Release from aboveground enemies increases seedling survival in grasslands

Plant enemies can influence plant community assembly and structure. However, it is unclear how insect herbivores and fungal pathogens affect seedling recruitment. Complex interactions with competition and resource availability make it difficult to isolate the effect of enemies. This uncertainty can impede understanding of community assembly drivers, species coexistence, and trophic interactions; and limits hypothesis testing such as the enemy release hypothesis, a key hypothesis in invasion biology. Using a novel species-specific approach, we examine how enemies affect seedling survival and recruitment of 16 grassland species.

We planted seedlings of 16 native 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 one growing season to enforce aboveground species-specific release from enemies, and tested whether it enhanced survival relative to untreated controls. Applying treatments to individuals allowed us to test for the effect of enemies on plant performance while controlling for contexts such as competition from the resident community and resource levels.

Release from insects increased seedling survival by 56% on average, with no additional benefit of release from fungal pathogens. This effect was observed for both forbs and C4 grasses across both sites, and was strongest in resource-acquisitive species. There was no effect of the mean phylogenetic distance between our target species and the resident plant community, or of light availability or soil moisture.

Synthesis. The significant positive effect of release from insect herbivores on survival early in colonisation – a trend that held across functional groups and types of resident communities – suggests insects play an important regulatory role in community assembly, especially for resource-acquisitive species. As variation between species could be explained by traits but not by phylogeny, we emphasise the importance of trait-based approaches to plant community ecology. Our results also support the key mechanism (increased performance following release from enemies) underlying the enemy release hypothesis. Enemy release may therefore aid initial recruitment of plants during the invasion process.

Methods
This dataset is based on an experiment carried out at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. In this experiment, seedlings of 16 different species were planted into two naturally occurring grasslands; one early-successional sites and one mid-successional site. The seedlings were treated with either insecticide, insecticide, and fungicide, or a control treatment, and seedling survival was monitored for two months. Relative seedling survival was compared among the three treatments, and correlated with environmental conditions, species-level traits, phylogenetic relatedness, and damage by insects and fungal pathogens.