Does the overabundance of native White-tail deer (Odocoileus virginianus) facilitate non-native earthworm invasions?

Background/Question/Methods

 Large herbivores, such as many ungulates, can have dramatic ecosystem 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 as well, 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, does the overabundance of native White-tailed deer (Odocoileus virginianus) facilitate non-native earthworm invasions?  We conducted a long-term field experiment with paired deer exclosure and control 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. Paired plots were located in four different focal regions. Earthworm abundances were measured within both control and exclosure sub-plots, while deer abundances were measured within each of the control sub-plots.  We then compared sub-plots to determine if there were greater earthworm abundances within control versus exclosure sub-plots.

Results/Conclusions

We found a positive correlation between deer abundance and abundances of the invasive anecic (deep vertical burrowing) earthworm Lumbricus terrestris, consistent with deer facilitating this earthworm's invasion. Further, control sub-plots had on average more endogeic (shallow horizontal burrowing) earthworms (e.g. Octolasion tyrtaeum) than paired deer exclosure sub-plots, providing the first experimental evidence that deer might facilitate the invasion of some non-native earthworms. In addition, greater earthworm abundances correlated with reductions in litter duff, an important measure of ecosystem effects of earthworm invasions, which is consistent with a large body of literature, demonstrating that the effects of these exotic earthworms are substantial. If deer overpopulation facilitates nonnative earthworm invasions, and if nonnative earthworm invasions have large ecosystem effects, this suggests that managing deer is important, not only for their above ground effects on plant communities, but also for their below ground effects on important ecosystem engineers like invasive earthworms.