untitled

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.)