PhD thesis: Plant invasions: a multi-disciplinary comparison between coexisting native and non-native plant pairs



Foto: Mohamad Abdallah en la defensa de su tesis doctoral (Autora: Suha Serhan)


Esporles, July 13, 2022. Mohamad Abdallah has defended his doctoral thesis supervised by the doctors' Anna Maria Traveset Vilaginés (from the Mediterranean Institute for Advanced Studies (IMEDEA (CSIC-UIB)) and Jaume Flexas (from the University of Balearic Islands). The event took place on July 12 at the University of Balearic Islands and was telematically followed on ZOOM.

Plant invasions are considered one of the most devastating ecological problems of the 21st century. The current world trends, in particular those related to international trades and human movements, are expected to increase dramatically the extent and frequency of the introduction of nonnative species. Although a high number of studies have been dedicated to analyzing how invasions by non-native plants occur, it is still not well understood why some non-native plant species become successfully established outside their native ranges while others do not. In this context, the overarching goal of this Ph.D. thesis was to understand the different mechanisms that underlie under such successful establishment, using a multidisciplinary approach: ecology, phytochemistry, and leaf physiology. We first evaluated differences between native and non-native species as two different groups of plants, and then we used multiple-species pair experiments (each pair formed by a native and a non-native coexistent species) to search if the observed general patterns can be applied to every particular non-native species in comparison with its native coexisting species. We combined field observations and experiments under controlled conditions to highlight the ecological relevance of our results while allowing us to better investigate the cause-effect mechanisms.

Direct observations revealed that the proportion of pollinators shared by native and non-native species fluctuated from 0 to 33%, and the pollinators most commonly shared in each plant pair were not the most generalized species. The non-native plants tended to acquire a largely different set of pollinators than their native counterparts, suggesting that non-native species caused little interference on the pollination success of the native species. Among the assessed floral traits, corolla length at opening and flower abundance showed to be important in determining the differences in flower visitation rate between natives and non-natives. Our results on the comparisons of morphological and chemical traits showed that only a few of them differed significantly between native and non-native species. On the other hand, the significance of such differences was inconsistent between different resource environments. Such findings indicate that the performance of non-native plants is ‘context-dependent’, and does not concur with the idea of non-natives species consistently outperforming native species in high resource environments and that the latter surpasses non-natives in low resource environments.

Regarding the phytochemical approach, the impacts of allelochemicals and antioxidant defense mechanisms on non-native plants and their native counterparts were not conclusive. The leaf-physiological approach showed that the assessed non-native species on a Mediterranean island tended to have advantageous values in their leaf physiological traits, such as possessing lower leaf construction cost as compared to the co-occurring native ones. Nonnative species also possess larger mesophyll conductance (gm) and lower mesophyll conductance limitation to photosynthesis (lm), two novel traits to be added to the ‘leaf physiological trait invasive syndrome’. When it comes to the leaf economics spectrum, our findings suggest that native and non-native species are placed on the opposite sides of the leaf economics spectrum, with the non-native species positioned on the fast return end.

Most of our findings support the general trend that the successful establishment of non-native species in new geographical zones could be influenced by one or multiple mechanisms. Yet, they also illustrate that this cannot be generalized for every particular non-native plant when comparing it with its native coexisting plant species. This fact highlights the importance of evaluating other mechanisms or approaches (e.g., evolutionary ecology, genetics) at multiple stages of the invasion process, and across a larger number of species, to assess whether there is a general pattern contributing to the invasion success of non-native species.