Plant enemies like insect herbivores and fungal pathogens eat or degrade plant tissue. How does this affect performance? Photo: Josh Brian

We are invasion biologists, studying how and why species become invasive. One of the biggest hypotheses in our field is the ‘enemy release hypothesis’, which says that species become invasive because they escape from enemies in their native range. This hypothesis is predicated on the assumption that enemies reduce the performance of plant populations in their native ranges. Surprisingly, there aren’t actually many studies directly validating this link between enemy release and performance under natural conditions.

The experiment: Can we make general statements about this link?

One set of plots staked out. Our seedlings were grown for a month in the greenhouse in their own little containers, then popped out and planted into the plots! Photos: Josh Brian.

We designed an experiment to test whether plants that are experimentally released from enemies (by painting with pesticides) show increased performance compared to co-occurring plants that are still faced with enemies. We were originally interested in performance in terms of plant growth and reproduction – we planned to monitor this over the course of three years. We used 16 different species and two different community contexts, to try make our conclusions as general as possible.

The set-up: A team effort!

Our experiment took place at Cedar Creek Ecosystem Science Reserve in Minnesota. Setting up a total of 288 plots and planting a total of over 1500 seedlings in undisturbed grassland was no mean feat! Thanks to an incredible team of interns at the site, it all got set up and underway on time, in the early summer of 2022.

The amazing Cedar Creek interns, hard at work planting over 1500 seedlings in the plots. Photo: Josh Brian.

The observation: Are there differences in survival?

As the experiment began, we took regular censuses of our planted seedlings, initially just to make sure that mortality wasn’t too low and so we knew where to replant if required. However, after the first two weeks or so, we noticed an interesting trend – the plants that had been treated with pesticides (enforcing ‘enemy release’) appeared to be surviving a bit better. We therefore made the effort to collect weekly census data throughout the growing season, and had a look at the results at the end of the summer.

One of our sets of plots from above. Gaps between plots weren’t mowed – the vegetation was just trampled from all the time we spent walking around the plots! Photo: Maggie Anderson, University of Minnesota.

The paper: An unplanned result!

The results we saw were incredibly strong: seedlings experiencing enemy release had much higher survival over the course of the growing season, a result that was broadly consistent across all 16 species and both types of community. We hadn’t planned to consider survival, as the literature on enemy release and plant performance was mainly focused on changes to plant growth and reproduction. After digging a bit further, we found that our results were quite novel: very few studies had reported enemy release effects on seedling survival, and none with the generality that we did. Our results suggest that enemy release may facilitate recruitment of exotic species very early in the invasion process!

The lesson: Keeping an eye on the ground

We feel like this paper, a ‘bonus’ from our experiment, wouldn’t have happened without close and regular contact with the field site. In the age of big data, it’s easy to get swept up by large, far-reaching syntheses, where data is removed from the context it was collected in. We hope this paper shows that “small-scale” experiments and listening to what the plants are telling you remains a vital way to understand the world.

Find out more! This post links to Joshua Brian, Harry Shepherd, María Pérez-Navarro, Jane Catford’s piece for the Journal of Ecology entitled: ‘Release from aboveground enemies increases seedling survival in grasslands’. Check the full piece out here: https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.14318

You can also follow the team on X: @DrJoshBrian @HarryShepherd_ @ma_perez_nav @AlienImpacts and check out their website: alienimpacts.com

One of the added bonuses of staying at the field site – opportunities for amazing rainbows! Photo: Josh Brian.