Biology Reference
In-Depth Information
analyses discussed in Clutton-Brock are restricted to 1961-67 and
1985-90. Between 1959 and 1968, most lambs in a certain area were
caught, weighed, sexed, and tagged within the first month of life. During
both periods selected for the study, population size varied from about 600
to nearly 1600, as a result of high over-winter mortality. Mortality was
particularly pronounced among lambs and rams, and was caused mainly by
starvation. There were four population crashes, but there is evidence for
additional crashes during the periods not included in the analyses. Winter
mortality increased with population density in a non-linear fashion. No
density dependence was found for neonatal survival or overall fecundity.
The authors conclude that the persistent instability is probably caused by
the consistently high fecundity due to the super-abundance of food in
summer. Consequently, population size rises above carrying capacity of
the island, leading to the crashes.
In summary, the results of this study show the frequent occurrence of
nonequilibrium conditions (i.e., a marked over- or under-saturation).
Indeed, periods during which the population was close to an equilibrium
point were extremely short, although the overall result was ''regulation'':
the population fluctuated between certain minima and maxima.
Many relevant examples of often strong and irregular fluctuations in popu-
lation sizes of plants and animals, and the factors that may be responsible, can
befoundinHassellandMay(
1990
), who organized and edited a Discussion
on ''Population, regulation and dynamics.'' I discuss two of the contributions
from that Discussion, one on fish, the other on plant populations.
Shepherd and Cushing (
1990
) drew attention to a large body of
evidence for long-term changes in the sizes of marine fish populations.
They discuss long-term changes in recruitment of North Sea sole, North
Sea plaice, Iceland cod, northeast Arctic cod, and North Sea herring in
detail. All these species are of great economic importance and have been
studied over many years. Fluctuations are great and appear to be quite
irregular, but the causes are not known, although climatic changes may
be partly responsible. The authors give the following circumstantial
evidence for the possible involvement of regulatory processes:
(1) fish populations are known to sustain levels of fishing mortality
several times greater than levels of natural mortality (possible evi-
dence for strong regulation);
(2) fish stocks have persisted for centuries with few extinctions or
explosions (possible evidence for regulation, but regulation is not
necessarily strong); and
