least within the studied timespan of one year.

The influence of crab burrows on sediment salinity in a Rhizophora-dominated mangrove forest in North Brazil during the dry season

“…The result corroborates the findings of Pülmanns et al (2015) in the rainy season, when sediment salinity in "rooted" areas was also higher than in "gap" areas (27 and 31 in gap areas and rooted areas, respectively, at the end of the rainy season) Findings by Smith III (1987), demonstrating a salinity of 57.5 in areas with high amounts of aerial roots versus 55.2 in gaps, are also in concordance with the present results. A microcosm experiment by Pülmanns et al (2016), showed lower salinity in treatments with (41) than without (47) artificial burrows after 6 months (both treatments started with sediment salinity of 37.5 at the first centimeter),…”

Population connectivity and larval dispersal of the exploited mangrove crabUcides cordatusalong the Brazilian coast

“…This decapod crustacean is also considered a key species for the ecological balance within benthic communities of mangrove forests, and have the largest biomass among mangrove invertebrates ( Wolff, Koch & Isaac, 2000 ). The species has a strong impact on litter turnover, nutrients cycling, soil aeration and forest productivity ( Pülmanns et al, 2014 , 2016 ). Despite its importance to the ecosystem and abundance throughout the year, U. cordatus is currently on the Brazilian list of species threatened by overfishing ( Pinheiro & Rodrigues, 2011 ).…”

Interference competition as a key determinant for spatial distribution of mangrove crabs

“…However, we are also aware that our findings need to be corroborate by other field observations and trials. In particular, we recognise the importance of performing manipulative removal or exclusion experiments as historically done in other intertidal habitats [83], despite such an approach is very difficult or even impossible to be applied in mangroves with large sesarmid crabs [84, 85]. Finally, this study adds interference competition for food as a key factor in the complex interplay of biotic (predation, competition for space) and abiotic environmental factors determining the spatial and distribution patterns of mangrove fauna.…”