Does Immigration “Rescue” Populations from Extinction?
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DOES IMMIGRATION “RESCUE” POPULATIONS FROM EXTINCTION? by MICHAEL CLINCHY B.Sc., University of Toronto, 1988 M.Sc., Queen’s University, 1990 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to the required standard: ____________________________ ____________________________ ____________________________ ____________________________ ____________________________ THE UNIVERSITY OF BRISTISH COLUMBIA July 1999 © Michael Clinchy, 1999 ABSTRACT I measured the rate of immigration by female common brushtail possums (Trichosurus vulpecula) in response to the removal of resident breeding females, in a landscape with no physical barriers to dispersal. I removed 10 residents from one 36 ha study grid and 9 from another, and monitored immigration over the next two years. Only one immigrant settled in one of the two removal areas. Sixteen breeding females resident on the periphery of the removal areas expanded their ranges into the removal areas. The one immigrant was a subadult that did not give birth in the breeding season following her arrival. Parentage analysis using microsatellite DNA indicated that the immigrant had moved only one home range away from her putative mother’s home range (@ 200 m). All of the known daughters of resident females settled beside their mothers. Parentage analysis indicated that 39 % of adjacent pairs of resident females were putatively mother and daughter, which is close to the 42 % expected if daughters always settle beside their mothers. The sex ratio of pouch-young was significantly male-biased, as predicted by the ‘local resource competition’ hypothesis, if most males disperse and most females settle beside their mothers. A deterministic, stage-based model of demography indicated that the birth rate was insufficient to balance the death rate (r = - 0.1), suggesting that the site was a ‘dispersal sink’. However, even with immigration, the projection was that density would decline by 84 %. Neither ‘old age’, starvation, predation or disease could explain 11 of the 24 deaths among resident females. Most of these females demonstrated prior symptoms of stress. Females that were captured and handled more frequently had a significantly lower probability of survival, and the estimated adult survival rate was significantly lower than that expected from the observed age distribution. I suggest apparent ‘dispersal sinks’ may often be of our own making. Deaths due to capture and handling are analogous to removals. Consequently, since there were evidently more than 19 ‘removals’, I conclude that the results of the experiment likely overestimate the importance of immigration in replacing losses among breeding females. ii TABLE OF CONTENTS Abstract ii Table of Contents iii List of Tables vi List of Figures x Preface xiii Acknowledgements xiv CHAPTER 1: Does immigration "rescue" populations from extinction? General introduction 1 References 8 CHAPTER 2: Does immigration "rescue" populations from extinction? Implications regarding movement corridors and the conservation of mammals Introduction 11 Alternative hypotheses 12 Behavioural data 13 Conclusions 17 References 18 CHAPTER 3: Methodological problems in the study of immigration and extinction Introduction 25 The correlational-survey approach 25 Identifying immigrants in open populations in the field 27 An integrated approach 30 References 33 CHAPTER 4: Does immigration "rescue" populations from extinction? Evidence from a large-scale field experiment on common brushtail possums Introduction 40 Methods Study species 46 Study site 48 Detection, capture, and handling 50 Definitions and experimental design 52 Radio telemetry and den mapping 56 Live-trapping 58 Spotlighting and darting 59 Complete enumeration 60 Adequacy of location data from radio telemetry 71 Collection of tissue samples and analysis of microsatellite DNA 73 iii Results Female immigration following removals 74 The vacuum effect and breeding dispersal 76 Potential immigrants captured in the core 77 Adverse effects of capture and handling and the measured rate of immigration 79 Locations of known mothers and daughters 82 Locations of mothers and daughters identified using microsatellite DNA 83 Direct observations of restricted movements between the grids 84 Restricted movements between the grids identified using microsatellite DNA 85 Sex ratios of pouch-young 85 Discussion 86 References 93 CHAPTER 5: Connectivity or carnivory: what's more important to the demography of common brushtail possum populations? Introduction 139 Methods 142 Age estimates from cementum annuli 142 Date of birth of pouch-young 143 Results Seasonality of births 144 Sex ratios of dependent young 145 Stages in the weaning of young 146 Pre-weaning survival of young 147 Pre-weaning loss of young as a result of capture and handling 149 Pre-weaning loss of young as a result of predator chases 154 Pre-weaning loss of young as a result of infanticide 154 Pre-weaning loss of young as a result of food shortage 155 Post-weaning survival of young 160 Age at first breeding 168 Reproductive success of first-time breeders 169 Discussion 174 References 179 CHAPTER 6: Dispersal sinks and handling effects: interpreting the role of immigration in common brushtail possum populations Introduction 212 Methods 215 Results Adult female survival 217 A model of possum demography 220 Wobbly possums 224 Other indices of poor condition 229 Weight loss associated with trapping, among apparently healthy animals 231 Deaths due to predation 232 Necropsies 234 Age at death 240 Survival and the frequency of handling 243 Population growth rates and "censored" survival estimates 246 iv Discussion 247 References 253 CHAPTER 7: Conclusions and implications for conservation General discussion 282 References 288 APPENDIX 1: Assumptions underlying the analysis of microsatellite DNA Introduction 290 Linkage disequilibrium and deviations from Hardy-Weinberg equilibrium (HWE) 290 Parentage analysis 292 Absolute likelihoods 298 References 300 APPENDIX 2: Estimating the date of birth of pouch-young Introduction 304 Derivation of methods 304 Protocols adopted 307 References 309 v LIST OF TABLES Table 2.1 Summary of results of examining populations illustrated in Figure 2.2 and 21 asking the question "do both populations persist from time 1 to time 2?" Table 2.2 Contingency table for analysis of relationship between proximity and 22 persistence using evidence from Table 2.1 and Figure 2.2. Table 3.1 Distance to the nearest occupied patch at time 1, as compared between 37 populations found to be extinct or extant at time 2, based on results from four surveys of the Bodie, California, pika metapopulation, as reported in Smith and Gilpin (1997, their Fig. 2) Table 4.1 Number and date of, and level of effort associated with, full-grid trapping 102 sessions on the West grid. Table 4.2 Number and date of, and level of effort associated with, full-grid trapping 103 sessions on the East grid. Table 4.3 Level of effort associated with, and number of animals seen during 104 spotlight transects in the core areas of the two study grids. Table 4.4 Level of effort associated with, and number of animals seen during 105 spotlight transects on the periphery of the two study grids. Table 4.5 Population size estimates regarding all adult and subadult females 106 on the West grid. Table 4.6 Population size estimates regarding all adult and subadult females 107 on the East grid. Table 4.7 Population size estimates regarding all adult and subadult males 108 on the West grid. Table 4.8 Population size estimates regarding all adult and subadult males 109 on the East grid. Table 4.9 Allele frequencies observed at six microsatellite loci. 110 Table 4.10 Putative mother-daughter relationships identified among adult (and subadult) 111 females on the West grid. Table 4.11 Putative mother-daughter relationships identified among adult (and subadult) 112 females on the East grid. Table 4.12 Differences in mean likelihood scores, as shown in Tables 4.10 and 4.11, 113 between adjacent and non-adjacent putative parent-offspring pairs of adult (and subadult) females. Table 4.13 Reported sex ratios of pouch-young. 114 vi Table 5.1 Differences in the reproductive biology of common brushtail possums 185 in disturbed and undisturbed populations. Table 5.2 Number of resident adult females producing a primary young in either autumn 186 or winter, compared to the number of females that did not reproduce. Table 5.3 Sex ratios (F:M) of dependent young, categorized by study area 187 and breeding season. Table 5.4 Pre-weaning survival of primary young. 188 Table 5.5 Mean interval, in days, between occasions on which females were handled, 189 while they were carrying a pouch-young, prior to 175 days after the birth of their young, as compared between instances where the young was known to have survived to the onset of weaning (at 175 days), and where it did not. Table 5.6 Pre-weaning loss of young in relation to ear-tagging. 190 Table 5.7 Survival of young to October-November, 1996, as compared between mothers 191 subject to repeated capture and handling (On-grid) and others that had not been previously trapped and handled (Off-grid). Table 5.8 Predator activity during the pre-weaning phase of development of primary 192 young, from mid-March to mid-December, in 1995 and 1996, based on sightings, sounds, signs, and diagnoses of the proximate cause of death of adult possums. Table 5.9 Survival of primary young to August 1, in both 1995 and 1996, 193 on both principal study grids. Table 5.10 Survival of pouch-young to the onset of weaning (175 days) in 1995 and 1996, 194 among females demonstrating any or all of four symptoms of poor condition. Table 5.11 Changes in the status of individual adult female possums examined in a 195 given season in 1995 for evidence of rump wear as compared to the same individual's status in the same season in 1996.