Temporal and spatial variation of exotic earthworm communities established across a north temperate forest undergoing secondary succession

Background/Question/Methods

The dynamics of incipient invasions by exotic earthworm species into northern U.S. temperate forests have been widely documented (frequently characterized as ‘invasion fronts’ moving across landscapes from centralized points of introduction). Understanding factors controlling earthworm species distributions across invasion stages (i.e., introduction, colonization, regional spread) is critical for assessing long-term impacts on northern temperate forests ecosystems, though studies describing exotic earthworm species distributions following regional spread and establishment within these ecosystems remain limited. We address this knowledge gap through study of exotic earthworm communities in a northern-lower Michigan forest landscape representative of forests of the upper Great Lakes region, and where exotic earthworm species presence was first documented in the early 1950s. Field surveys of earthworm community composition were conducted over two years using 10 permanent forest plots, with concurrent measurements of environmental variables divided into ‘effect factors’ determined by large-scale ecosystem processes (leaf litter inputs, soil physical properties, soil C and N content), and ‘response factors’ likely impacted by earthworm activity over short time scales (litter decomposition, soil isotopic values). Non-metric multidimensional scaling was used to characterize spatial and temporal variation in earthworm community composition; co-inertia analyses were used to characterize multivariate relationships between earthworm biodiversity and environmental factors.

Results/Conclusions

Earthworm communities were dominated by five exotic species with varying population densities: Lumbricus rubellus ≥ L. terrestris >> Dendrobaena octaedra ≥ Aporrectodea spp. (A. trapezoides + A. caliginosa). Earthworm community composition did not show strong seasonal or annual trends, though unique associations between species densities and environmental factors were observed. L. terrestris and L. rubellus densities were positively associated with soil C and N content, A. rubrum inputs, and soil moisture; and negatively associated with P. strobus inputs. D. octaedra, and Aporrectodea spp. densities were positively associated with % sand; and negatively associated with plot-to-road distance. Total earthworm density and biomass were strongly correlated and positively associated with soil moisture and litter C inputs. Surface soil del-13C depletion was associated with Aporrectodea spp. and total earthworm density; surface soil del-15N enrichment was associated with D. octaedra density. Increased leaf litter decomposition rates were associated with L. terrestris and Aporrectodea spp. densities, and total earthworm biomass. Our results characterize exotic earthworm biodiversity patterns at scales relevant to forest ecosystem processes (i.e., 100m – 1000m). Understanding factors controlling earthworm distributions are critical for assessing long-term impacts of earthworm introductions and predicting earthworm responses to successional-shifts in forests of the upper Great Lakes region.