Island Fox Monitoring

Santa Cruz Island fox standing on top of a grassy bluff, looking towards the camera and into the sun
Santa Cruz Island's endemic island fox.

NPS / Jessica Weinberg McClosky

Why We Care

The island fox (Urocyon littoralis) is one of the smallest canids in the world. Occurring only on six of California's eight Channel Islands, each island population is recognized as a separate endemic subspecies. Although foxes have always existed at low population sizes, four island fox subspecies underwent catastrophic declines in the 1990s.

On San Miguel, Santa Rosa and Santa Cruz Islands at Channel Islands National Park, the decline was attributed to predation by golden eagles (Aquila chrysaetos). The presence of non-native ungulates as a food source in addition to the DDT caused decline of bald eagles (Haliaeetus leucocephalus), a natural competitor, facilitated the establishment of golden eagles as resident breeders on the islands. By 2000, predation on island foxes resulted in the population decline to 15 individuals on San Miguel and Santa Rosa Islands, and less than 80 on Santa Cruz Island.

In 2004, each of the park's island fox subspecies were federally listed as endangered. A recovery program was begun at Channel Islands National Park that included captive breeding and reintroduction as well as removal of resident golden eagles, reestablishment of bald eagles, and removal of non-native ungulates. Population and mortality are subsequently monitored to ensure that recovery proceeds apace and future threats to the park's island fox subspecies are identified.

How We Monitor

For island foxes, recovery, or the probability of persistence, depends on population size and mortality rate.

  • Population size is estimated annually on each island by extrapolating density from multiple small grids.
  • Each grid is trapped for 5 nights and foxes are marked with passive integrated transponder (PIT) tags.
  • The mortality rate, or conversely, the annual survival rate, is estimated by continual monitoring of a fairly large (40+) sample of radio-collared foxes on each island.
Remote camera image of a radio-collared island fox by a box trap.
Remote camera image of a radio-collared island fox by a box trap.

Our Goals

  • Identify trends in island fox populations.
  • Ensure recovery program makes progress in increasing wild island fox populations to viable levels.

Why It Matters for Park Managers

  • Survival of reintroduced foxes has remained high (above 80%) on San Miguel and Santa Cruz Islands and has increased on Santa Rosa since reintroduction began in 2003.
  • Excellent reproduction in the wild combined with high annual survival has allowed recovering island fox populations to increase dramatically.
  • Demographic modeling incorporating the population and survival estimates now predicts a very low risk of extinction for the recovering island fox populations, provided that catastrophic mortality sources (predation and disease) are successfully mitigated.
Line graph showing the San Miguel island fox population plummet in the mid 1990s followed by an overall population rise since the mid 2000s
Decline and recovery of San Miguel island foxes (U. l. littoralis), as indicated by estimated island wide population size from 1993 to 2014, with 80% confidence intervals.

For More Information

Download 1-2 page PDFs about the Mediterannean Coast Network's monitoring programs.

Source: Data Store Saved Search 3539. To search for additional information, visit the Data Store.

The Mediterranean Coast Network documents its findings in reports published in the NPS Natural Resource Publication Series.

Source: Data Store Saved Search 1507. To search for additional information, visit the Data Store.

Protocols describe in detail the procedures used to collect, manage, analyze and report monitoring data. They follow strict guidelines for content and format, and are reviewed and revised by subject-matter experts in each field.

Source: Data Store Saved Search 1519. To search for additional information, visit the Data Store.


Laura Shaskey

Last updated: December 6, 2018