Abstract
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.
Highlights
As a direct consequence of human activity, island ecosystems worldwide are being transformed by invasive plant and animal species [1]
Five months after the last herbicide application, there was an abundant regeneration of Scalesia pedunculata in the study area of the Scalesia forest, but only very few were still alive towards the end of the study period (Fig 2)
Our results showed that while control of Rubus niveus in the Scalesia forest on Santa Cruz facilitated an abundant natural regeneration of Scalesia pedunculata, only 5% of these new plants survived the 5-year study period (13 out of 259 individuals)
Summary
As a direct consequence of human activity, island ecosystems worldwide are being transformed by invasive plant and animal species [1]. Many efforts are underway to control and eradicate invasives and restore island ecosystems, intending to conserve native and endemic species diversity [2]. Evaluating the success of these restoration projects is often constrained by a paucity of long-term ecological monitoring data [3, 4]. Several long-term restoration projects have been undertaken in the Galapagos Islands (e.g., mammal eradication [8]; plant eradication [9]), demonstrating that even in archipelagoes with comparatively late onset of human settlement like Galapagos [10], active ecological restoration is necessary to protect native ecosystems and conserve biodiversity
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