untitled

Just by chance there may be for a time a correlation between density and environ-

mental factors. However, if we take many separate populations, the probability that

all of them are simultaneously correlated with an environmental factor by chance

alone is very small. Therefore, if we find a correlation between mean densities from

independent populations with an environmental factor, there is a strong inference

that weather is influencing some resource for which animals are competing, and which

results in regulation about some equilibrium point.

An example of this approach is shown in Schluter’s (1988) study of seed-eating

finches in Kenya (Fig. 8.8): finch abundance from various populations is correlated

with seed abundance. Other examples of density correlated with weather factors are

given in Sinclair (1989).

As we discussed in Section 8.3.7, density dependence is a necessary but not sufficient

requirement to demonstrate regulation. There are an increasing number of studies

in the bird and mammal literature demonstrating density-dependent stages in the

life cycle. For birds (Fig. 8.9a), the long-term study on great tits (Parus major)

at Oxford, England has shown that winter mortality of juveniles was related to the

number of juveniles entering the winter (McCleery and Perrins 1985). In contrast

(Fig. 8.9b), it was early chick mortality in summer that was density dependent for

the English partridge (Perdix perdix) (Blank et al. 1967).

For mammals, density-dependent juvenile mortality has been recorded for red deer

on the island of Rhum, Scotland (Clutton-Brock et al. 1985) (Fig. 8.10a), for rein-

deer in Norway (Skogland 1985) (Fig. 8.10b), for feral donkeys (Equus asinus) in

Australia (Choquenot 1991), and for greater kudu in South Africa (Owen-Smith 1990).

Adult mortality was density dependent for African buffalo in Serengeti (Sinclair 1977).

In each case, the cause was lack of food at critical times of year. Reproduction is

known to be density dependent in both birds (Arcese et al. 1992) and mammals

(Clutton-Brock et al. 1991). Figure 8.10c shows that the proportion of Soay sheep

that give birth at 12 months of age declines with density. Fowler (1987) reports over

118 Chapter 8

5.5

4.5

3.5

2.5

1.5

0.5

1.0 2.0 3.0 4.0 5.0 6.0 7.0

Logn seed abundance (mg / m^2 )

Log

finch abundance (numbers)n

Fig. 8.8The total
abundance of seed-
eating finches in
savanna habitats of
Kenya is related to
the abundance of the
food supply. Such a
positive relationship
in unconnected
populations may
demonstrate regulation.
(After Schluter 1988.)

8.4.3Examples of
density dependence