POPULATION REGULATION, FLUCTUATION, AND COMPETITION WITHIN SPECIES 129
0.8
0.6
0.4
0.2
0
0 50 100 150 200 250 300
Population density
Age-specific survival
Lambs
Yearlings
Adults
Fig. 8.21Sigmoid
survival functions in
relation to the
population density of
adult+yearling Soay
sheep, for adults
(dashed-dotted line),
yearlings (dotted line),
and lambs (solid line),
estimated by Clutton-
Brock et al. (1997).
800
600
400
200
0
0 1020304050
Nt
t
Fig. 8.22Simulated
dynamics of an age-
structured population
with sigmoid survival
and fecundity functions
with the same
parameters as those
of the Soay sheep
population on the island
of Hirta. (After Clutton-
Brock et al. 1997.)
1750
1500
1250
1000
750
500
1955 1960 1965 1970 1975 1980 1985 1990 1995
Year
Total polulation size
Fig. 8.23Observed
variation in the
population of Soay
sheep over the entire
St Kilda archipelago.
(After Clutton-Brock
et al. 1997.)