Priority effects occur when species that arrived early during assembly affect the establishment, growth, or reproduction of species arriving later. Depending on the direction of the effect that early-arriving species have on the late ones, priority effects can be either positive (late species favoured by early species) or negative (late species inhibited by early species).
Priority effects are not just a matter of competition for limited resources. In a paper published in 2015, Tad Fukami defined two classes of mechanisms leading to the creation of priority effects: niche pre-emption and niche modification. Niche pre-emption is linked to resource capture by early-arriving species: species arriving first at a site decrease the amount of essential resources (e.g., water, nutrients, light) that will be available for species arriving later. Niche modification occurs when early-arriving species modify the types of niches available for late-arriving species. In grasslands, niche modification includes mechanisms such as allelopathy (via root exudation) and biotic plant-soil feedbacks (selection of a particular soil microbiome that can affect the identity of the species able to colonize the community). Since the beginning of my postdoc, I have been particularly interested in understanding the mechanisms leading to positive and negative priority effects in grasslands. Using a combination of techniques including root image analysis, planar optode imaging, and rhizosphere metabolomics, I aim to understand how priority effects created by manipulating plant order of arrival can affect root productivity, root traits, rhizosphere processes, and vertical root distribution (species complementarity). |
Both diversity and priority effects are important drivers of the structure and functioning of grassland plant communities, but they have been rarely studied in combination. One possible reason for this is that experiments that simultaneously manipulate plant species richness (diversity) and plant order of arrival (priority) are quite challenging to set up because of the large number of treatment combinations that such experiment would have. However, I believe that long-term experiments that consider historical contingency in plant species order of arrival alongside plant species richness are now needed in order to gain a better understanding of how diversity and priority effects interactively drive the functioning of plant communities and the mechanisms allowing species to coexist.
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