Testing the relative roles of competition and plant-soil feedbacks in explaining commonness and rarity of native and of alien plant species
Final Report Abstract
Commonness of native and invasiveness of alien plant species may both be driven by competitive ability, and specifically the ability to maintain plant performance while growing at higher intraspecific densities. Species that are rare or non-invasive may have low competitive ability, and are unable to maintain high plant performance when at higher intraspecific densities, while common and invasive species may be more successful through suffering less negative density dependence. Recently, plant-soil feedbacks have been put forward as another potential driver that explains both abundance of native species and invasiveness of alien species. Common native and invasive alien species may suffer less from soil pathogens, and therefore show higher plant performance than rare native and non-invasive species. These plant-soil feedback effects may act separately from, underpin, or interact with the ability of plant species to compete intraspecifically. In this project, we assessed the relative roles of plant-soil feedbacks and competition in explaining commonness of native and invasiveness of alien plant species. We did this using three multispecies experiments, each of which targeted a different aspect of plant life-history- 1) individual plant growth, 2) plant population growth, and 3) plant establishment in grassland communities in the field. To test whether individual plant growth was less affected by competition or plant-soil feedbacks in common and invasive species, compared to rare and non-invasive ones, we used a greenhouse experiment with 30 common and rare alien and native species from nine plant families. We tested whether plant biomass decreased less for common than rare species, regardless of origin, when grown at higher relative frequencies in a community and in soil previously conditioned by the same species at different frequencies in an orthogonal design for these two factors. Our findings suggested that alien species but not natives showed a decrease in performance when grown in soil pre-conditioned with a higher frequency of conspecifics. In general, our findings showed that soilborne pathogen accumulation might be weak in its effects on plant performance compared to intraspecific competition, with neither being clearly linked to species commonness. To test whether plant population growth was less affected by competition or plant-soil feedbacks in common and invasive species, compared to rare and non-invasive ones, we used a mesocosm experiment in which we grew 20 annual common and rare species of alien and native origins from three different families over two years. Species communities were initially sown at two relative abundance levels (high/low) and half of them were regularly treated with fungicide. Our main finding was that alien species might gain an advantage through greater recruitment into populations when at low relative abundance, allowing them to invade native plant communities. Notably, native and alien species similarly profited from fungicide treatments, suggesting that both origins may be affected by generalist pathogens to a similar degree. Finally, to test whether establishment of invasive and common species was less affected by natural enemies above or belowground, or by competition from a resident community than for non-invasive and rare species, we used a two-year field experiment where we introduced 10 alien and 10 native plant species that are either common or rare in Germany, to grassland communities. We found that less disturbed grassland communities exhibited strong biotic resistance to establishment success of species, whether alien or native. However, we also found evidence that alien species may benefit weakly from soil-borne enemy release, but that this advantage over native species is lost when the latter are also released by biocide application. Thus, disturbance was the major driver for plant species establishment success and effects of pathogens on alien plant establishment may only play a minor role. Taken together, our findings strongly suggest that the effects of soil microbes on alien plant invasiveness and native plant commonness are not generalizable. Competition and disturbance are of prime importance in affecting plant performance, while plant-soil interactions seem to be secondary. Common/invasive species appear to be affected in a similar way to rare/non-invasive species, but alien species seem to have a disadvantage compared to natives when colonising and competing with native communities and this may result from maladapation to environmental factors which have yet to be identified. However, alien species may have an advantage over natives in low-competition environments through having higher per capita population growth rates compared to native species.
Publications
- (2016). Alien and native plant establishment in grassland communities is more strongly affected by disturbance than aboveand below-ground enemies. Journal of Ecology 104(5): 1233-1242
Müller, G., Horstmeyer, L., Rönneburg, T., van Kleunen, M. & Dawson, W.
(See online at https://doi.org/10.1111/1365-2745.12601) - (2016). Commonness and rarity of alien and native plant species - the relative roles of intraspecific competition and plant - soil feedback. Oikos 125: 1458-1466
Müller, G., van Kleunen, M. & Dawson, W.
(See online at https://doi.org/10.1111/oik.02770)
