Overview:
We selected this manuscript for our lab meeting because we are interested in biotic determinants of plant coexistence, particularly using Chesson's framework, as part of our group is working on this topic.
The paper aims to test the effects of competition and pathogens in stabilizing Californian perennial grassland communities invaded by exotic plant species. For that, the authors develop a population dynamical model which accounts for the particularities of the lifespan of 2 native (perennial) and 3 exotic (annual) plant species. Estimates of each parameter of the model are obtained from field observations and experiments. The dynamical population model predicts that native species will be able to persist within the invasion of the exotic grass species. In this model, priority effects are to a large extent responsible for explaining the persistence of native species. Importantly and contrarily to expects pathogens
play a minor role in determining the coexistence of native and exotic plant species.
Positive feedback:
We agreed that the paper was very interesting in exploring the role of pathogens in coexistence. Also, it presents a nice framework to apply principles from coexistence to species with different life-history (annual and perennial plants), with the theoretical and methodological challenges that is supposes. Overall, this makes a very nice contribution to the scientific literature.
Main concerns:
The main concerns that we discussed in our lab meeting were:
- We wondered whether the paper is able to calculate niche and fitness differences explicitly. Right now it uses the GRWR (growth rate when rare) instead, which is a combination of both. So far, the calculation of niche and fitness differences has become reasonable straighforward when working with species with annual life cycle, but this paper among a few others (Chu and Adler 2015 Ecol. Mon.) show the difficulties when species with longer and more complicate life cycles are included.
- Perhaps the model is too complex (too many parameters) for the low accuracy that achieves explaining the patterns found natural communities. We discussed that there is a trade-off between simplicity and realism when building models that must be considered depending on each particular case of study. Simple models can be useful to infer mechanisms even when its predictive power is low, on the other end complex models can be useful when they have high predictive power. But in between this two extremes, models enter a grey area where they may stop being useful. This is acknowledged by the authors who provide alternative explanations to put in agreement their results with what is observed in the field at larger scales.
Other thoughts and future prospects:
Overall, we think that in the years to come it would be awesome to advance the development of mathematical tools to estimate niche and fitness differences for other communities besides the annual plant communities as the example of the present native perennial Californian grasslands. We also felt that given the theoretical nature of the paper, the direct conservation implications of this work do not marriage well in the paper, and this section could be shortened. Finally, we congratulate the authors for their work and we hope seen it published in an international journal soon.