COS 18-5 - Aboveground enemy release increases grassland seedling survival

Abstract: The enemy release hypothesis is the most popular hypothesis to explain the success of invasive plants. However, whether enemy release can enhance invader survival and therefore establishment shortly after invasion (i.e. at the seedling stage) remains to be sufficiently tested. Further, the generality of studies on enemy release are typically limited, by focusing on single species or community contexts. These shortcomings have practical implications for managing invasions, as the effectiveness of tools such as biocontrol remain unclear.

We planted seedlings of 16 different species from two functional groups (C4 grasses and forbs) into replicate plots in two different field communities (early and late succession). We hand-painted over 1500 individual seedlings with pesticides (insecticide and fungicide) over the course of a growing season to enforce enemy release, and tested whether it enhanced the survival of our planted seedlings relative to controls. We found that release from insects increased the survival of seedlings by an average of 80%, while there was no additional benefit of release from fungal pathogens. This effect was broadly consistent across functional groups and community contexts, and was still observed even after accounting for plot-level characteristics of light availability, moisture, total species richness and the mean phylogenetic distance between our target species and the rest of the community. There was some species-specific variation in the size of the survival benefit, but this could not be explained by phylogeny, damage rates or plant traits.

Previous studies have suggested that the benefits of enemy release depend on the specific identity of the invader, its traits and the recipient community. However, we reveal a generalizable positive effect of enemy release from insect herbivores early in the invasion process. We suggest that the higher vulnerability of seedlings led to this more ubiquitous benefit, which may be consistent across invader functional groups and types of native community. Initial enemy release may therefore aid invader establishment, even if enemies rapidly accumulate. Plant survival, particularly seedling survival, is an understudied metric of performance in the context of enemy release, but we show that it is vital to consider in global plant invasions. These results have positive implications for managers of invasions, as it suggests that biocontrol could effectively limit invasive populations if they are detected early enough.