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.
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| 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.
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| 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 effecting 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.
