Something new under the ground: Ecosystem consequences of earthworm invasions

Background/Question/Methods

Joan Ehrenfeld's distinguished career spanned subjects as diverse as the ecosystems she studied. My interests overlapped strongly with hers, including wetland, soil and urban ecology, and it is remarkable that she made significant contributions to all those fields.  In her work on invasive shrubs in forest ecosystems, Joan noted that many shrub-invaded areas had abundant populations of invasive foreign earthworms.  She proposed an excellent experiment to examine the interactive effects of the invasive shrubs and earthworms, which, although it was never funded, prompted us to share our insights on the ecosystem consequences of biological invasions.  Joan understood that biological invasions affected ecosystem processes through a variety of interacting, mutually reinforcing pathways.  In the case of earthworm invasions, these effects are a consequence of the role of earthworms as ecosystem engineers, the specific behavioral and physiological characteristics of different earthworm species, and the interactions of earthworms with plants and soil microbes as the ultimate arbiters of nutrient processing and fluxes in soil.  Earthworm invasion of north temperate forests previously devoid of earthworms has become a classic example of how an ecosystem engineer can alter nutrient fluxes, change nutrient pools, and transform soil foodwebs via many of the interacting pathways and mechanisms that Joan emphasized in her own work.

Results/Conclusions

The most significant ecosystem effects of earthworm invasion occurs at undisturbed sites where earthworms completely transform the soil surface horizons, eliminating the organic forest floor and altering the location and nature of soil microbial activity.  At sites in New York, earthworms reduced total carbon storage in the surface horizon, reduced soil C:N ratios, and affected the loss and distribution of different soil P fractions.  Earthworm shifted the distribution and function of fine roots, and the composition and function of the microbial community.  Rates and timing of litter disappearance varied among areas dominated by different earthworm species.  Studies both within and across sites showed that the earthworm effects were context dependent, and varied with landscape position, disturbance history and other factors.  Stable isotope studies with leaf litter double-labeled with ¹³C and ¹⁵N and in plots with and without earthworms provided insight into processing of detrital C and N into SOM.   Results reinforced the theory that microbial C:N stoichiometry has a primary role in shaping soil C:N, and that earthworms exert their effects on soil organic matter processing ultimately through their interaction with the soil microbial community.