The stoichiometry of two invasive earthworm species (Dendrobaena octaedra and Lumbricus terrestris) consuming leaf litter of different C, N, and P content

Background/Question/Methods

Ecological stoichiometry (ES) describes how elemental imbalances between organisms and their food resources influence organismal physiology, population dynamics, and ecosystem processes. Elemental imbalances between detritivores and their food resources are among the largest in nature, yet ES research on detritivores is limited and most examples are for aquatic species. In many temperate deciduous forests of the Midwestern U.S., invasive earthworms have become the dominant detritivore. Earthworm consumption of leaf litter is endangering understory plants, reducing carbon pools, and altering N and P cycling. Our objective was to use ES to understand how detritus (leaves) of different nutritional quality (C, N, and P) affects invasive earthworm feeding, growth, and excretion. We fed two common invasive earthworms (Dendrobaena octaedra and Lumbricus terrestris) one of 7 species of deciduous leaves [sugar maple (SM), red maple (RM), quaking aspen (QA), bigtooth aspen (BTA), northern red oak (RO), yellow birch (YB), and basswood (BA)]. Juvenile D. octaedra were placed in bags with ~0.5 g of leaves (n=8 per leaf species) and allowed to feed for 45 days. Juvenile L. terrestris were placed in cups of soil with ~0.5 g of leaves (n=5 per leaf species) on the soil surface and allowed to feed for 21 days.
Results/Conclusions

Leaf litter quality differed among species, with YB and QA leaves having the lowest C:N and C:P and RO and SM leaves having the higest. For D. octaedra, growth rate was related to leaf mass loss (p<0.001, r²=0.60). D. octaedra fed YB and QA grew rapidly (0.015 and 0.012 mg·d^-1, respectively) whereas mass declined when fed RO and SM (-0.0028 and -0.014 mg·d^-1, respectively). D. octaedra had the highest assimilation efficiency for QA (8.4%) and YB (7.4%). Overall, D. octaedra growth was best explained by leaf C:N ratio (p<0.001, r²=0.27). For L. terrestis, growth rates were highest when fed YB and BA (1.67 and 1.34 mg·d^-1 respectively) and earthworms lost mass when fed RM and SM (-1.99 and -2.68 mg·d^-1 respectively). Assimilation efficiencies for L. terrestis were highest for BS (18%) and QA (15%). Faster growth and consumption rates by L. terrestris may reflect the more “invasive” nature of this earthworm species. These results suggest that in forest stands dominated by litter with low C:N and C:P leaves (e.g., YB, QA, and BA), invasive earthworms will feed and grow faster and as a consequence have more rapid impacts on litter and nutrient dynamics.