Aboveground enemy release increases seedling survival in grasslands

Abstract

The enemy release hypothesis is a popular hypothesis to explain the success of invasive plants. Enemy release studies typically focus on single species or types of communities, feature indirect experimental manipulations that apply pesticides to whole communities not individual species, and only examine responses of established plants or plant populations, limiting their generality. Using a novel species-specific approach, we examine whether enemy release can enhance seedling survival and recruitment of 16 grassland species by experimentally linking enemy release with enhanced plant performance.We planted seedlings of 16 native grassland species from two functional groups (C4 grasses and non-legume forbs) into two grassland sites (early and mid succession). We hand-painted 1,548 individual seedlings with pesticides (insecticide and fungicide) over the course of a growing season to enforce aboveground species-specific enemy release, and tested whether it enhanced survival relative to untreated controls. Using native species enabled us to directly test effects of enemy release, while avoiding confounding factors like unknown invasion histories. Of the 16 native study species, 13 are naturalised/invasive outside of their native ranges.Release from insects increased seedling survival by 80% on average, with no additional benefit of release from fungal pathogens. This effect was consistent across functional groups and community successional stages, and was strongest in resource-acquisitive species. The size of species’ performance benefits from enemy release were positively correlated with the number of regions globally where each species has been introduced and naturalised.Synthesis. Previous studies of enemy release have centred on adults and findings have varied among species. We found a positive effect of release from insect herbivores early in colonisation – a trend that held across functional groups and types of resident community. We posit that the consistent vulnerability of seedlings vis-à-vis later life stages leads to this more ubiquitous benefit of enemy release. Enemy release may therefore aid initial recruitment of most, if not all, plants during the invasion process, even if enemies rapidly accumulate. The positive correlations between the benefits of enemy release for seedlings, species’ life history strategies and global naturalisation patterns provide compelling hypotheses for future research.