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