The ‘enemy release hypothesis’ argues that when an exotic species is introduced to a novel habitat, release from regulation by herbivores results in increased vigor, abundance, and distribution.Lonicera maackii is an invasive shrub responsible for a reduction in biodiversity in the forests of the Midwestern United States that appears to exhibit enemy escape in North America. We assessed natural herbivory rates and responses to simulated herbivory of this shrub in the field and greenhouse. Herbivory rates were determined on leaves collected in 2008 and 2009 from edge and interior habitats throughout Ohio. Growth and phytochemical responses to simulated herbivory of seedlings were examined in a full-factorial greenhouse experiment in which herbivory, light levels and nutrient levels were manipulated. Likewise, effects of simulated herbivory on growth were examined on mature plants in the forest understory in the Wright State University woods.
Results/Conclusions
Leaf area removal averaged 1.84±0.09% across 8 populations in Ohio (2008) and 2.33±0.08% (2009) across 3 populations, with edge plants receiving slightly more damage than forest interior plants. Damage tended to accumulate steadily throughout the season and, of those experiencing herbivory, 85% were damaged by chewing suggesting damage was the result of generalist herbivores. In the greenhouse, a one-time loss of 50% of the leaf area of seedlings caused the relative growth to decline from 1.82±0.62 mm to 1.30±0.53 mm and the root to shoot ratio to increase from 0.14±0.07 to 0.39±0.25. These effects were more noticeable when nitrogen was limiting and most dramatic when light was limiting along with nitrogen. Effects of removal of 50% of the leaf area in the field on branch length growth rates were not noticeable during the year of defoliation, but effects may appear in later seasons. Overall, results indicate that L. maackii currently receives insufficient damage to impact fitness in the field, but heavy defoliation could affect growth, particularly in seedlings, and under conditions where vital resources such as light and nutrient availability are restricted. These findings support the view that ‘enemy release’ may contribute to the success of L. maackii in Ohio.