|
|
Counting Foxes:
From 1993 to
1999 the National Park Service monitored the population of Island
Fox on San Miguel Island. Each summer, foxes were trapped and tagged.
When each fox is first captured, it is implanted with a device about
the size of a grain of rice called a passive integrated transponder
tag, or "PIT" tag. The tag gives each fox a permanent, unique number
that can be read with a special scanner and allows scientists to
monitor individual foxes over the years.
|
|
|
|
The
Warning:
Park biologists estimated there to be over 400 foxes on
San Miguel in 1994, but by 1995, an alarming decline had begun. With each
passing year, fewer foxes were trapped in the monitoring grids. At the
same time, rangers and visitors reported seeing fewer live foxes and more
fox skeletons and carcasses. By 1998, the San Miguel population was down
to a few dozen. Similar declines occurred simultaneously in the island
fox populations on neighboring Santa Rosa and Santa Cruz Islands.
|
|
|
|
|
Looking for Answers:
In fall 1998,
National Park Service biologists initiated a radiotelemetry study
of island foxes on San Miguel Island to determine causes of mortality.
Of 15 radiocollared foxes tracked from 1998 to 1999, 5 were believed
to be killed by golden eagles and 2 died of other causes. This confirmed
the results of an earlier study on Santa Cruz Island where golden
eagle predation was identified as the cause of death for 21 of 29
fox carcasses. This level of golden eagle predation is considered
by scientists to be unnatural because golden eagles were never known
from the Channel Island before the 1990's.
|
|
|
The
role of Predation:
Until recent years, the island fox occupied the
top position in the island food chain with no natural predators.
Scientists found the first golden
eagle nest ever recorded for the Channel Islands on Santa Cruz
Island in 1999. Golden eagles on the Channel Islands are supported
by an alien prey base: feral pigs on Santa Cruz Island, and introduced
deer and elk on Santa Rosa Island.
Most likely deterred in the past by the native bald
eagle, golden eagles did not begin their visits to the islands
until their larger fish-eating relatives were wiped out by DDT,
hunting, and egg collection. With sharp talons, swiftness of flight,
and 4 times the body mass of a fox, golden eagles can easily prey
upon the unsuspecting fox. Interestingly, the first documented decline
on San Miguel in 1995 coincides with the first golden eagle sighting
on the island.
|
|
|
|
|
|
Additional Threats:
Predation by golden eagles is the primary mortality factor for foxes
on the northern Channel Islands, but there are other threats as well.
Introduced diseases or parasites can decimate island fox populations.
In just one year, on privately-owned Santa Catalina Island, about
90% of the fox population was recently lost due to canine distemper
virus.
Due to their insular existence, island foxes have no built up immunity
to parasites and diseases brought in from the mainland and are especially
vulnerable to those a domestic dog might be carrying. In addition,
it is extremely difficult to vaccinate against or treat foxes for
these parasites and diseases in the wild. For this reason, pets are
not permitted in Channel Islands National Park [36CFR2.15(a)].
|
|
|
