From forest to farmland: Effects of land use on understorey birds of Afrotropical rainforests Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultäten der Georg-August-Universität zu Göttingen vorgelegt von M.Sc. Ing. Kadiri Serge BOBO aus Kamerun Göttingen 2007 D7 Referent: Prof. Dr. Michael Mühlenberg Korreferent: Prof. Dr. Rainer Willmann Tag der mündlichen Prüfung: 31.10.2007 PREFACE After working on waterbirds in the Waza-Logone floodplain (1995-1998), I continued my interest in birds through involvement in field surveys to identify ‘Important Bird Areas’ of Cameroon as part of an African BirdLife International programme (1999-2000). I was introduced to the Korup area thanks to consultancy work offered by the WWF-CARPO in 2001 to elaboration on the ‘Wildlife Management Plan for the Nta-Ali Forest Reserve, extended by the GTZ-Cameroon on bird surveys of this same reserve. These experiences prepared me well for my Master of Science (2004) and the present PhD research. The Korup area belongs to the Guineo-congolian rainforest zone; its flora is less degraded by humans and its vegetation cover is still impressive. Slash-and-burn agriculture is recurrent in the vicinity of villages. Land use systems are created by rural people searching for subsistence by clearing forest for farms. These habitats are home to a unique avifauna community, due to their strategic position at the most eastern limit of the Upper-guinean- and the western limit of the Lower-guinean- ecoregions. This created an exciting opportunity to contribute to the debate on birds in land use systems and to understand how environmental factors affect the entire avian community in our study area. Development process of this thesis Following a previous study which focused on the indicator properties of various taxa, namely birds through call-based methods, fruit-feeding butterflies, trees and understorey plants in the Korup area (see list of publications), the idea arose to further our understanding of the living conditions of bird communities in this area. The initial title “From forest to farmland: Effects of land use on birds of tropical rainforests” has been changed to “From forest to farmland: Effects of land use on understorey birds of Afrotropical rainforests” as we focused more on understorey birds in Cameroon, Central Africa. The presented works on ‘Understorey bird community structure, species richness and abundance’ (Chapter V.), ‘Ground and shrub nest predation risk, and availability of cavity nesting sites’ (Chapter VI.), ‘Food resources: Invertebrates, abundance and species richness of fruiting and flowering trees, and leaf litter area and weight’ (Chapter VII.) and other indicators of habitat quality like ‘Body weight, i ectoparasites, fault bars, fluctuating asymmetry’ (Chapter VIII.) were developed together as systematic research, in the same temporal and spatial scales, using almost the same people. These have enabled us to discover some relationships and to interpret observed patterns in understorey birds (Chapter IX.). Remarks on terminology The present study has involved a large number of people with whom I worked in the field or as reviewers. Throughout this dissertation, “we” is used to stress common work and ideas. Throughout this thesis, when comparing patterns along the gradient of forest disturbance, ‘natural habitats’ is used for near-primary forest and secondary forest habitats, and ‘modified habitats’ or ‘land use systems’ for agroforestry systems and annual cropfarms. The latter is also sometimes called ‘annual cultures’ or ‘annual croplands’. Also, ‘understorey birds’ is referred to birds trapped with the mist-net; another synonym used is ‘mist-netted bird community’. Valid tree species names follow the Missouri Botanical Garden's VAST (VAScular Tropicos) nomenclatural database. Names of birds follow Borrow and Demey (2001). Invertebrate orders follow Steyskal et al. (1986). ii TABLE OF CONTENTS PREFACE………………………………………………………………………………………i TABLE OF CONTENTS……………………………………………………………………..iii SUMMARY……………………………………………………………………………………ix ZUSAMMENFASSUNG…………………………………………………………………….xiv PART I: GENERAL INTRODUCTION I. BACKGROUND……………………………………………………………………….1 I.1. Notes on the Guinea-Congolian rainforest avifauna……………………………………….1 I.2. Notes on the avifauna of Cameroon………………………………………………………..1 I.3. Threats to Cameroon avifauna……………………………………………………………...2 I.4. Species of global conservation concern in Cameroon……………………………………...2 II. STATUS OF CURRENT RESEARCH…………………………………………………5 II.1. Land use and biodiversity change in tropical forests………………………………………5 II.2. Birds and land-use systems………………………………………………………………...6 II.3. Knowledge on biodiversity in land use systems from our study area……………………..7 III. IMPORTANCE, OBJECTIVES AND HYPOTHESES………………………………...9 III.1. Importance………………………………………………………………………………...9 III.2. Objectives………………………………………………………………………………....9 III.3. Research questions……………………………………………………………………….10 III.4. Hypotheses……………………………………………………………………………….11 IV. THE STUDY AREA…………………………………………………………………...12 IV.1. Korup National Park……………………………………………………………………..12 IV.1.1. General…………………………………………………………………………12 IV.1.2. Brief land use history…………………………………………………………..13 iii IV.1.3. Threats to KNP…………………………………………………………………13 IV.2. The Support Zone………………………………………………………………………...14 IV.3. The studied plots………………………………………………………………………….14 IV.4. Biophysical conditions…………………………………………………………………...15 IV.4.1. Climate, topography, geology and soils………………………………………..15 IV.4.2. Hydrology and drainage………………………………………………………..17 IV.4.3. Flora and fauna…………………………………………………………………17 IV.4.4. Human population, cultural and linguistic groups……………………………..17 IV.5. The studied land use systems…………………………………………………………….18 PART II: UNDERSTOREY BIRD COMMUNITY STRUCTURE, SPECIES RICHNESS AND ABUNDANCE ALONG A GRADIENT OF FOREST CONVERSION V. UNDERSTOREY BIRD COMMUNITY STRUCTURE, SPECIES RICHNESS AND ABUNDANCE ALONG A GRADIENT OF FOREST CONVERSION………20 Abstract………………………………………………………………………………………...20 V.1. Introduction……………………………………………………………………………….21 V.2. Methods…………………………………………………………………………………...22 V.2.1. Data collection………………………………………………………………….22 V.2.2. Data analysis……………………………………………………………………23 V.2.3. Theory…………………………………………………………………………..25 V.3. Results…………………………………………………………………………………….27 V.3.1. Abundance and species richness (spot diversity)……………………………….27 V.3.2. Species similarity between study sites (beta diversity)…………………………33 V.3.3. Species-level response………………………………………………………….35 V.3.4. Understorey bird community structure…………………………………………36 V.3.5. Habitat preference and geographic range………………………………………45 V.3.6. Habitat preference and body mass……………………………………………...48 V.3.7. Habitat preference and vertical foraging niches..................................................49 V.3.8. Habitat preference and feeding guilds………………………………………….50 iv V.3.9. Comparison of the overall species richness pattern between the call-based and mist-netting methods…………………………………………...52 V.4. Discussions………………………………………………………………………………..53 V.4.1. General notes on bird community studies………………………………………53 V.4.2. Overall understorey bird species richness and abundance……………………...54 V.4.3. Insectivorous birds……………………………………………………………...55 V.4.4. Ant-followers…………………………………………………………………...56 V.4.5. Omnivorous……………………………………………………………………..56 V.4.6. Nectarivorous…………………………………………………………………...56 V.4.7. General feeding groups comparison……………………………………………57 V.4.8. Biome-restricted species………………………………………………………..57 V.4.9. Forest canopy birds in land use systems………………………………………..58 V.4.10. Size class foliage gleaners and habitat modifications…………………………58 V.4.11. Comparing species-level responses of the two methods………………………59 V.5. Conclusion...........................................................................................................................59 PART III: INDICATORS OF HABITAT QUALITY AND DIRECT FACTORS INFLUENCING UNDERSTOREY BIRD COMMUNITY PATTERNS ALONG A GRADIENT OF FOREST CONVERSION VI. GROUND AND SHRUB NEST PREDATION RISK, AND AVAILABILITY OF CAVITY NESTING SITES……………………………………61 Abstract.......................................................................................................................................61 VI.1. Introduction........................................................................................................................62 VI.2. Methods..............................................................................................................................63 VI.2.1. Data collection....................................................................................................63 VI.2.2. Data analysis.......................................................................................................65 VI.3. Results................................................................................................................................65 VI.3.1. Nest predation.....................................................................................................65 VI.3.2. Cavity nesting sites..............................................................................................68 VI.4. Discussions.........................................................................................................................71 v VI.5. Conclusion..........................................................................................................................74 VI.6. Limitations to artificial nest predation experiment………………………………………74