BIODIVERSITY AND ECOSYSTEM RESTORATION ä ä GALAPAGOS REPORT 2015 - 2016 Restoration of the blackberry-invaded Scalesia forest: Impacts on the vegetation, invertebrates, and birds Heinke Jäger1, Sascha Buchholz2, Arno Cimadom3, Sabine Tebbich3, Jacqueline Rodríguez1, Denisse Barrera1, Anna Walentowitz4, Mareike Breuer2, Alonso Carrión5, Christian Sevilla5 and Charlotte Causton1 Photo: © Steven Frank 1Charles Darwin Foundation 2Technische Universität Berlin, Germany 3University of Vienna, Austria 4University of Greifswald, Germany 5Galapagos National Park Directorate The majority of studies on the impact of invasive plants have focused on single- species invasions despite the prevalence of co-occurring invasive species in many ecosystems that have the potential to interact (Kuebbing et al., 2013). Understanding the role of plant invaders in an ecosystem as well as interactions between and among species is important and can significantly affect the outcome of restoration programs (D’Antonio & Meyerson, 2002). A holistic approach toward restoration of the ecosystem is essential, though seldom undertaken (McAlpine et al., 2016). In the present study in the highlands of Santa Cruz Island in Galapagos, we addressed these shortcomings by investigating interactions among different invaders and by simultaneously assessing impacts of chemical blackberry control on the vegetation, invertebrates, and on bird numbers and breeding success in a forest dominated by the endemic Scalesia pedunculata tree at Los Gemelos. The Scalesia forest, housing the highest number of plant and animal species in the highlands of Santa Cruz, has been drastically reduced by agricultural activities in the past and more recently, by invasive plants (Rentería & Buddenhagen, 2006). On Santa Cruz, about 100 ha remain, less than 1% of its original distribution, with the largest concentration at Los Gemelos (Mauchamp & Atkinson, 2011). One of the worst invasive plants at Los Gemelos is blackberry (Rubus niveus, Rosaceae), which grows vigorously and prevents recruitment of native species, thus changing the surrounding plant community (Rentería et al., 2012). Over more than ten years, the Galapagos National Park Directorate (GNPD) has successfully controlled blackberry at Los Gemelos by applying herbicides. However, there is concern that this intensive management has changed the structure of the forest, and is impacting the invertebrates and birds that live there (Cimadom et al., 2014). To evaluate this, we compared vegetation composition, insect abundance, bird numbers, and breeding success of two finch species, the green warbler-finch (Certhidea olivacea) and the small tree-finch (Camarhynchus parvulus), in two areas, one with a high density of blackberry and the other where the GNPD is actively controlling it. Methods To study the impacts of blackberry density and the chemical control of blackberry 143 GALAPAGOS REPORT 2015 - 2016 on the plant and animal communities, we defined two Controlled Area were first cut down manually using a study areas at Los Gemelos, and, in 2014, established 34 machete and after two months the regrowth was sprayed permanent 10×10 m plots. All plots contained blackberry with herbicides (applying a mixture of glyphosate and and other dominant invaders, such as sauco (Cestrum Combo©). After another two months, re-sprouting auriculatum) and wandering jew (Tradescantia fluminensis; invasive species were removed by a weed whacker and Figure 1). Of the 34 plots, 17 were within an 8-ha area by hand. Monitoring of the vegetation, invertebrates, and that was left untouched (referred to as the Reference birds was carried out once before control measures were Area), and 17 were within a 6-ha area where blackberry applied, and then again in 2015 and 2016. was controlled (Controlled Area). Invasive plants in the Los Gemelos Controlled Area Reference Area Figure 1. Study site at Los Gemelos, Santa Cruz: 17 permanent plots in the Reference Area (lower left corner) and 17 plots in the Controlled Area where blackberry was chemically controlled (upper right corner). Results and Discussion facilitate the germination and/or growth of blackberry. Plants Percent cover of the non-dominant plant species (herbs, grasses, shrubs, and trees) also increased after chemical Control measures were successful in reducing blackberry control of blackberry (Figure 2), which could be an cover; however, blackberry cover also decreased, though indication of recovery. However, the number of non- to a lesser degree, in the Reference Area over the same dominant introduced species and species of unknown period (Figure 2). This may be because blackberry goes origin increased after control actions were carried out through a natural die-back cycle and then recovers again (Figure 3 A, D) and could have accounted for a higher (H Jäger, pers. observ.). Wandering jew and sauco cover total plant cover. At the same time, total number of increased in the Controlled Area and it is probable that species remained roughly the same, whereas the total the presence of these species suppresses germination number of endemic species slightly decreased (Figure and/or growth of blackberry. Because of this, it might be 3 B, C). This trend was not observed in the Reference beneficial to leave them in areas that they have invaded. Area (Figure 3 A-D) indicating that disturbance caused Although these are also invasive species with a potential by the control measures might have facilitated the negative impact on the surrounding vegetation, they are expansion of invasive species and establishment of other less harmful than blackberry and their removal seems to introduced species, as was also shown in the case of 144 GALAPAGOS REPORT 2015 - 2016 quinine control (Jäger et al., 2009; Jäger & Kowarik, 2010). of Santa Cruz during February 2016. No new seedlings Future monitoring is needed to show whether this is an were found in this area. During the same time, 47 out of ephemeral phenomenon or not. 255 (18.4%) trees died in the Controlled Area; however, the chemical control of blackberry had a spectacular Percent cover of Scalesia pedunculata in the Reference Area effect on the regeneration of Scalesia pedunculata. Only decreased by a third over the monitoring period (Figure 2 five months after the last herbicide application by the A), which could be due to tree mortality. Over the course of GNPD, up to 280 Scalesia seedlings per plot were found two years, 71 out of 200 (35.5%) trees died in the Reference at the Controlled Area where there had been none during Area. The reason for this is not clear but could have been our first monitoring. This was reflected by an increase in caused by strong winds that occurred in the highlands Scalesia cover in the Controlled Area (Figure 2 B). 200 200 2014 2016 150 150 100 100 % Cover % Cover 50 50 0 0 no dom Ces_aur Rub_niv Tra_u Sca_ped no dom Ces_aur Rub_niv Tra_u Sca_ped Reference Area Controlled Area Figure 2. Percent cover of the dominant invasive species sauco (Ces_aur), blackberry (Rub_niv), wandering jew (Tra_flu), and the endemic Scalesia pedunculata (Sca_ped), and of the non-dominant, remaining vegetation (non_dom) at the study sites (N=17 each for the Controlled and Reference Areas). 30 30 A B Reference Area 25 25 Controlled Area 20 20 15 15 10 10 Total #NaQ species Total 5 species # native Total 5 0 0 2014 2014 2014 2014 30 30 C D 25 25 20 20 15 15 10 10 5 5 Total # endemic species Total Total # introduced species # introduced Total 0 0 2014 2014 2014 2014 Figure 3. Total number of non-dominant plant species at the two study areas: A – Questionable native species (NaQ; origin unknown, could be native or introduced); B - native species; C - endemic species, and D - introduced species (N=17 each for the Controlled and Reference Areas). 145 GALAPAGOS REPORT 2015 - 2016 We also determined that the threshold of blackberry Analyses at species level are pending. At this stage, it cover that the resident plant species (native and endemic) is difficult to reach conclusions about the dynamics can tolerate, and under which they can still thrive, is about of invertebrates in both study areas. Preliminary data 60%. These results confirm those of Rentería et al. (2012), on spider abundance (Araneae) showed similar trends who also determined a 60% blackberry cover as (barely) in the Reference Area and Controlled Area (Figure 4), tolerable for the persistence of the resident plant species suggesting that climate might influence invertebrate in the understory. While this percentage is far from ideal, it numbers at both sites. However, this does not explain the can serve as a guideline for management actions. difference in abundance between the two study areas prior to and after control efforts. Further analyses and Invertebrates continued monitoring are needed to determine whether management efforts have affected invertebrate diversity Around 16,184 specimens were collected from Malaise and abundance. traps and 766 from Pitfall traps set out in the study areas. Reference Area Controlled Area 250 250 200 200 150 150 100 100 Total # Spiders Total # Spiders Total 50 50 0 0 2014 2015 2016 2014 2015 2016 Malaise traps Pitfall traps Figure 4. Total number of spiders caught in Malaise traps (results shown are sums of individuals caught in two Malaise traps placed in each area for four weeks), and in 51 Pitfall traps (three traps per plot placed in each area). Birds area before and after the control measures was influenced by drought conditions in 2015. Breeding success in this The breeding success of the green warbler-finch and small year was extremely low for both species in both areas. The tree-finch did not differ significantly between the two areas overall very low breeding success may have masked the in any of the three years (Figure 5). The comparison within short-term effect of the mechanical and chemical control 60 60 Reference Area Controlled Area 50 50 40 40 30 30 20 20 10 10 % Successfull nests Warbler-Finch nests % Successfull 0 Tree-Finch nests Small % Successfull 0 2014 2015 2016 2014 2015 2016 Figure 5.