Analysis of the feeding ecology of earthworms (Megascolecidae) in Japanese forests using gut content fractionation and [formula omitted] and [formula omitted] stable isotope natural abundances

Abstract

Differences in food resource use by earthworms (Megascolecidae) were studied at three Japanese forest sites with different latitude in cool-temperate (TOEF), temperate (MFF) and subtropical (Yona) zones. The vertical distribution and the fractionation of gut contents of earthworms were analysed to determine ecological categories. At TOEF and MFF, earthworm communities were composed of species living mainly in litter and topsoil layers. At TOEF, Amynthas vittatus (Goto and Hatai, 1898), Metaphire hilgendorfi (Michaelsen, 1892) and Eisenia japonica (Michaelsen, 1891) (Lumbricidae) were dominant, the former two species being mainly observed in the litter layer whereas the latter was in soil layers. At MFF, ‘ Metaphire’ soulensis (Kobayashi, 1938), Amynthas purpuratus (Ishizuka, 1999), A. vittatus and Amynthas distichus (Ishizuka, 2000) were dominant and the former three species were observed in the litter layer whereas the latter one occurred in soil. The gut contents of those living in the litter layer had more organic fractions, whereas the latter had more inorganic fractions. Therefore these species were regarded as epigeic species and polyhumic-endogeic species, respectively. At Yona, Amynthas yambaruensis (Ishizuka and Azama, 2000) had burrows through 20 cm depth with casts and middens on the forest floor, and their fraction ratio of gut content was intermediate between the epigeic and endogeic earthworms at TOEF and MFF. Therefore A. yambaruensis was classified as anecic. In addition, at Yona, there were two unidentified Amynthas spp.; one epigeic species in the midden of A. yambaruenesis, and one endogeic species found in B-layer soil. Natural abundances of the stable isotope pairs 13 C/ 12 C and 15 N/ 14 N ( δ 13 C and δ 15 N ) were measured in soil organic matter and earthworm tissue. δ 15 N values showed epigeic < polyhumic-endogeic at TOEF and MFF, whereas at Yona, δ 15 N values were in the order anecic < epigeic < endogeic. The observed 15 N enrichments thus appear to indicate that the functional niches of earthworm species are due to resources available to them in the decomposition process from fresh litter to humus. δ 13 C values showed similar trends, but the differences between ecological group was smaller compared to δ 15 N .