No way back changes in biotic interactions promoted by invasive alien plants

Resumo

Plant invasions are one of the main direct drivers of species loss in nature, seriously altering the habitat structure and leading to substantial effects on ecosystem functioning. Globalization, habitat fragmentation, and climate change facilitate the migration and colonization of many exotic species that induce changes on native communities through disruption of biotic interactions. These changes are currently taking place at rates unprecedented in the period newly defined as the Anthropocene. Many questions related to plant invasions have been answered in the last decades. However, directions of most recent studies tend to be very global or only focus in a particular assemblage of species, with few considering the many functional and trophic roles of entire groups. It is widely known that environmental disturbances clearly facilitate the spread of invasive plants. Nevertheless, the changes in the plantherbivore interactions and the alteration of the composition of invertebrate communities have received little attention, as well as which environmental factors may favour the invasion success of unaltered ecosystems. To carry out this doctoral dissertation, a greenhouse experiment and several fieldwork studies were developed on vulnerable native ecosystems in the northwestern Iberian Peninsula. This region presents some of the most problematic plant species worldwide: three invasive woody Acacia species (Acacia dealbata Link, Acacia longifolia (Andr.) Willd. and Acacia melanoxylon R. Br.) and the invasive Carpobrotus edulis (L.) N.E. Br. The main objective of this doctoral dissertation is to elucidate whether the impact generated by these plant invasions on the native plant communities alter the biotic interactions, especially affecting the plant-herbivore interactions, and changing the composition of invertebrate communities due to the presence of these invasive plants. The results showed that A. dealbata can emerge and growth in unaltered plant communities, favouring their expansion under natural conditions and adjusting the growth to the ecosystem characteristics. Besides, the microhabitat conditions of areas invaded by A. dealbata play an important role for the establishment of native species, even being the main factor affecting seedling establishment rather than changes in soil properties or allelopathy. Moreover, the results obtained indicate that A. dealbata and C. edulis can accumulate new natural enemies in the introduced range, but the damages found did not seem enough to exert an effective natural control. Carpobrotus edulis may also alter the associated herbivore community structure by changing the plant-herbivore interactions and influencing the occurrence of herbivore assemblages. Finally, it is noted that the invasive plants studied alter the biodiversity and ecosystem functioning by reducing the invertebrate assemblages and functional diversity. Overall, the results presented here provide evidence that invasive plants alter the biotic interactions, especially affecting the plant herbivore interactions and changing the composition of invertebrate communities. It is important to explore the direct and indirect effects produced by plant invasions on the taxonomic and functional diversity to detect alterations of the associated organisms at the ecosystem scale. Predicting such changes poses a serious challenge to conserve the ecosystems in a time of rapid environmental disturbance. This information is vital to define long-term management strategies for reducing the spread of these invasive plants.