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golden eagles (Aquila chrysaetos) have recently immigrated into the Channel Islands

from the mainland and begun to breed successfully. Rising eagle abundance has led

to the rapid decline of an endemic prey species, the island fox (Urocyon littoralis)

(Fig. 17.9). Foxes have completely disappeared from two of the islands and have expe-

rienced a 10-fold decline in abundance on the largest island (Santa Cruz) following

colonization by eagles in the early 1990s (Roemer et al. 2002; Courchamp et al. 2003).

The decline in fox abundance has led to a parallel increase in a competitor, the island

spotted skunk (Spilogale gracilis amphiala). Although there has been a successful effort

to translocate some eagles away from the islands, several pairs still remain on the

island, thwarting fox recovery. This presents a conservation conundrum: do we harm

or remove a protected species (the golden eagle) in order to save an endangered species

(the island fox)? An obvious countermeasure is to cull feral pigs. The model predicts

that, without timely reduction in eagle abundance, pig eradication could inadvertently

lead to heightened predation pressure on foxes, perhaps even doom them rather than

helping them (Courchamp et al. 2003). This is a clear demonstration of the utility

of trophic models as a means of evaluating alternative conservation actions.

For most big game species, harvesting does not pose a conservation threat. Indeed,

such species usually become entrenched as “game” because their life-history attributes

(high reproductive capacity, broad geographical distribution, ability to tolerate inter-

ference by hunting humans) make them relatively robust. Modern exceptions to this

308 Chapter 17

60

50

40

30

20

10

0

40

35

30

25

20

15

10

5

0

1950 – 59 1960 – 69 1970 – 79 1980 – 89 1990 – 94 1995 – 99

1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

% trap success

Number of sightings

(b)

Fig. 17.9(a) The (a)
number of golden eagle
sightings on the
Channel Islands,
California, increased as
the feral pig population
increased. (b) The
abundance of the
Channel Island fox
(
), as indicated by
trapping, declined due
to depredation by
eagles, and this allowed
the increase of the
endemic skunks ().
(Data from Roemer
et al. 2002.)

17.8.2Extinction
threat due to
unsustainable
harvesting