|photo courtesy of M. Solomon|
However, four of six islands saw dips or declines in population in 2014. The historic drought and its impacts on food resources is believed to be the cause. Biologists on Santa Catalina have documented a direct connection between rainfall levels and island fox reproduction. Years with drought-level precipitation result in reduced plant food and prey, thereby causing fewer island fox pups to be born or to survive. The extent of drought impact varies significantly by island.
|Minimal drought impacts on Santa Cruz Island, 2015|
Introduced disease continues to threaten all island foxes and a lack of canine distemper virus (CDV) vaccine safe for use in island foxes has been a major concern for two years. The Working Group took urgent steps, and Friends of the Island Fox participated, to determine the effectiveness of a newly available CDV vaccine.
|courtesy of E. Gotthelf|
Island Fox Update 2015 pdf a specific island-by-island summary
- San Miguel Island - 520 (low of 15 in the year 2000). Population recovered with over 500 individuals since 2010. Slight population dip. Concern: Threat from new parasites continues, as do drought impacts (see below)
- Santa Rosa Island - 874 (low of 15 in the year 2000). Population recovery stalled by drought, but stable.
- Santa Cruz Island - 1,750* (low of 62 in the year 2002). Population recovered and robust with over 1,000 individuals since 2009. No discernible drought impacts.
- Santa Catalina Island - 1,717 (low of 103 in the year 2000). Population recovered and stable with over 1,000 individuals since 2010. Slight population dip. Concern: Drought impacts and human related impacts, including car strike, and high risk for introduced disease.
- San Clemente Island - 1,230* (not Endangered). Population stable. Concern: Continued fatalities to automobile strike.
- San Nicolas Island - 263* (not Endangered). Population has declined ~41% since 2012 because of drought impacts. Concern: Habitat destruction has left island foxes dependent on non-native plants and prey which have declined dramatically in the drought, see below.
The graph above shows population numbers on even years, therefore the dip from 2013 is not represented. For more detailed graphs see the Island Fox Update document. The numbers represented are the official 2014 population figures reported at the Island Fox Conservation Working Group Meeting 6/16/15. *Population figures represent adults only.
Healthy plant and animal populations fluctuate normally with available resources. Recovered island fox populations naturally dip when there is not enough food, water, or territory. There is a natural limit to the number of island foxes an island can support, also known as carrying capacity.
Low rainfall has impacted food resources across the Channel Islands. However, islands have not been impacted equally. Some islands appear to have greater natural drought tolerance.
As recently discovered by analyzing island-fox diet, Santa Catalina and Santa Cruz Islands have greater native plant biodiversity than the other smaller islands. Native island vegetation evolved with periods of drought. Native plants are better able to survive and produce fruit vital for the survival of island animals. The greater the native plant biodiversity, the greater the survival options for island foxes.
|island fox footprints among ice plant on San Nicolas|
Restoring native vegetation is the best long term solution for healthy island fox populations on these two islands. (for more on this topic, see the Island Fox Update 2015 pdf document above)
A secondary impact of the drought has been increased complications with parasites. Changes in diet and decreased general health leave island foxes more vulnerable to internal and external parasites. Here again, each island has its own specific parasite challenges. (See Island Fox Update 2015 pdf for details) (Spiny-headed worm on San Miguel Island first detected in 2013).
Introduced disease continues to pose a threat to all island foxes, especially on islands visited by people. Dogs, cats, and introduced species, like raccoons, are all avenues for disease to be transported to isolated and disease-naive island foxes. Efforts to verify the effectiveness of a new CDV vaccine for island foxes is currently underway.
|courtesy of D. Mekonnen|
Monitoring with radio collars continues to provide important information to land managers. Radio collars have helped verify that no golden eagles have returned to eat island foxes on the northern islands. Radio collars were instrumental in determining the cause of island-fox decline on San Nicolas Island. And they continue to provide early warning of disease introduction. Radio monitoring collars will also enable land managers to ethically determine the effectiveness of the new CDV vaccine.
Annual island fox health checks, serology testing, and counting identify threats to island fox survival early, so that actions can be taken to protect island populations.
‘Fox-Saver’ bins on Santa Catalina and the Navy's education outreach on San Clemente are all helping to reduce the number of island foxes hit and killed by cars.
Thank you to the Island Fox Conservation Working Group and all of the important work that has helped island foxes recover from the brink of extinction. Thank you also to the many participants on our June Santa Cruz Island Trip that have allowed us to show you their photographs.