The potential for indirect negative effects of exotic insect species on a liverwort, Bazzania trilobata (Lepidoziaceae), mediated by the decline of a foundation tree species, Tsuga canadensis (Pinaceae).
In many ecological communities, the effects of exotic species are likely to extend beyond their direct interactions with natives, due to indirect effects. This dynamic might be particularly consequential in cases where invasive insects or other exotic herbivores target foundation plant species in the communities they invade. In this study at a site in western Massachusetts, we used experimental transplants to gauge the potential effects of a decline in the evergreen conifer Tsuga canadensis due to ongoing spread of two exotic insect pests on a liverwort, Bazzania trilobata, whose distribution is closely linked to dense stands of this conifer in the northeastern USA. After 4 years, transplanted B. trilobata samples moved to forest areas with lower abundance of T. canadensis declined significantly, particularly on sites with higher solar radiation, as determined by local slope and aspect. In addition, samples manually cleared of deciduous leaf litter were - 17% larger than those exposed to natural accumulation of leaf litter, indicating a direct negative effect of deciduous trees on the liverwort that might increase as these tree species replace T. canadensis lost to invasive pests. A parallel experiment documented high mortality of B. trilobata (55%) when subjected to open canopy conditions similar to those resulting from selective "salvage" logging of hemlock in the region. These results indicate that the spread of exotic insect pests targeting T. canadensis is likely to produce strong indirect negative effects on the liverwort B. trilobata, via diminished commensal interactions with the conifer and increased amensalistic effects from the deciduous tree species that commonly replace it.