Changes in earthworm density and community structure during secondary succession in abandoned tropical pastures

Plant community succession alters the quantity and chemistry of organic inputs to soils. These differences in organic input may trigger changes in soil fertility and faunal activity. We examined earthworm density and community structure along a successional sequence of plant communities in abandoned tropical pastures in Puerto Rico. The chronological sequence of these plant communities were pasture, grass-vine-fern, shrub-small tree, and forest. Earthworm density was the highest in pasture (831 worms m−2 in top 0.25 m soil), decreased as secondary succession proceeded, and reached the lowest (32 worms m−2) in the forest. Whereas only soil feeding Pontoscolex corethrurus was present in the pasture and grass-vine-fern communities, both soil and litter feeding worm species were found in the shrub-small tree and forest communities. Ground litter biomass correlated negatively with earthworm density. Soil water content differed slightly among the successional communities, but was unlikely to play an important role in triggering differences in worm density among these abandoned lands. Soil pH values did not differ along the successional changes. These results suggest that decrease in earthworm density and increase in worm community diversity during secondary succession may result from changes in the chemistry of organic inputs, rather than in soil properties and litter quantity. We conclude that successional development from grass-dominated pastures to woody species-dominated forests reduces earthworm density and diversifies worm community structure in humid tropical soils.