Common or rare - all about the soil?

The question why some species are rare while others are common has puzzled ecologists since decades and it has not lost any of its relevance. Especially in times of globalization, species invasions are among the most pressing issues in ecology. More recently it has been proposed that soil-biota and more specifically pathogens may play an important role in driving the success of plant species, through so-called ‘plant-soil feedback’.

In our study „Commonness and rarity of alien and native plant species- The roles of intraspecific competition and plant-soil feedback“, we used a multi-species greenhouse study to test if differences in density-dependent plant-soil feedback or intraspecific competition could explain species success. Since invasiveness of alien and commonness of native species may be driven by the same mechanisms, we used an experimental setup that included common and rare alien and native plant species.

We found an overall decrease in performance with increasing intraspecific competition, but no differences based on origin or commonness of the species. Contrary to the enemy release hypothesis we also detected a weak negative plant-soil-feedback effect for alien species, but not for natives, across our set of 30 species. Again, plant-soil feedback effects did not differ between common and rare species. While these findings may be unexpected at first sight, they highlight that alien species in general may not always benefit from enemy release and that novelty may be claimed for both sides of the plant-pathogen interaction, which can eventually lead to more negative responses of alien species.

On a more personal note, I think that multi species experiments are a powerful and useful approach to test if assumptions and hypotheses are generalisable (and they are work- intense but fun to do). However, species responses may prove to be very variable, and simple and appealing hypotheses are unlikely to be applicable across all species, communities, and types of interaction.