Effect of boardwalks on the semaphore crab Heloeciuscordiformis in temperate urban mangrove forests

Abstract

Boardwalks are used in management of mangrove forests because they are thought to solve problems of access by people while promoting recreational and educational opportunities. Nevertheless, boardwalks and the people using them may have negative effects on plants and animals of the mangrove habitat. This study investigated the potential impact of a boardwalk on the distribution and abundance of the semaphore crab Heloecius cordiformis in a mangrove forest near Sydney (New South Wales, Australia). To examine the generality of these findings, the hypotheses tested at this location were tested in another mangrove forest in the same catchment. At each location, there were more H. cordiformis in areas near the boardwalk than further away. Also, density of pneumatophores, cover of leaf litter and proportion of root-material in the sediment were significantly greater away from than near the boardwalk and all these variables were significantly negatively correlated with the abundance of crabs. Experimental transplantations of orthogonal combinations of sediment, root material, algae and pneumatophores demonstrated that the crabs respond to the softer sediment associated with boardwalks by increasing in abundance. Changes in density of pneumatophores, cover of algae, density of roots and type of mud did not influence the abundance of H. cordiformis. Minimising the effects of future anthropogenic disturbances can be best achieved by understanding the processes responsible for existing disturbances of a similar type. Experimental studies can demonstrate which aspects of boardwalks influence the abundance of H. cordiformis. Such studies should be more widely used in environmental management.