Don’t leave me behind: viability of vegetative propagules of the clonal invasive Carpobrotus edulis and implications for plant management

Managing the invasive plant Carpobrotus edulis: is mechanical control or specialized natural enemy more effective?

Carpobrotus edulis is an invasive clonal plant with drastic effects on biodiversity and functioning of coastal ecosystems. In recent years, authorities and land managers have implemented diverse management strategies that usually focus on mechanical removal and chemical control. However, applying mechanical control to remove C. edulis may cause indirect adverse effects since it could increase the probability of spreading new propagules, which do not lose their physiological activity. Therefore, reducing the physiological activity of these plant fragments should be a priority to avoid their spread and re-rooting. Our goal was to assess the plant regeneration capacity after applying mechanical control (i) when placing the plant material on different types of ground surface (on sand, on stones and using rooted plants as control) and (ii) combined with the attack of specialized herbivores (the soft scale Pulvinariella mesembryanthemi). To achieve this, we evaluated how these two factors (ground surface and herbivory) affected the plant physiological activity, its survival and re-rooting, biometric measurements, shoot and root nutrient composition and biochemical parameters (total phenols and tannins). Regardless of the ground surface type, our results indicated that the specialist herbivore greatly affected the C. edulis parameters studied. The attack of P. mesembryanthemi stimulated the plant defence mechanisms, even in those individuals with less photosynthetic activity. Furthermore, P. mesembryanthemi severely reduced the biomass and volume of plant material. Decomposition of C. edulis was accelerated by the combination between the inoculation of P. mesembryanthemi and placing the plants on the stones ground surface. Overall, preventing plant re-rooting by avoiding connection to the soil is an effective method of reducing its viability after the eighth-tenth month. After applying mechanical control, we recommend placing C. edulis fragments over an inert ground surface to avoid re-rooting, which would favour its death. We conclude that the combination of mechanical control and P. mesembryanthemi or even direct inoculation with this specialist herbivore could help authorities and land managers to improve management strategies for C. edulis.

Biogeographical comparison of terrestrial invertebrates and trophic feeding guilds in the native and invasive ranges of Carpobrotus edulis

Plant invasions impact on biodiversity by altering the composition of native communities by disrupting taxonomic and functional diversity. Non-native plants are often released from their natural enemies, which might result in a reduction of the attack of primary consumers. However, they can also be exposed to the attack of new herbivores that they might not be able to tolerate. Hence, invertebrate communities can be influenced by invasive non-native plants, which in turn modify interactions and change environmental conditions. In this study, we examined the compositional and trophic diversity of invertebrate species, comparing ecosystems with and without the plant species Carpobrotus edulis in coastal areas in its native (South Africa) and introduced (Iberian Peninsula) ranges. Results show that C. edulis has a clear impact on invertebrate communities in its non-native range, reducing their abundance in invaded areas, and particularly affecting certain trophic groups. Invasive C. edulis also alters the invertebrate diversity by not only reducing abundance but also by altering species composition. Overall, the physical dominance of C. edulis modifies the co-occurrence of invertebrate assemblages, reducing the number of trophic groups and leading to substantial effects on primary consumers. Results suggest that the lack of natural enemies might be an important driver of the expansion of C. edulis in its introduced range. Further work is needed to examine long-term changes caused by non-native plants on invertebrate assemblages and the subsequent modification of biological interactions.

Drifting away. Seawater survival and stochastic transport of the invasive Carpobrotus edulis

Coastal areas are vulnerable and fluctuating habitats that include highly valuable spaces for habitat and species conservation and, at the same time, they are among the most invaded ecosystems worldwide. Occupying large areas within Mediterranean-climate coastlines, the "ecosystem engineer" Carpobrotus edulis appears as a menace for coastal biodiversity and ecosystem services. By combining the observation, current distribution, glasshouse experiment, and dispersion modeling, we aim to achieve a better understanding of the successful invasion process and potential dispersion patterns of C. edulis. We analyzed the response of plant propagules (seeds and plant fragments) to seawater immersion during increasing periods of time (up to 144 h). After 2 months of growth, plant fragments showed a total survival rate (100%) indicating high tolerance to salinity. During this time, fragment length was increased (up to 60%) and root length was higher than control in all cases. Also, immersed fragments consistently accumulated more biomass than control fragments. After two months of growth, photosynthetic parameters (F_v'/F_m', Φ_NO, and Φ_II) remained stable compared to control fragments. Physiologically, osmolyte and pigment content did not evidence significant changes regardless of immersion time. Based on the capacity of propagules to survive seawater immersion, we modeled the potential transport of C. edulis by combining an oceanic model (ROMS-AGRIF) with a particle-tracking model. Results indicated that propagules may travel variable distances maintaining physiological viability. Our model suggested that short-scale circulation would be the dominant process, however, long-scale circulation of propagules may be successfully accomplished in <6 days. Furthermore, under optimal conditions (southerly winds dominance), propagules may even travel large distances (250 km alongshore). Modeling transport processes, in combination with the dynamics of introduction and expansion, will contribute to a better understanding of the invasive mechanisms of C. edulis and, consequently, to design preventive strategies to reduce the impact of plant invasion.

Herbivory induced non-local responses of the clonal invader Carpobrotus edulis are not mediated by clonal integration

The anthropogenic displacement of species around the world results in new environmental situations where native and exotic species coexist. Exotic plants have to face native herbivores, and interactions between introduced plants and native herbivores seem to play an important role in the invasiveness of some exotic plant species. We studied the role of clonal integration in induce morphological, physiological, and biochemical responses in the clonal invader Carpobrotus edulis against the attack of the native snail Theba pisana. Our results demonstrated the presence of labour division mediated by physiological integration, with a significant increase of photosynthesis potential (both at morphological and physiological) in un-attacked integrated ramets. This response could be especially important under herbivory, as the negative impact of T. pisana on the photosynthetic structures of attacked C. edulis ramets could be buffered by transferring the dependence of photosynthetic activity to the un-attacked ramets. Our results also showed a constitutive resistance in un-attacked apical ramets, showing a similar amount of defence compounds to those exhibited in the basal branches attacked by snails. Results reported a non-local compensatory response, which there was an increase of total biomass in apical ramets when their basal ramets were attacked by the herbivore. We interpret this result as a compensatory response, with these apical ramets increasing shoot biomass to compensate for the biomass loss due to a potential attack from herbivores. However, this non-local response was not mediated by physiological integration but probably due to belowground communication, with the presence of alarm signals released by root exudates. We conclude that the attack of this snail is not enough to be a possible biological control due to the compensatory response to this snail by C. edulis, favouring their expansion. Future studies should focus on unravelling the role of belowground communication in the defensive responses of C. edulis.