Stable Isotopic Studies of Earthworm Feeding Ecology in Tropical Ecosystems of Puerto Rico

Hendrix, PF, SL Lachnicht, MA Callaham, and XM Zou. 1999. Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of puerto rico. Rapid Communications in Mass Spectrometry 13 (13): 1295-9.

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used 13C and 15N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less 15N enrichment (delta 15N = 3–6%) than exotic Pontoscolex corethrurus (15N = 7–9%) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in 15N enrichment (delta15N = 6–9%), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in 15N than those in the forest sites, but more highly enriched in 13C suggesting assimilation of C from the predominant C4 grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright # 1999 John Wiley & Sons, Ltd.

Interactive effects of native and exotic earthworms on resource use and nutrient mineralization in a tropical wet forest soil of Puerto Rico

Lachnicht, SL, PF Hendrix, and X. Zou. 2002. Interactive effects of native and exotic earthworms on resource use and nutrient mineralization in a tropical wet forest soil of puerto rico. Biology and Fertility of Soils 36 (1) (AUG): 43-52.

Investigation of single or mixed assemblages of native Estherella sp. and exotic Pontoscolex corethrurus from a rain forest in Puerto Rico was undertaken to understand resource use patterns, and linkages with C and N mineralization in a 19-day incubation. Resource use was explored with addition of 15N-enriched leaf litter and 13Cenriched glucose to reconstructed organic and mineral soil horizons. Juvenile Estherella sp. became at least 6.06‰ more enriched in 13C than sub-adult Estherella sp. or adult P. corethrurus. Sub-adult Estherella sp. became >3.6‰ enriched in 13C over P. corethrurus. δ15N acquired by P. corethrurus was greater by 0.83–1.56‰ in the mixed-species than the single-species assemblages. δ15N of subadult Estherella sp. was enriched by 0.73–0.81‰ over juvenile Estherella sp. in the single-species assemblage. Net N immobilization occurred in the organic layer of all 15Nenriched treatments. Net N mineralization in mineral soil layers was significantly greater in microcosms with P. corethrurus than in those containing only Estherella sp.. Cumulative respiration was greatest in P. corethrurus assemblages, however, assemblages with only Estherella sp. released more 13C in respiration. P. corethrurus assimilated different N resources when incubated with, as compared to without, native Estherella sp.. δ13C and δ15N signatures acquired by assimilation of 13C and 15N differed by species, developmental stage, and competitive interactions. The results showed that alone, exotic P. corethrurus induced higher mineralization rates than native Estherella sp., but that the interaction of exotic and native species impinged on resource use by P. corethrurus, reducing the effect of the exotic species on C and N mineralization. Invasion of exotic P. corethrurus may change the mineralization potentials of C and N and their biogeochemical cycling in soils.

Earthworm communities along an elevation gradient in Northeastern Puerto Rico

Gonzalez, Grizelle; Garcia, Emerita; Cruz, Veronica; Borges, Sonia; Zalamea, Marcela; Rivera, Maria M. 2007. Earthworm communities along an elevation gradient in Northeastern Puerto Rico.. European Journal of Soil Biology 43 .

In this study, we describe earthworm communities along an elevation gradient of eight forest types in Northeastern Puerto Rico, and determine whether their abundance, biomass and/or diversity is related to climatic, soil physical/chemical and/or biotic characteristics. We found that the density, biomass, and diversity of worms varied significantly among forest types. The density of earthworms was highest in the Pterocarpus forest. In terms of biomass, both elfin and the Pterocarpus forests had the highest values. The number of earthworm species significantly increased as elevation and annual rainfall increased and air temperature decreased. We conclude that differences in earthworm species richness along this elevation gradient may be due to a combination of biotic and soil physical and chemical factors. Soil pH and root length density are important predictors of number of worm species along this elevation gradient.


Exotic Earthworms Accelerate Plant Litter Decomposition in a Puerto Rican Pasture and a Wet Forest
Z. G. Liu and X. M. Zou
Ecological Applications
Vol. 12, No. 5 (Oct., 2002), pp. 1406-1417

Tropical land-use changes can have profound influence on earthworms that play important roles in regulating soil processes. Converting tropical forests to pastures often drastically increases the abundance of exotic earthworm populations such as Pontoscolex corethrurus. We initiated this study to examine the influence of exotic earthworms on the decomposition of plant leaves and roots in a tropical pasture and a wet forest of Puerto Rico. We employed two treatments: control with natural earthworm population, and earthworm reduction using an electroshocking technique. Decomposition rates of plant leaves on the ground surface and root materials within the surface mineral soil were estimated using a litterbag technique. To understand the role that exotic earthworms play in altering plant litter decomposition, we also compared soil CO2 evolution rates, soil microbial biomass, and physical and chemical soil properties between the controls and earthwormreduced plots during a one-year period. Earthworm populations in the electroshocked enclosures were reduced by 85% and 87% as compared with pasture and forest controls by the end of the experiment. Earthworm reduction significantly decreased the annual decay rates of plant leaves but had no effects on those of plant roots in both pasture and forest sites. Although the control plots had less mass remaining on every litterbag collecting date, significant treatment effects on leaf decomposition occurred only after 240 d in both sites. The decay rates were greater when organic materials had low carbon to nitrogen or phosphorus ratios. Soil respiration was also decreased in the earthworm-reduced plots. In contrast, soil microbial biomass C was not affected by earthworm reduction. Furthermore, there were no significant differences between the two treatments in soil bulk density, moisture content, pH, or temperature at either site. Our results suggest that exotic earthworms may accelerate leaf litter decomposition by elevating rates of litter consumption/digestion or microbial activity, rather than by improving soil physical/chemical conditions or altering microbial biomass.


Borges, S. and M. Alfaro. 1997. The earthworms of
Baño de Oro, Luquillo Experimental Forest, Puerto
Rico. Soil Biol. Biochem. 29:231-234.

The earthworm fauna of the four forest types at the Batio de Oro region of the Luquillo Experimental Forest was sampled over 6 months. Eight species were found: two Glossoscolecidae, one Ocnerodrilidae, four Megascolecidae, and one Eudrilidae. The total species density and biomass showed no significant correlation to forest type or soil depth, but some species did show different distribution patterns according to certain soil properties. Exotic species were more abundant, but native species accounted for most of the biomass. Species number and density were low when compared with similar studies in other neotropical forests. This could be due to the high moisture content of the soils and its effects on other soil properties.
Syndicate content