Are grapsid crabs keystone species in tropical mangroves?

Grapsid crabs have been referred to as “keystone species” in the mangrove forests of tropical Australia and south-east Asia. We used experiments and sampling studies to test the effects of grapsids, primarily Neosarmatium meinerti, on ecological processes in the mangrove forests around Darwin Harbour. Although information on other grapsids is limited, there was clear evidence that N. meinerti, as a consumer, did have significant effects. It selectively attacks propagules of particular mangrove species, reduces the survival of seedlings in some assemblages and appears to reduce the amount of litter on the forest floor. Thus, N. meinerti appears to influence key ecological processes structuring these mangrove ecosystems, but the question of whether or not it is a “keystone species” requires more information regarding the strength of the interactions and effects, and determination of the necessity of the species to the ecological organization of the system