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Island forests are becoming increasingly fragmented and colonized by invasive species, which can eventually lead to local species extinctions. In the Galapagos Islands, invasive species pose a serious extinction threat to the endemic daisy tree Scalesia pedunculata, formerly the dominant habitat-forming species of the unique Scalesia forest. This forest has been reduced to fragments due to land use changes in the past and is now increasingly invaded by introduced plants. We conducted a field experiment on Santa Cruz Island to assess the impacts of blackberry (Rubus niveus) and two other invasive plant species, Cestrum auriculatum and Tradescantia fluminensis, as well as the effects of the removal of two of these (R. niveus and C. auriculatum) on cover, composition and diversity of the resident plant communities. Particular attention was paid to effects of the invasive species on the S. pedunculata population. Annual vegetation monitoring was carried out in a total of 34 permanent plots (10 m × 10 m) over 10 years (2014–2023), using the line-intercept method. Seventeen of these plots were established in an invaded area and 17 plots in an area with continuous invasive plant removal since 2014. Results indicated that there were significant changes in both the species composition of the plant communities and average percent cover of species over time, comparing removal plots with invaded plots. Species composition in removal plots changed significantly more than in invaded plots, towards a plant community with greater percent cover of endemic species. A significant negative relationship between the three invasive species and cover of S. pedunculata suggested that multiple invader species may have additive negative impacts. Natural recruitment of S. pedunculata by seeds was observed in the removal but not in the invaded plots. These results, as well as the striking decrease of 71% in cover of adult S. pedunculata in the invaded plots indicate that this threatened species will be driven to local extinction on Santa Cruz Island, Galapagos, in less than 20 years if invasive plant species are not removed on a large scale.

More than 60% of the flora of the Galapagos Islands is introduced and some of these species have become invasive, severely altering ecosystems. An example of an affected ecosystem is the Scalesia forest, originally dominated by the endemic giant daisy tree Scalesia pedunculata (Asteraceae). The remnant patches of this unique forest are increasingly being invaded by introduced plants, mainly by Rubus niveus (blackberry, Rosaceae). To help large-scale restoration of this ecologically important forest, we seek to better understand the natural regeneration of S. pedunculata after invasive plant control. We monitored naturally recruited S. pedunculata saplings and young trees over five years in an area where invasive plant species are continuously being removed by manual means. We measured survival, height and growth of S. pedunculata saplings and young trees along permanent transects. Percent cover of surrounding plant species and of canopy shade directly above each S. pedunculata individual were determined, as well as distance to the next mature S. pedunculata tree. We identified potential factors influencing initial sapling survival and growth by applying generalized linear models. Results showed a rapid growth of saplings and young trees of up to 0.45 cm per day and a high mortality rate, as is typical for pioneer species like S. pedunculata. Sapling survival, growth and mortality seemed to be influenced by light availability, surrounding vegetation and distance to the next adult S. pedunculata tree. We concluded that natural regeneration of S. pedunculata was high only five months after the last herbicide application but that 95% of these recruits had died over the 5-year period. Further studies are needed to corroborate whether the number of surviving trees is sufficient to replace the aging adult trees and this way maintain remnants of the Scalesia forest. Urgent action is needed to help improve future restoration strategies to prevent further degradation of this rapidly shrinking threatened forest ecosystem.