Island Fox Status Update 2025

In 2024, island foxes remained healthy. Normal rainfall levels led to population balancing; high density populations decreased in number and low density populations increased. San Clemente Island, however, realized a third consecutive year of population decline. Concern for these island foxes grows because a specific cause for the decline remains unknown.

island fox on San Clemente Island

The following is an update across the Channel Islands from notes compiled by Friends of the Island Fox at the Island Fox Conservation Working Group Meeting May 15, 2025 hosted at the Santa Barbara Zoo (hybrid meeting).

Reported population numbers are the official estimates submitted by the land managers and calculated by them from island foxes captured for health checks June 2024–January 2025. (How island foxes are captured and given health checks) 

Overview: While food and shelter resources for island foxes were good across the islands in 2024, dry conditions were more prevalent on San Clemente Island and a wildfire burned a third of the island. Monitoring systems documented island foxes surviving the fire, but it is possible the fire contributed to the low number of pups documented. Weather extremes are becoming more frequent across the islands. 

Fox Density: Because island size, weather, and habitat varies from island to island, comparing island fox density can be more informative than comparing population sizes. Fox density reflects the number of island foxes found within a square kilometer (per sq km). (More on density).

A density of 8–12 island foxes per sq km, is the fairly high density of individual animals typically found across the Channel Islands. When a density rises above this range, as San Miguel Island did in 2023 (see graphic above), it is not surprising when the population adjusts down the following year, especially if weather conditions are not optimum for growing native fruit and supplying large amounts of insect and rodent prey.

The two smallest islands with island foxes, San Miguel and San Nicolas, remain most stable when their population density is around 8 island foxes per sq km. Food resources on these smaller islands are never as plentiful as on the larger islands. While the population number decline on San Miguel may seem concerning, the downward adjustment in density creates a more sustainable population. 

The larger islands, Santa Rosa, Santa Catalina, and Santa Cruz provide greater biodiversity of resources and can maintain higher island fox population densities. Densities over 12 foxes per sq km, however, can mean that some island foxes are pushed to very small territories with lower quality resources. Drought, flooding, or the introduction of disease, often has a greater negative impact when foxes are living in high density.

The density graphic above also shows how thin the island fox density was on San Clemente Island in 2023 and 2024. Everyone is watching this island closely and investigating leads on the causes. 

Pup on Northern Islands in August 2024

 

In 2025, the leading threats to island foxes are:

• reduced funding for Federal lands: Monitoring island foxes requires staffing. Reductions in Federal funding for National Park staffing, especially biologists, and unknown budgetary cuts affecting U.S. Navy base environmental efforts threatens island fox health checks and monitoring in 2025. Across Federal lands health checks began early in hopes of completing them before funding cuts were enacted. The ability to continue monitoring on Santa Rosa, San Miguel, San Clemente, and San Nicolas Islands remains in jeopardy for 2026 and into the future. 
• biosecurity: Annual vaccination of island foxes for the deadly canine distemper virus remains a priority. If health checks are discontinued on Federal lands, there will be no annual opportunity for vaccination. Additionally, canine adenovirus (dog flu) has swept across several islands and may be impacting pup survival.
• climate change: Weather extremes not only influence island fox health and resources, they also impact island fox monitoring. The heavy rains of 2023 limited access to Santa Cruz Island and the data collected that year led to a false representation of an upward spike in population numbers. Quality monitoring requires consistency in data collection. Monitoring data in 2024 revealed a stable population on Santa Cruz–mitigating the spike. This highlights the importance of annually collected data to provide the most accurate depiction of island fox population trends across the islands.    

island fox having teeth examined during a health check

 

Greatest Concern: We know how quickly an entire population of island foxes can be impacted by disease, resource loss, or an introduced threat. Quality monitoring is the cornerstone of island fox recovery success. Millions of dollars invested in island fox recovery could be swept away to save a few $100,000 in annual monitoring costs. 

 

Smaller Islands

Because of the amount of years tracked, the graph above shows population estimates for even years only. While smaller populations can be more volatile, San Nicolas and San Miguel Islands remain stable. 

San Miguel Island (lime green line): The downward adjustment of the population from an estimated 525 individual island foxes to 322 reflects a rebalancing to a more sustainable population density. Trends suggest the resources on the island can support 300–450 foxes. Pup numbers were typical in 2024 and individual health appeared good.

San Nicolas Island (pink line): The population remained stable. General health remained good and pup numbers increased. Forty percent of the population shows antibodies for exposure to adenovirus. Ear mites are prevalent. In 2025 foxes receiving health checks will be treated with a topical intervention, following protocols developed on Catalina Island. The hope is to reduce ear mite infestation and increase overall health.

San Clemente Island (blue line): Adult survival dropped from an 89% chance of surviving the year to 69%, and only 7 pups were documented in 2024. Investigations continue into pup survival, the impacts of adenovirus (exposure now prevalent in 80% of the population), and the quality of food resources. The wildfire may have also contributed to population decline. Car strikes add an additional threat to survival. Providing necropsies of individuals killed by vehicles may be vital to identifying health concerns that have gone undetected. 

Larger Islands

Because of the amount of years tracked, the graph above shows population estimates for even years only. As all of these islands hover at carrying capacity, their populations remain stable.

Santa Catalina Island (red line): This population increased slightly in 2024. Treating island foxes for ear mites has reduced cancerous tumors and increased overall health. Introduced adenovirus, corona, herpes, and parvo viruses all continue to decline. Biosecurity threats are greatest to this island, but no raccoons or other invasive species were detected on the island in 2024. The greatest threats to island foxes on this island are people and their pets. Eighteen island foxes were killed by cars and two were attacked by dogs. Rodenticide and unintended poisoning is a growing threat on all human-inhabited islands.

Santa Rosa Island (pink line with black centers): This island currently has the greatest island fox density–13.2 island foxes per sq km. As the density has increased, adult annual survival has declined slightly to a 72% chance of surviving the year. How this high density impacts available resources and fox territory is under investigation this summer with a group of known individual foxes wearing GPS tracking collars and being monitored for diet and health.

Santa Cruz Island (black line): This island continues to have the largest island fox population. As expected, the estimate of over 4,000 individuals in 2023 was an anomaly created by not being able to count foxes at historic counting locations because of stormy weather and flooding. Returning to count in the historic spots in 2024 resulted in a scientifically supportable and healthy population estimate of 3,086. 

Your donations help fund vaccinations and radio collars for monitoring island foxes. 

As July and August unfold, biologists are in the field vaccinating island foxes and fitting them with radio-tracking collars funded by Friends of the Island Fox.

Healthy island foxes depend on people like you supporting conservation efforts. 

 

Island Fox Status Update 2024

In 2023, island fox populations remained healthy and stable across all six islands. Weather extremes, however, effected islands in different ways.

Island fox on San Clemente Island courtesy of J. Sanchez

The following is an update across the Channel Islands from notes compiled by Friends of the Island Fox at the Island Fox Conservation Working Group Meeting May 8, 2024 hosted at the Santa Barbara Zoo (hybrid meeting).

Reported population numbers are the official estimates submitted by the land managers and calculated by them from island foxes captured for health checks July–December 2023. (How island foxes are captured and given health checks)

Overview: Rainfall was abundant in 2023, but the impact on plant and prey species varied by island. Island fox populations on smaller islands grew, while island fox numbers on larger islands adjusted downward slightly. The anomaly was Santa Cruz Island, which appears to have experienced a significant population increase, creating a historic record for a single island population.

Range Size and Diet Research: Ongoing research by FIF Research Grant recipient Katie Elder revealed that fox home ranges on Santa Rosa Island have declined in size by 83% since 2009. It's not surprising that home range territories have gotten smaller as the fox population has recovered from less than 400 individuals to over 2,500. The surprise is how strictly the 15 male foxes monitored for a year with GPS collars stayed confined within their small territories. This means that individuals in poor habitats have less access to varied resources; inland foxes aren't accessing beach foods and individuals with grassland home ranges may have less access to fruiting plants. How will individual survival be challenged if drought conditions return?

Density: On the northern islands, island fox density has become high: 11.35–14 island foxes per square kilometer. What is island fox density? While it's great to see increased populations of island foxes, high density means individual fox territories are close together. Disease can be transmitted more easily through a dense population. As the Range Size research reveals, high population density may also mean island foxes have less flexibility to survive fluctuations in their available resources.  

Climate Impacts: For the first time, islands documented increased rainfall that did not coincide with island fox population growth. The timing of storms or extreme high levels of rain may have negatively impacted deer mouse or insect populations resulting in fewer resources for island foxes.    

In 2024, the leading threats to island foxes are:

• biosecurity: the threat of introduced viruses, accidental transportation of a deadly parasitic worm from San Miguel Island to other islands, disease, and/or non-native animals
• reduced funding for monitoring
• climate change: extreme weather effects food resources, heightens parasites, challenges health, and can make it more difficult for biologists to access the islands
  

Greatest Concern: Biosecurity threats are heightened by the high density of island foxes on northern islands and rising tourism on Catalina Island. Canine distemper and other dog diseases remain threats. A new investigation has identified the parasitic spiny-headed worm on San Miguel Island that has caused fox fatalities in the past, especially in drought years. The investigation to find the prey species that acts as an intermediary host continues. New findings reveal that this lethal intestinal parasite was most likely transported to the island by humans. It is vitally important that this parasite not be moved to other islands.

Be a BioSecurity Guardian–Clean, Check, and Close everything you take to the islands. Watch Biosecurity Video.

Smaller Islands

San Miguel and San Nicolas Islands appear to have equally benefited from additional moisture and available resources. Their populations grew at the same rate in 2023. San Clemente Island remained stable at its lower population size.

San Miguel Island (lime green line): While adult annual survival declined, healthy pups stabilized and increased the population. This island currently has the greatest island fox density: 14 foxes per square kilometer.

San Nicolas Island (pink line): General health remains good, but lifespan appears to be 7–8 years, rather than the 10–12 years on larger islands. Density remains moderate at 8.4 foxes per sq km. Dog flu has declined, but remains prevalent. Car strike causes the greatest island fox mortality and more days of foggy weather increased fox fatalities.

San Clemente Island (blue line): Adult island foxes on this island had a high 89% chance of surviving the year, but pup survival continues to be investigated. This island currently has the lowest fox density: 4.06 foxes per sq km. Approximately 5% of the population is lost annually to car strike and inexperienced young island foxes are more likely to be hit.

 

Larger Islands

The larger islands were all thought to have reached carrying capacity. Santa Catalina and Santa Rosa Islands adjusted downward at nearly the same small rate in this extremely wet year. Because their populations are so large, this adjustment is not unexpected. Santa Cruz Island, however, increased at the same high rate as the small islands, creating a historic population estimate.

Santa Catalina Island (red line): The population remained stable with a density of 9.22 foxes per sq km. Analysis of tick samples collected from island foxes from 2019–2022 showed no tick-borne disease. This is important because 60% of Catalina Island foxes carry ticks. Both dog flu and corona virus declined in 2023, but biosecurity remains a huge concern on this island. From car strike and dog attack to foxes being drown in scuba wash tanks and entrapped in human structures, humans and pets currently pose the greatest threat to island foxes. In 2023, over 62% of known fox fatalities were human caused.

Santa Rosa Island (pink line with black centers): This population remains stable with a high density of 11.35 foxes per sq km and adults had an 89.5% chance of surviving the year. Research is showing that Rosa foxes are becoming dietary specialists. How will this impact individual foxes and the island ecosystem?

Santa Cruz Island (black line): With an adult annual survival rate of 94.7%, this island was estimated to have a historic population size of 4,057. The fox density is currently 12.9 foxes per sq km. Due to a record 35 inches of rain, foxes could not be counted in some of the traditional grid locations. There is a possibility that the high population calculation may be slightly inflated by data fluctuations.

Weather has a profound effect on island resources. Changing weather patterns–tropical summer storms, extreme winter rainfall, increased summer heat, denser fog–all impact island habitats and island foxes. 

Your donations help monitor island fox survival with radio collars and provide vaccinations to protect them from introduced disease. Research is investigating health, behavior, and how best to support stable island fox populations in a changing world.

Healthy island fox populations depend on people like you supporting conservation efforts. 

 

Island Fox Status Update 2023

Island fox populations are healthy, but the differences between each island subspecies are becoming more apparent. 

 

The following is the update from across the Channel Islands as compiled from notes by Friends of the Island Fox at the 25th Annual Island Fox Working Group Meeting held May 18, 2023 at the Santa Barbara Zoo (hybrid meeting).

Reported population numbers are official estimates from each land manager as calculated during the 2022 counting period. (How are island foxes counted)

Overview: Island foxes appeared healthy across the islands, but while some populations increased toward near record highs, others declined. Increasingly, island fox populations are revealing different health risks, climate impacts, and island specific challenges.

Microbiome research. Each island ecosystem effects the natural bacteria of its island fox population. Three islands have ear mites: Nicolas, Clemente, and Catalina. Both Clemente and Catalina Island foxes have reduced bacterial biodiversity, but only Catalina Island foxes develop chronic inflammation in conjunction with out of balance bacteria and develop cancer if ear mites are not treated. More about microbiome research

Island fox life span may vary by island. The age of many island foxes is documented because most individuals are microchipped. In 2022, two island foxes on San Miguel Island died naturally of old age at 11 and 12 years old. At the same time, the oldest island fox captured on San Nicolas Island was 9. Santa Rosa currently has the greatest density of island foxes and collared foxes that died during the year were 2–7 years old. Island fox life span may be influenced by multiple variables. Determining age from tooth cementum

Island size influences fox population limits. Smaller islands, like San Miguel and San Nicolas can only support 300–450 island foxes. When drought, catastrophic heatwaves, or other survival challenges occur, these smaller populations are more vulnerable to loss. 

The leading threats to island foxes in 2023 are:

• biosecurity: the threat of introduced viruses, disease, and/or non-native animals
• reduced funding for monitoring
• climate change: increasing regional temperatures and decreasing annual rainfall, which reduce resources, increase wildfire, and promote greater parasite numbers

 

Greatest Concern: While other island fox populations remained stable or even increased, San Clemente Island foxes declined by an estimated 30%, without a known cause. Individual island foxes on San Clemente appear healthy and pups were documented in numbers similar to other islands. This decline is being investigated; it's a strong reminder that these island populations are vulnerable and can be effected quickly by a negative force. 

Disease threats remain high for all island foxes. As populations reach carrying capacity and island foxes live closer together, it becomes easier for viruses to pass from fox-to-fox. Friends of the Island Fox is responding to this increased concern by supporting canine distemper and rabies vaccinations across four islands in 2023.

You can help protect island foxes by being a biosecurity guardian; watch the video

 

Smaller Islands

The two smallest islands San Miguel and San Nicolas remain stable, but you can see in the graph above that the trend was completely different on San Clemente Island.

San Miguel Island (lime green line) - This population continues to increase from a drought impact low in 2018. In 2022, an individual island fox on San Miguel had an 82% chance of surviving the year, the highest survival rate reported across the islands for the year. 

San Nicolas Island (pink line) - Easing drought also benefited San Nicolas Island foxes and their individual health improved. However, island foxes have begun congregating in areas where people offer food. As a result Adenovirus (dog flu) and Parvovirus are spreading and increasing across the island; fortunately, no fatalities have been attributed to the diseases. Being hit by a car remains the greatest threat to these island foxes. FIF is working with the Navy on an education program to reduce this threat.

San Clemente Island (blue line) - The population dropped lower than it has been since 2008. Car strike is especially frequent on this island–42 fatalities in 2022–with the majority being young foxes. This, however, doesn't account for the overall decline. Researchers are evaluating the survival rate for pups and other potential causes.

Larger Islands

The current population sizes of the larger islands have helped provide greater stability and resilience against climate change impacts. All of the island fox populations on the larger islands are at or near documented population highs.

Santa Catalina Island (red line) - Catalina Island foxes benefited from easing drought; their general health improved and viruses in the population declined. Currently, human activity poses the greatest threat. Not only are island foxes hit by cars, they get trapped in man-made structures and are killed by pet dogs. Catalina has the greatest threat of canine distemper or rabies being introduced. FIF funded vaccinations for 350 island foxes on this island and health testing.

Santa Rosa Island (pink line with squares) - This island currently has the greatest known island fox density, with 12.4 foxes per sq km. The calculated rate for an individual to survive the year declined to 75%, but the population remained unchanged. While several collared foxes aged 2–7 died during the year, the cause of death remains unknown because of a lack of staffing in the National Park to recover the bodies for testing. The two GPS radio collar studies of island fox territory supported by Friends of the Island Fox continue. Data should be available later this summer.

Santa Cruz Island (black line) - Santa Cruz Island has the largest island fox population, but increasing competition has edged down the chance of an individual island fox surviving through the year to 55%. Still, island foxes appear healthy. Ten sentinel foxes died during the winter, but their bodies were not recovered for testing due to stormy weather. 

Across the Channel Islands, island foxes are stable and healthy because people like you are investing in their future. Your donations to Friends of the Island Fox fund vaccinations, radio collars for monitoring, health efforts, education, and important research.