The efficiency of earthworm extraction methods is determined by species and soil properties in the Mediterranean communities of Central-Western Spain

Abstract

Given the well-known role of earthworms in the functioning and health of soils and whole ecosystems, feasible and reliable studies of their abundance and diversity in agricultural lands are essential for the effective design of best agricultural practices. However, previous work has shown that the extraction efficiency of different methods proposed seems to depend on species and size of earthworms and presumably on soil type, which makes creating an earthworm inventory difficult. In the present study, we compare the efficiency of five earthworm extraction methods combining hand-sorting with chemical expellants (hand-sorting, formalin, allyl isothiocyanate (AITC), formalin + hand-sorting and AITC + hand-sorting) over a wide range of soil properties (depth, texture and water regime) in cultivated and semi-natural habitats found in a Mediterranean region (CW-Spain). Sampling efficacy was measured in terms of number of earthworms extracted, taking into account different species, ecological groups, development stages, size of individuals, and soil properties. We found 20 species, only 6 endogeic and 1 anecic species being abundant. The anecic Aporrectodea trapezoides responded reasonably to chemical expellants, as did certain soil surface dwelling endogeic species (Microscolex phosphoreus and Microscolex dubius), with above 50% of specimens of these species sampled after chemical application. For other endogeic species, such as Allolobophora molleri and Aporrectodea rosea, chemical expellants gave poor results (<15% and 5% of specimens, respectively), and combined methods produced similar results to hand-sorting alone. Hand-sorting appears necessary for sampling the total earthworm community in particular for endogeic species, but when only species richness is of interest, the application of a chemical expellant can be a time-efficient method. Response to different methods was irrespective of the earthworm size within species, but depended on the maturity stage of the specimens, habitat type and soil properties, making difficult the adoption of a simple sampling protocol for large surveys in highly fragmented Mediterranean earthworm communities.