Direct and indirect effects of the land crab Gecarcinus quadratus (Gecarcinidae) on seedling density, organic carbon distributions and rooting profiles in Corcovado National Park, Costa Rica.

The land crab Gecarcinus quadratus (Gecarcinidae) lives in densities exceeding 10,000 adults ha$\sp{-1}$ in the coastal forests of Corcovado National Park, Costa Rica. Crabs, living solitarily in half-meter deep burrows, forage nocturnally, transporting plant propagules and fallen leaves to subterranean chambers. Experimental manipulations, including exclusion and inclusion plots, were established from 1994 to 1996 to test whether crab foraging decreased seedling density and surface litter accumulation, and affected distributions of soil organic carbon and root densities. G. quadratus populations decreased seedling densities. After two years of experimental crab exclusion, seedling (3-25 cm tall) densities were 144% higher than control densities. Smaller ($$25 cm) seedling densities were unaffected. In preference tests, crabs killed transplanted seedlings originating from outside the crab-zone (foreign) ten to one over seedlings originating from within the crab-zone (local), indicating additional selectivity. G. quadratus significantly decreased forest leaf-litter mass by relocating up to 85% of fallen leaves belowground. During two years of crab exclusion, 5.6 $\pm$ 3.9 (sd) times more leaf-litter accumulated in exclosures relative to control plots (in the wet season). Dry season exclosures, when crabs were inactive, accumulated 1.5 $\pm$ 0.3 times more litter than controls. After two years, exclosure surface soils contained 18 $\pm$ 8% more organic carbon than controls, although trends of changing surface carbon over time suggest no treatments effects. Sampling of organic carbon from deeper soils failed to reveal treatment effects. Sampling of litter, soil, and roots from excavated burrow chambers, however, reveal that crabs store decomposing litter and create subterranean pockets containing 60% more carbon and 96% greater very-fine root density than same-depth soils unassociated with burrows. Crab litter relocation may also indirectly affect seedling survival. Foreign and local seedlings transplanted into the crab-zone but protected from crabs survived better in litter conditions mimicking those of the seedlings' origins (litter-filled: foreign; bare-ground: local). By selectively consuming seedlings and altering leaf-litter distributions, Gecarcinus quadratus demonstrates that a single faunal population can simultaneously create top-down and bottom-up directional forces. The relevance of these bi-directional influences on plant community composition is discussed.PhDBiological SciencesBotanyEcologyZoologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130834/2/9811192.pd