Plants don’t exist within isolation. They live in a web of competition and cooperation with other plants. Plants succeed if they find a niche to occupy. But how do they find that niche? Yuanzhi Li and Bill Shipley carried out a series of experiments on herbaceous plant mesocosms. The mesocosms faced varying degrees of stress and disturbance. Dr Li said: “We expect that importance of assembly processes (i.e. environmental filtering and limiting similarity) should depend on environmental conditions and thus modify the patterns of functional niche occupation differently.”
Habitat filtering is the process that weeds out plants with unsuitable traits for specific locales. This means the plants in habitats tend to converge on certain traits. So plants in a desert tend to share an ability to handle water stress. Limiting similarity is the factor that stops plants occupying exactly the same niche.
To see what role habitat filtering and limiting similarity play, Li and Shipley set up 24 mesocosms. These were plots 112.5cm × 90cm × 36cm big at the University of Sherbrooke, Quebec, Canada. The authors then gave the mesocosms three levels of stress and four levels of disturbance. This meant that two mesocosms would share the same conditions. They then looked to see how the plants were affected.
The scientists found that habitat filtering and limiting similarity both played equal parts in how the communities changed. Dr Li was surprised. “Habitat filtering and limiting similarity seemed equally important under different environmental conditions. However, we are not confident about this result as we mentioned in the discussion that it might be due to the low statistical power with only two mesocosm replicates or due to the narrow gradients of stress and disturbance.”
“Maybe investigating functional niche occupation in different natural systems with wider environmental gradients at a larger scale would give us a more comprehensive and robust a picture and the underlying mechanisms.”
The experiment has already run a long time, as Li explained. “The experiment was actually maintained for 7 years before it ended, which is much longer than a PhD. Fortunately, My supervisor (Bill Shipley) had already started it and dealt with most problems before I came to do my PhD. When I came, the experiment was well established.” This allowed the authors to use five years of plant succession to draw their conclusions.
Li and Shipley say in their paper that part of that more comprehensive picture will be developing ‘a general and comparable measure of “stress” and “disturbance”….’ This will allow researchers to compare results between experiments.
If people can agree what stress and disturbance are then there should be plenty of opportunity for productive experiments in the future. The authors conclude in their paper: “…[O]ur study does provide an experimental basis for the underlying processes of habitat filtering and limiting similarity structuring community functional niches along environmental gradients of stress and disturbance.”
Li, Y., & Shipley, B. (2019). Functional niche occupation and species richness in herbaceous plant communities along experimental gradients of stress and disturbance. Annals of Botany. https://doi.org/10.1093/aob/mcz140