Few studies have investigated the influence of herbivores on soil nutrient and plant rooting profiles in tropical forest ecosystems.  Depauperate rainforest soils are replenished by nutrient return through leaf fall and decomposition.  The land crab Gecarcinus quadratus (Gecarcinidae), however, alters the dynamics of this process through their nocturnal foraging and burrowing activities.  In Costa Rica’s Corcovado National Park, G. quadratus achieve densities up to six crabs per square meter in populations that extend along the Park’s coastline and inland for approximately 600 meters.  Living in single occupancy burrows that descend down to one meter, crabs emerge to selectively forage for fallen leaves within the crab-zone.

Collections of leaf-litter fall and experimental determinations of litter relocation rates of individual crabs were combined to develop an estimate of seasonal rates of litter relocation by the crab population.  Data suggest that G. quadrataus are capable of relocating up to 85.6% of the falling leaf-litter to their burrow chambers during the wet season and up to 63% in the dry season.  Experimental crab exclosures (25 m2) were established within a repeated-measures randomized block design to test for changes in leaf litter and soil organic carbon as a function of crab removal.  Exclosures collected an average (sd) of 5.6?3.9 times more leaf-litter than did control treatment quadrates over the two year study.  Excavated burrow chambers, from elsewhere in the crabzone, revealed leaf-litter collections of up to 11.75 g dry mass, a value 2.5 times greater than the average amounts of leaf-litter collected by one-m2 leaf-fall traps over seven days.  This extensive litter relocation by land crabs may partially explain the low organic carbon contents and rooting densities found in surface soils of the crab-zone relative to nearby crabless zone soils.  Crab-zone surface soils had 36% less organic carbon, 72% fewer fine roots, and 50% fewer very fine roots than crabless surface soils.  The relocation of litter by crabs to burrow chambers may also help to explain the more homogeneous rooting profiles found in crab-zones.  Twice as many fine roots and five times as many very fine roots were found in crab-zone soils at depths from 75 to 100 cm relative to nearby crabless zone soils.  After two years, exclosures had surface soil organic carbon values 18?8 (sd)% higher than control treatment soils.  Trend analyses of surface organic carbon changing over time as a function of experimental treatment, however, failed to reveal significant treatment effects suggesting that longer-term experiments may be required to detect crab-mediated changes in soil organic carbon.  After two years of crab exclusion, no treatment effects were detected for organic carbon content of soils from 32 and 72 cm depths.  Organic carbon content from burrow chambers, however, was 60% higher than the value predicted by chamber depth alone and not significantly different from surface values, demonstrating the capacity of land crabs to create isolated subterranean pockets of elevated organic carbon at the burrow chambers.  Very fine rooting densities were also two times higher in soils associated with burrow chambers relative to same-depth soils not associated with burrows.  Data presented in this study suggest that Gecarcinus quadratus is capable of altering leaf-litter distribution patterns and potentially the distributions of both soil organic carbon and plant roots.