The role of native ungulates and invasive earthworms in shaping plant and soil communities in northeast Ohio
Large herbivores, such as many ungulates, can have dramatic above- and below-ground effects. These effects can be both positive, such as bison shaping the grassland through grazing and trampling, or negative such as overabundant deer reducing native plant abundance and diversity. Invasive earthworms have also been shown to have large effects on both the above and belowground communities. The negative effects of both overabundant native deer and invasive earthworms individually on the forest ecosystem can be substantial; however, their interactions with each other are poorly understood. We thus asked, do the overabundance of native White-tailed deer (Odocoileus virginianus) and invasive earthworms interact to affect both plant and soil communities? We sampled a long-term field experiment with paired deer exclosure and control sub-plots over a large spatial extent in northeastern Ohio. Our study site encompasses twenty-nine different deer exclosures and paired control sub-plots that have been in place between five and fifteen years. Earthworm abundances and plant richness were measured within both control and exclosure sub-plots, while deer abundances were estimated for each of the control sub-plots. Soil microbial and fungal communities were analyzed from soil cores taken within sub-plots using the DNA fragmentation technique Terminal Restriction Fragment Length Polymorphism (TRFLP).
We found that plant richness was reduced with earthworm presence and varied with earthworm functional group and deer presence. This is consistent with a growing body of literature showing that invasive earthworms reduce plant richness in North American forests. However, this result also shows that the effects of earthworms may depend on whether deer are present. We also found that earthworms altered the bacterial community in the soil. A generalized least squares model, incorporating spatial distance between plots into the error structure, showed that earthworm density, earthworm functional group, and deer exclosure treatment all interacted to alter the bacteria community. The anecic functional group (Lumbricus terrestris) significantly altered the bacterial community diversity within deer exclosures. Deer browse, which was used as a proxy for deer density, did not interact with the other predictors in our model and had no effect on the microbial communities. These results show that both deer and earthworms are interacting to affect both the above-ground plant community and the below-ground microbial community. Invasive earthworms, and in particular Lumbricus terrestris, interact with deer in shaping plant and soil microbial communities.