untitled

the next-but-one level, and non-linear effects where competitors within the predator

or prey levels respond to changes in one of those levels (Wootton 1994a,b). Two

examples are illustrated here.

Top predators can affect the diversity of their prey through changes in the abund-

ance of primary prey. This is particularly important where both exotic predators and

their primary prey result in increased predation of endemic species. The introduc-

tion of feral cats and European rabbits on islands has resulted in catastrophic

declines of many seabird populations (many examples are given in Courchamp et al.

2000b). In New Zealand, introduced ferrets (Mustela furo), cats, and rabbits have resulted

in increased predation on two endemic skink species. Predation on skinks was inversely

density dependent, as one expects for secondary prey (see Section 10.7.2) (Norbury

2001). We saw in Section 17.8.1 what happened when feral pigs (Sus scrofa) were

released onto the California Channel Islands in the early twentieth century (Roemer

et al. 2002). As they became more abundant, mainland golden eagles (Aquila

chrysaetos) were able to colonize the islands in the 1970s, building to high numbers

by the 1990s (see Fig. 17.9). These raptors caused a rapid decline in the endemic

island fox (Urocyon littoralis). In turn, this caused a change in the competitive

balance of the fox with the endemic spotted skunk (Spilogale gracilis amphiala) that

was able to increase. Thus, hyperpredation, where top predators consume lower pre-

dators, has resulted in a change in competition between them.

The opposite process to that of hyperpredation occurs when top predators are

removed and a linear trophic cascade results in the increase of lower predators and

a decrease in some of their prey. Consequently, there is a change in species compo-

sition. Again on islands where both top predators, such as cats, and their prey, such

as rats, are present, removal of cats can increase the rats that then prey on seabird

nests (Courchamp et al. 1999). In southwest Spain, the Iberian lynx (Felis pardalis)

depredates both Egyptian mongooses (Herpestes ichneumon) and rabbits. In areas where

lynx were absent, mongoose predation resulted in rabbit densities being 2– 4 times

lower than in areas where lynx occurred (Palomares et al. 1995). In southern

California the disappearance of coyotes (Canis latrans) has allowed an increase in

ECOSYSTEM MANAGEMENT AND CONSERVATION 367

Predators

10

Detritus

10

Herbivores

25

Nutrients

5

Plants

5

0.2

3.5

0.5 4.0

5.0

20.0

28.3

30.0

0.5

0.2

0.5

9.5

Fig. 21.1Hypothetical
flow diagram showing
the passage of nutrients
through the trophic
levels (boxes). The
numbers are in
units/ha/year. Inputs
and outputs for each
box and for the whole
system must balance.

21.5.1

Hyperpredation and
apparent competition

21.5.2Meso-
predator release