In this Insight, Benjamin Delory, a Postdoctoral research associate at Leuphana University Lüneburg, talks about his recent paper on when history matters: The overlooked role of priority effects in grassland overyielding

The story behind the paper

Plant communities are historically contingent. This means that past events such as the sequence of plant species arrival during community assembly can affect species’ relative abundance and ecosystem processes (e.g., biomass production). Ecologists refer to this phenomenon as a priority effect. Priority effects are widespread in nature and, depending on the direction of the effect that the early-arriving species have on the ones arriving later, they can be either negative or positive. Although negative priority effects are (very) common, positive priority effects can be observed as well, notably when facilitation mechanisms favour the establishment of late-arriving species in plant communities.

In biodiversity – ecosystem functioning (BEF) experiments designed to study the effect of plant species richness on ecosystem processes, assembly history is usually not manipulated: all target species are sown at the same time in the plots and unsown species are periodically removed (weeding) to maintain a plant diversity gradient. Despite that, both diversity and priority effects can have profound consequences for the structure and functioning of plant communities. However, we did not know to what extent plant order of arrival during assembly of plant communities would affect the direction and magnitude of biodiversity effects. It was this idea that led to our paper, and to be able to explore it further, we needed data to work on.

In 2012, three of my colleagues (Emanuela Weidlich, Philipp von Gillhaussen, and Vicky Temperton) set up a grassland field experiment close to Jülich (Germany) in which the order of arrival of three plant functional groups (forbs, grasses, and legumes) was manipulated to create priority effects. Because they measured the productivity of each species sown in mixture and monoculture plots for two consecutive years (2013-2014), their dataset was a very good starting point to investigate how plant arrival order affects grassland overyielding, as well as its drivers (complementarity and dominance effects). Using a new framework developed for the quantification of priority effects, I also investigated if the magnitude of biodiversity effects was dependent on the strength and direction of priority effects.

Our paper has two key messages. First, we found that plant order of arrival during assembly affected the mechanisms driving positive biodiversity effects in grasslands, but not grassland overyielding per se. Second, we found a positive relationship between priority and diversity effects: in plots where priority effects were positive we observed the greatest positive biodiversity effect values. Importantly, this increase in grassland overyielding when moving from negative to positive priority effects was solely due to increased complementarity effects, thus suggesting that increased species complementarity might have played a role.

What’s next?

These findings are important because they demonstrate the strong interlinkage of biodiversity and order of arrival effects in grasslands. However, much work still needs to be done, particularly to gain a better understanding of how priority and diversity effects interactively drive the structure and functioning of plant communities aboveground and belowground, as well as the mechanisms allowing species coexistence in nature. To be able to do this, I strongly believe that long-term experiments that simultaneously manipulate plant arrival order and plant species richness are needed. Such experiments would not only allow us to test how the positive relationship between diversity and priority effects reported in our paper change across a plant diversity gradient (which was not possible to do in our study), but also to investigate what are the ecological mechanisms at play. To sum up, I believe that the predictive power of plant community ecology can be increased by explicitly linking assembly and biodiversity approaches in future experiments. This is an area of research on which I intend to focus my attention in the coming years. 

About the author

I am currently a postdoctoral researcher at the institute of ecology of Leuphana University Lüneburg (Germany). I am a chemical ecologist by training and started to work on plant community ecology during my postdoc. During my PhD, I focused mainly on volatile-mediated plant-plant interactions. Since the beginning of my postdoc, I have been working on interactions between plants in grassland ecosystems, with a particular emphasis on the consequences of priority effects for the structure and functioning of plant communities.

As far as I can remember, I have always been interested in ecology. In addition to being in close contact with nature, what I like the most about being an ecologist is certainly the multidisciplinarity of the field. To answer complex ecological questions, ecologists often have to rely on many different, yet complementary, scientific disciplines such as plant physiology, microbiology, analytical chemistry, statistics, or mathematical modelling. This multidisciplinarity encourages us to work collaboratively, which I really appreciate (after all, there is always more in two heads than in one).

If I have to give one piece of advice for someone starting to do research in ecology, I would say the same thing I am saying to students starting a project with me: be curious and passionate about your work, always explore and plot your data before doing any kind of statistics, and learn how to code! I am often surprised to see how much time can be saved by a simple R loop.

When I am not working, I like spending time with my friends or going outside for a run, a bike tour, a hike, or a backpacking trip across gorgeous landscapes. Ecology is never far away.

You can read the paper in full here and the free plain language summary here.