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Earthworm invasion, white‐tailed deer and seedling establishment in deciduous forests of north‐eastern North America

Earthworm invasions and high deer populations are among many stressors threatening long‐term population viability of forest understorey plants in north‐eastern North America. Stressor effects are typically tested one at a time; however, stressors often co‐occur and plants respond to effects of multiple stressors simultaneously. We used a factorial design to test independent and combined effects of non‐native earthworms and native white‐tailed deer on survival of seedling transplants of 15 native understorey plants in five forests in New York State. Earthworm biomass was negatively correlated with survival of 12 of 15 species. We found no interactive effect of deer and earthworms, but did find a positive, non‐consumptive effect of deer on Geranium maculatum and Polygonum virginianum survival. Deer and earthworm presence/absence indirectly influenced other trophic levels: earthworm presence increased the likelihood of insect attack, and deer exclusion increased the likelihood of rodent disturbance of transplants. Synthesis. Invasive earthworms negatively affected seedling survival of many understorey plants, including species previously thought to benefit from earthworm associations. This effect was a function of earthworm biomass, a surrogate for earthworm activity. We expect deer herbivory to increase in importance, including indirect effects, as seedlings grow into browse height over the next years. Investigations of co‐occurring stressors can result in ‘ecological surprises’ including previously overlooked non‐consumptive effects or effects on other trophic levels.