Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2006 Functional integrity of plant-pollinator communities in restored habitats in Mauritius Kaiser, Christopher Niels Abstract: Die vorliegende Dissertation befasst sich mit den Wechselbeziehungen zwischen in Mauritius eingeführten und einheimischen Tier- und Pflanzenarten und den damit verbundenen mutualistischen Pflanze-Tier-Interaktionen. Es wurden vollständig quantifizierte Besucher-Netze verwendet, um Pflanze- Tier-Interaktionen auf der Ebene der Bestäuber- und Pflanzengemeinschaft zu untersuchen. Die Ergeb- nisse dieser Arbeit deuten darauf hin, dass die Mechanismen der biotische Bestäubung und Samenverbre- itung einheimischer Pflanzen durch die Anwesenheit eingeführter Pflanzenarten indirekt beeinträchtigt werden. Die lokale Bestäuberfauna wurde von eingeführten Insekten dominiert, was auf eine erhebliche Störung der einheimischen, co-evolvierten Pflanze-Tier-Beziehung schliessen lässt. This thesis investi- gates the effects of introduced plant and animal species on native mutualistic interactions in Mauritius. The Mauritian ecosystem is highly degraded by invasive alien plant species. Conservationists have estab- lished a habitat restoration scheme to conserve endemic plants. The consequences of this restoration for associated animals and the role of mutualists in fulfilling their previous ecosystem functions are largely unknown. The main results of this thesis include: 1. Bird pollination behaviour may be affected by restoration schemes, resulting in reduced pollination success of the endangered endemic S. mamillatum in restored habitat. 2. The Janzen-Connell model describes seedling establishment of S. mamillatum, and ecological analogue species may serve as seed dispersers. 3. There is little evidence for indirect com- petition for pollination between the invasive P. cattleianum and the endemic B. zaluzania. 4. Visitation webs of restored habitat were more complex than those of unrestored habitat, which may be critical for ecosystem stability. The dominance of introduced pollinators suggested strong competition with native pollinators, which may have resulted in the disruption of co-evolved plant-pollinator interactions. 5. Fluc- tuations in species diversity, connectance, degree distribution and nestedness in pollinator communities suggest that network stability may vary considerably both temporally and spatially. Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-163460 Dissertation Published Version Originally published at: Kaiser, Christopher Niels. Functional integrity of plant-pollinator communities in restored habitats in Mauritius. 2006, University of Zurich, Faculty of Science. FUNCTIONAL INTEGRITY OF PLANT–POLLINATOR COMMUNITIES IN RESTORED HABITATS IN MAURITIUS Dissertation zur Erlangung der naturwissenschaftlichen Doktorwürde (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaflichen Fakultät der Universität Zürich von Christopher Niels Kaiser aus Deutschland Promotionskomitee Prof. Dr. Christine B. Müller (Vorsitz und Leitung der Dissertation) Dr. Jane Memmott Prof. Dr. Bernhard Schmid Prof. Dr. Jens M. Olesen Zürich, 2006 THIS THESIS IS DEDICATED TO MY PARENTS AND GRANDPARENTS FOR THEIR LOVE AND SUPPORT Contents 1 CONTENTS CHAPTER 1 General introduction 3 CHAPTER 2 Habitat structure affects reproductive success of the rare endemic tree Syzygium mamillatum (Myrtaceae) in restored and unrestored sites in Mauritius 13 C. N. Kaiser, D. M. Hansen and C. B. Müller CHAPTER 3 Endangered endemic plants on tropical oceanic islands: seed dispersal, seedling establishment, and ecological analogues 35 D. M. Hansen, C. N. Kaiser and C. B. Müller CHAPTER 4 Are there negative indirect effects between endemic and invasive flowering plant species via shared pollinators? 71 C. N. Kaiser and C. B. Müller CHAPTER 5 Exotic pest insects: another perspective on coffee and conservation 87 C. N. Kaiser, D. M. Hansen and C. B. Müller CHAPTER 6 Visitation webs of plant–pollinator communities in restored and unrestored Mauritian heathland habitats 93 C. N. Kaiser, J. Memmott and C. B. Müller CHAPTER 7 Temporal dynamics in plant–pollinator network structure 171 C. N. Kaiser, J. Memmott and C. B. Müller Summary 197 Zusammenfassung 200 Acknowledgements 203 Appendix 206 Curriculum vitae 251 Chapter 1: General introduction 3 CHAPTER 1 General introduction Habitat restoration in Mauritius The oceanic island of Mauritius, which belongs to the Mascarene archipelago, is situated some 800 km east of Madagascar in the south-western Indian Ocean between 19°50’S and 20°51’S, and 57°18’ and 57°48’E (Fig. 1a), and has a land area of 1865km2. Mauritius’ unique and diverse fauna and flora have been subject to a wave of extinctions since human colonisation about 400 years ago, a pattern typical of many island ecosystems. The primary forces driving these extinctions have been, and continue to be, habitat destruction, fragmentation and degradation by invasive alien plant and animal species (Mauremootoo et al. in press-a). As a result, only about 1.9% of Mauritius is covered in the native Mauritian forest today (> 50% native canopy), the rest of the island’s forest cover is either plantation forestry or native forest heavily degraded by introduced plant species (Fig. 1b; Page & d'Argent 1997). Currently, 94% of flowering plant species endemic to Mauritius is threatened to some degree according to IUCN criteria (Mauremootoo et al. in press-b) making the flora of Mauritius one of the most threatened island floras in the world. Both habitat degradation and vertebrate extinction have been covered in great detail elsewhere (Vaughan & Wiehe 1941, Lorence & Sussman 1986, Cheke 1987, Strahm 1994, Cheke & Hume in press, Mauremootoo et al. in press-a). However, very little is known about the invertebrate diversity of Mauritius. Species inventories on some selected invertebrate groups exist (Vinson 1938, Williams 1981, 1982, 1983, 1989, Griffiths 2000) and a list of economically important, introduced invertebrate species has been compiled (Williams & Ganeshan 1999), but this work is far from complete. Given such lack of basic taxonomic information on many invertebrate groups, we can only speculate about the conservation status and the probable decline of invertebrate diversity in Mauritius. 4 Chapter 1: General introduction Figure 1 (A) The Mascarene archipelago includes the islands of Réunion, Mauritius and Rodrigues, which are located between approximately 600 and 1300km east of Madagascar in the south-western Indian Ocean. The inset (B) shows the forest on the island of Mauritius. The Black River Gorges National Park is outlined in red. Different shades of light green reflect different level of invadedness, and light blue areas show plantation forest. Dark green areas depict the last remnants of native vegetation (forest with >50% native canopy) (Source inset: Mauritian Wildlife Foundation, 2006). Chapter 1: General introduction 5 The current knowledge of Mauritian biodiversity reflects the focus of conservation management on single-species recovery programmes of large vertebrate species in Mauritius. Much traditional conservation effort and resources have gone into saving target species such as the Mauritius kestrel Falco punctatus, the pink pigeon Columba mayeri, the echo parakeet Psittacula eques and the Mauritius fody Foudia rubra from the brink of extinction. More recently, more resources have been directed towards plant restoration schemes, the most significant of which is a network of 11 conservation management areas (CMAs) covering 44ha in the south west of the island. This network was set up by the Government conservation service in collaboration with the non-governmental Mauritian Wildlife Foundation (Mauremootoo et al. in press-b). CMAs are fenced – against deer and pigs – plots of various sizes (0.1–24 ha) where introduced plant species are hand-weeded twice a year. This management aims to preserve a representative sample of the native flora. If fences are maintained in good condition, regeneration of native forest does occur. For example, in Brise Fér Old Plot, a CMA initially weeded in 1987, the efforts have resulted in a slow but gradual regeneration of 53–68% of native plant species compared to 32–40% in a comparable unrestored, degraded area (Mauremootoo et al. in press-b). As habitat restoration in Mauritius aims to restore whole ecosystem functioning it is crucial to determine whether this type of management also benefits the native, co-adapted animals. However, most native plant species rely on mutualistic interactions, such as pollination and seed dispersal, for successful regeneration, and it is not known whether these ecosystem functions are being fulfilled by the extant fauna. Habitat restoration can only be successful in the long-term when natural plant reproduction is ensured, and the absence of native mutualists may severely threaten this restoration aim. In this thesis, I compare plant–animal interactions of restored and unrestored, heavily degraded areas to increase our understanding on how invaders alter plants’ interactions. Pollination studies: from single species to community approach Just as conservation schemes in Mauritius are moving from a single species focus to a more holistic approach, pollination biology is also following the same route. Pollination biologists have long been focusing on the reproductive biology of single plant