Effects of Associated Fauna on Growth and Productivity in the Red Mangrove

Abstract

The ability of herbivores to influence growth in marine plant communities is well documented. However, the role of nonherbivorous fauna in directly affecting growth and productivity in marine plants has only recently begun to emerge. The boring isopod Sphaeroma peruvianum and encrusting barnacle Balanus spp. are closely associated with prop roots of the red mangrove Rhizophora mangle on the Pacific coast of Costa Rica. This study first examines the impact, both spatially and temporally, of these two invertebrates on the growth and productivity of mangrove prop roots. Second, the ability of benthic or water column predators to mediate the extent of the faunal impacts on growth and production of prop roots is assessed. Roots were tagged and monitored seasonally for isopod and barnacle inhabitation and settlement. Experimental caging techniques established that the presence of burrowing isopods on prop roots can cause a 50%, and of encrusting barnacles, a 30%, decrease in root growth rate and a 62% (with isopods) and 52% (with barnacles) decrease in net root production. The magnitude and spatial extent of these effects are mediated indirectly by predation. Aerial roots not yet in contact with the substrate are susceptible to isopod and barnacle recruitment. Aerial roots in contact with the substrate (ground roots) are free from encrusting barnacles primarily because of the foraging activity of a benthic predator, the hermit crab Clibanarius panamensis, which climbs these roots during high tide to forage indiscriminately on newly settled barnacles. Isopod absence from lateral roots that originate from ground roots (lateral—G roots) is explained best by predation rather than dispersal barriers. Isopods burrowed and survived for extended periods in lower lateral—G roots when predators were excluded. Isopods are probably excluded from older roots that have entered the substrate because they cannot burrow into the tough, woody tissues of these roots.