Patterns of abundance and habitat associations of earthworms in remnant forests of the Willamette Valley, Oregon
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Among the most pervasive introduced animal taxa in North America are earthworms in the family Lumbricidae. Understanding their patterns of abundance is important in evaluating their potential impact on the native earthworm fauna. We examined the relationship of the abundance of introduced and native earthworm species, and compared soil and vegetative attributes associated with their abundance in five remnant forests (Alderman, Belleview, Helmick, Mallard and Mission Bottom) within the Willamette Valley, Oregon, located between the Cascade Range and the Coast Ranges, USA. We detected two genera of native earthworms, Toutellus (78.5% of native earthworms) and Argilophilus (21.5%), collectively, in 7% of the excavated soil samples. We detected non-native lumbricids at all five remnants, in 86% of the samples, and we estimated an average of 1136 kg of lumbricids/ha in the top 25cm of soil. We found a positive association between counts of lumbricids and the percent of organic matter and moisture content. Greater depth of surface organic matter and sloped terrain were associated with the presence of Toutellus. We found a positive association between abundance (counts/sample) of lumbricids and Toutellus. We found no direct evidence that the introduced earthworm fauna was impacting native earthworm species, although we failed to detect numerous native species that are presumed to be present in the Willamette Valley. We recommend an experimental approach, coupled with field observations that allow the estimation of species-specific detection probabilities, for future research into the potential impacts of introduced earthworms on the native fauna. This topic deserves further attention given the ubiquity of introduced earthworms in North America.