Effects of an invasive herbivore at the single plant scale do not extend to population-scale seedling dynamics

A complex of nine invasive weevils has established in the northern hardwood forests of the Great Lakes Region. These weevils have become the numerically dominant arthropod fauna in the lower vegetation strata of this ecosystem. Effects of these folivorous adults and rhizophagous larvae on seedling survival and density are unknown. We measured the impact of adult weevil defoliation on individual sugar maple (Acer saccharum Marshall) seedlings, the numerically dominant flora in these stands. Over 14 500 seedlings were measured to examine the effects of larval abundance, adult abundance, and adult defoliation on seedling dynamics during 2005-2009. Heavy defoliation led to individual sugar maple seedling mortality. However, at the stand level there was no measureable net effect of invasive weevils against the high natural background rates of seedling mortality. Seedling growth and biomass were reduced by high adult or larval populations, which were associated with heavy leaf and root herbivory, but not by moderate adult or larval populations. We conclude that high levels of redundancy and compensatory processes in this environment allow sugar maple seedling populations to tolerate high levels of weevil damage, although additional stresses could reduce this resilience.