untitled

Population regulation, fluctuation,

and competition within species

8

In this chapter we first describe the theory and evidence for the stability of popula-

tions through regulation. We then analyze the processes that can cause fluctuations

and population cycles, using models to develop our understanding of the processes.

Finally, we examine one of the major causes of regulation, namely competition between

individuals for resources, or intraspecific competition. Other causes of regulation

such as predation will be dealt with in Chapter 10. Chapter 12 outlines an alterna-

tive approach to analyzing resource use.

If we look at long-term records of animal populations we see that some populations

remain quite constant in size for long periods of time. Records of mute swans (Cygnus

olor) in England from 1823 to 1872 (Fig. 8.1) illustrate that although the popula-

tion fluctuates, it remains within certain limits (190–1150). Other populations, such

as those of insects or house mice (Mus domesticus) in Australia (Fig. 8.2), fluctuate

to a much greater extent and furnish no suggestion of an equilibrium population

size. Nevertheless such populations do not always go extinct and they remain in

the community for long periods. Occasionally one finds unusual situations where

populations show regular cycles. The snowshoe hare (Lepus americanus) in northern

109

8.1 Introduction

8.2 Stability of populations

1200

1000

800

600

400

200

0

1820 1840 1860 1880 1900 1920 1940 1960 1980

Number of swans

Year

Fig. 8.1Some
populations remain
within relatively close
bounds over long time
periods. The mute swan
population of part of the
river Thames, England
(estimated by total
counts) shows a steady
level or gentle increase
despite some
perturbations due to
severe winters, for
example in 1946–47
and 1963–64. (Data
from Cramp 1972.)