Towards an Optimal Management of the Invasive Plant Rubus Niveus in the Galapagos Islands

Towards an Optimal Management of the Invasive Plant Rubus Niveus in the Galapagos Islands

Towards an optimal management of the invasive plant Rubus niveus in the Galapagos Islands Jorge Luis Rentería Bustamante Faculty of Natural Sciences, Division of Biology, Silwood Park Campus, Ascot, Berkshire, SL5 7PY, U.K. A thesis submitted for the degree of Doctor of Philosophy Imperial College London October 2011 Abstract Management actions to mitigate the impacts of invasive plant species require knowledge of the mechanisms influencing invasion success and anticipating interactions with various control options. To meet this need, I examined the impacts of the invasive plant Rubus niveus on the native communities of the Scalesia forest of Santa Cruz Island; its competitive abilities compared to some native, woody, species; and, factors affecting the invasion process. This knowledge was then used to evaluate and understand the failure of a five year eradication attempt of R. niveus on Santiago Island. Increasing densities of R. niveus had a negative effect on plant diversity and abundance also resulting in changes of forest structure. Experimental plots were used to elucidate mechanisms of how it displaced native species. Rubus niveus showed a faster growth rate and biomass production than native woody species; it also had a vastly larger seed bank. Increasing sunlight positively affected the growth, biomass production and reproduction of adult plants whereas germination was optimal at intermediate light conditions. Conversely, water stress affected mainly the performance of R. niveus whereas native species were more resilient. Although increasing native canopy cover negatively affected density of R. niveus, it still survived under low light conditions. The implication is that R. niveus rapidly invades after individual tree-falls or stand dieback but also is capable of invading undisturbed forest. After five years of intensive management of R. niveus in Santiago Island eradication seems unlikely. The invasion area continues to expand because: a failure to find all plants before they fruit, bird dispersal over long distance and the ability to colonize undisturbed areas and outcompete native vegetation. Furthermore, management actions have altered ecosystem processes. A more strategic paradigm in needed for R. niveus in Galapagos. 2 Declaration I declare that this thesis: “Towards an optimal management of the invasive plant Rubus niveus in the Galapagos Islands”, is entirely my own work, and it has not been submitted for any other academic qualification. Any other material is accordingly referenced and co-authors on published and submitted chapters are listed as footnotes at the start of each chapter. People who provided less formal help and advice are listed in the acknowledgements. Jorge Luis Rentería Bustamante 3 Acknowledgements Firstly, I extend my deep gratitude to Galapagos Conservation Trust for providing a scholarship to fund my PhD studies and to the Charles Darwin Foundation and the Rufford Small Grants Foundation for covering field work costs. I also thank the Galapagos National Park Directorate for allowing me to carry out this research project, for logistical support, and access to their data for work in Santiago. I would like to acknowledge Mark Gardener and Rachel Atkinson for their contributions in conceiving this research, carrying it out, and refining its interpretation and presentation. They must be credited also for their constant support and cultivating me as a researcher; I am enormously appreciative. My thanks also go to Mick Crawley for his guidance and supervision. Dane Panetta also gave considerable input to both chapters two and five. I am very grateful to many people who assisted me in the field, greenhouse, and lab work; their help was critical to making this research possible. I am especially grateful to Claudio Crespo for his hard work and commitment to this project. My thanks go to Cristina Banks, Bernardo Garcia and Ana Bento for their help with R and the statistical analysis. Alan Tye, Chris Buddenhagen, Reite Molotsane, Mandy Trueman, Heinke Jäger and Phil Saunders made helpful comments on the different chapters of this research. Finally, I am also thankful to Graham, Tim, Alex, Todd, Tariq, Paula, Gebre and Spencer for their constant support during this long journey, and my family and friends for always believing in me. 4 Table of Contents Abstract ........................................................................................................................2 Declaration ........................................................................................................................3 Acknowledgements ............................................................................................................4 List of Tables ......................................................................................................................7 List of Figures.....................................................................................................................8 Chapter 1 General introduction ................................................................................10 1.1 Invasive species and conservation.......................................................................10 1.1.1 Impacts of invasive plant species................................................................ 12 1.1.2 Managing invasive plant species ................................................................ 12 1.2 The Galapagos Islands ........................................................................................14 1.2.1 Invasive plant species in the Galapagos Islands ......................................... 15 1.2.2 The Scalesia Forest ..................................................................................... 16 1.3 Invasive Rubus niveus .........................................................................................17 1.3.1 Rubus niveus in the Galapagos Islands....................................................... 19 1.3.2 Management of Rubus niveus in Galapagos............................................... 20 1.4 Research Outline .................................................................................................22 Chapter 2 Impacts of the invasive plant Rubus niveus on the native vegetation of the Scalesia forest in Galapagos .....................................................................................23 2.1 Introduction .........................................................................................................24 2.2 Methods...............................................................................................................26 2.2.1 Study area on Santa Cruz............................................................................ 26 2.2.2 Sampling biological parameters.................................................................. 27 2.2.3 Sampling abiotic parameters....................................................................... 27 2.2.4 Analysis....................................................................................................... 28 2.3 Results .................................................................................................................29 2.3.1 Plant species richness.................................................................................. 29 2.3.2 Plant species diversity................................................................................. 32 2.3.3 Forest vertical structure............................................................................... 36 2.3.4 Abiotic parameters...................................................................................... 38 2.4 Discussion ...........................................................................................................40 Chapter 3 Competitive ability in early growth stages of the invasive plant Rubus niveus ......................................................................................................................44 3.1 Introduction .........................................................................................................45 3.2 Methods...............................................................................................................48 3.2.1 Relative growth rate.................................................................................... 48 3.2.2 Stress tolerance ........................................................................................... 49 3.2.3 Intra-interspecific competition.................................................................... 49 5 3.2.4 Seed bank and competition between recruits.............................................. 50 3.3 Results .................................................................................................................51 3.3.1 Relative growth rate.................................................................................... 51 3.3.2 Stress tolerance ........................................................................................... 55 3.3.3 Intra-interspecific competition.................................................................... 57 3.3.4 Seed bank and competition between recruits.............................................. 57 3.4 Discussion ...........................................................................................................61 Chapter 4 Effects of canopy gaps and light conditions on occurrence and growth of the invasive plant Rubus niveus in the Scalesia forest .............................................67

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