PhD thesis: The role of global warming on the ecology of exotic macrophytes in the Mediterranean Sea



Foto: (De izq. a dcha.) Iris Hendriks, Marlene Wesselmann y Nuria Marbà allebrera


Esporles, May 16, 2022. Marlene Wesselmann has defended her doctoral thesis supervised by the doctors Nuria Marbà and Iris Hendriks from the Mediterranean Institute for Advanced Studies (IMEDEA (CSIC-UIB)). The event took place on May 13 at the University of Balearic Islands and could be telematically followed on ZOOM.



Ocean warming and the establishment of exotic species (also known as introduced or non-native species) are two evident imprints of global change, with both phenomena showing similar increases. The effect of ocean warming on ecosystems is unequivocal, changing species phenology and driving a global redistribution of species with economic and ecological consequences. Conversely, the effect of global warming on exotic species is still unclear, despite scientific evidence indicating that the increase in temperature could favour their expansion and impact.  This is particularly important in the Mediterranean Sea, which is warming faster than the global ocean and it is a hotspot for exotic species, especially exotic macrophytes which rank also among the taxa with the greatest ecological impacts. Therefore, given this lack of knowledge and considering that exotic species and warming will increase further in the coming future, it is increasingly necessary to understand how exotic macrophytes are likely to respond to global warming over the coming century in the Mediterranean Sea and determine if they will maintain or shift ecosystem functioning in recipient sites.



Through the compilation of a database of geographic records of exotic macrophytes in the Mediterranean Sea reported across more than two centuries and thermal conditions for each exotic species at the range of origin and at the invaded location we found evidence for exotic thermophilic macrophytes (from tropical and subtropical origins) expanding faster during the last decades in the Mediterranean Sea compared to those from a temperate origin (Chapter 1). Subsequently, we experimentally studied the effect of temperature on native and exotic populations of one of the tropical exotic macrophytes invading the Mediterranean (the seagrass Halophila stipulacea) and used the variability in the thermo-tolerance responses in modelling approaches to forecast the species response under future climate scenarios (RCP2.6, RCP8.5) across its native and exotic range by 2050 and 2100. In general, we found that H. stipulacea presents a broad thermal tolerance and a high capacity to cope with warm waters, but populations from the colder Mediterranean regime (exotic) were more tolerant to cold temperatures than populations from warmer areas (Red Sea), demonstrating that the species is able to adapt to new thermal conditions within few decades (Chapter 2.1). The application of species distribution models highlighted the importance of accounting for the variability of species thermal tolerance across different thermal regimes (native and exotic), as the model run with the thermal tolerance of the native population failed to predict the current distribution of the species in the Mediterranean Sea. Conversely, it was accurately predicted when the model considered the thermal tolerance of the exotic Mediterranean populations, which projected the expansion of H. stipulacea through the western Mediterranean by 2100 (RCP 8.5), increasing its distribution in the Mediterranean by 50% (Chapter 2.2). The effect of the expansion of exotic macrophytes like H. stipulacea on ecosystem functioning at the recipient location was assessed in Chapter 3 by comparing the magnitude of key functions (carbon sequestration, biodiversity) provided by neighboring exotic vs. native species. The analysis revealed mixed results, with H. stipulacea showing moderate capacities for sediment organic carbon burial (similar or higher than neighboring meadows of native seagrasses) but low capacities to maintain the macrofaunal diversity in P. oceanica and similar capacities as C. nodosa meadows.



Finally, this thesis also demonstrates that some exotic macrophytes, like H. stipulacea, may become more relevant in the future due its capacity to cope with warm waters and to the warming associated decline of native species not so tolerant to warming. Hence, the research presented in this dissertation significantly contributes to the knowledge on the role of ocean warming on the expansion of exotic macrophytes in the Mediterranean Sea and their capacity to maintain ecosystem functioning.