Soay Sheep: Dynamics and Selection in an Island PopulationT. H. Clutton-Brock, J. M. Pemberton Unlike most other large mammals, the Soay sheep population of Hirta in the St. Kilda archipelago show persistent oscillations, sometimes increasing or declining by more than 60% in a year. This study explores the causes of these oscillations and their consequences for selection on genetic and phenotypic variation within the population, drawing on studies over the past twenty years of the life-histories and reproductive careers of many sheep. It will be essential reading for vertebrate ecologists, demographers, evolutionary biologists and behavioral ecologists. |
Contents
Individuals and populations | xv |
12 Individuals and populations | 4 |
13 Population density and selection | 8 |
14 Adaptation in a changing environment | 9 |
15 Soay sheep as a model system | 11 |
The sheep of St Kilda | 15 |
22 The islands of St Kilda | 16 |
23 Human settlement on St Kilda and the history of St Kildas sheep | 18 |
55 Discussion | 152 |
Mating patterns and male breeding success | 164 |
62 Measuring male breeding success | 166 |
63 Molecular paternity inference | 169 |
64 Breeding success among age classes and years | 170 |
65 Lifetime breeding success and cohort effects | 173 |
66 Opportunity for selection and the maintenance of genetic variation | 178 |
67 Discussion | 182 |
24 Physical characteristics of Soay sheep on St Hilda | 23 |
25 Feeding behaviour and habitat use | 27 |
26 Reproduction | 32 |
27 Early development and neonatal mortality | 36 |
28 Juvenile and yearling mortality | 39 |
29 Adult mortality and the costs of reproduction | 41 |
210 Discussion | 48 |
Population dynamics in Soay sheep | 50 |
32 Changes in population size | 51 |
33 Densitydependence in fecundity | 56 |
34 Variation in birth weight and neonatal mortality | 59 |
35 Winter mortality | 62 |
36 Cohort variation in growth survival and reproductive performance | 68 |
37 Modelling the impact of densitydependence | 73 |
38 Incorporating the effects of climate and age structure | 76 |
39 Synchrony in dynamics between populations | 79 |
310 Discussion | 80 |
Vegetation and sheep population dynamics | 87 |
42 Grazing in theory and practice | 90 |
43 Methods | 94 |
44 Spatial distribution of grazing | 95 |
45 Effects of grazing on flowering botanical composition and plant species richness | 103 |
46 Grazing and infection by fungal endophytes | 106 |
47 Grazing and longterm changes in plant communities | 108 |
48 Discussion | 109 |
Parasites and their impact | 111 |
52 The parasite community | 112 |
53 Variation in parasitism rates | 121 |
54 Costs of parasitism | 144 |
Selection on phenotype | 188 |
72 The opportunity for selection | 189 |
73 Selection on body size | 190 |
74 Selection on horn phenotype | 201 |
75 Selection on coat colour | 206 |
76 Discussion | 210 |
Molecular genetic variation and selection on genotype | 215 |
82 Loci screened | 219 |
83 Amount and distribution of molecular genetic variation | 222 |
84 Selection against inbred individuals | 224 |
85 Selection at specific loci | 229 |
86 Discussion | 236 |
Adaptive reproductive strategies | 241 |
92 Optimal lifehistories for female Soay sheep | 247 |
93 Optimal lifehistories for male Soay sheep | 252 |
94 Discussion | 268 |
The causes and consequences of instability | 274 |
102 Comparative demography | 275 |
103 Hard conditions and soft selection | 291 |
104 Adaptation in changing environments | 299 |
105 Extrapolations to other populations | 302 |
106 Summary | 305 |
107 Acknowledgements | 308 |
The flora of StKilda | 309 |
Inheritance of coat colour and horn type in Hirta Soay sheep | 319 |
How average life tables can mislead | 326 |
References | 330 |
Index | 378 |
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Soay Sheep: Dynamics and Selection in an Island Population T. H. Clutton-Brock,J. M. Pemberton Limited preview - 2004 |
Common terms and phrases
adult females age classes Albon alleles analysis Animal Ecology behaviour bighorn sheep biomass birth weight body weight born Cambridge Chapter coat colour cohorts Coltman correlated Coulson density is high density-dependent domestic sheep Ecology effects estimates Evolution ewes faecal egg counts fecundity Festa-Bianchet fluctuations frequency genetic variation genotype grazing Grenfell Gulland heritability heterozygosity Hirta host inbreeding inbreeding depression increase individuals infection interactions J. M. Pemberton Journal of Animal juveniles lambs life-history lifetime breeding success loci male breeding success mammals mating microsatellite Milner nematode oestrus offspring over-winter parasite parasite loads parasite resistance Parasitology paternity phenotype plant population crashes population density population dynamics proportion red deer relatively reproductive success scurred scurred males sheep numbers sheep on St sheep population Soay sheep species St Kilda strategy studies summer survival T. H. Clutton-Brock tion traits tussocks twins ungulates Village Bay Village Bay population wild winter mortality yearlings