Friday, May 06, 2022

Island Foxes and Carrying Capacity

"Carrying capacity" is a term used in biology to describe when a habitat has reached the maximum population of a specific species that can be supported by the habitat's resources.

Think of a table with a platter of cookies. If there are more cookies than people, individuals will recruit others to enjoy the cookies; the population at the table increases. If there is just one cookie for each person, carrying capacity has been reached. If there are more people than cookies the table has exceeded its carrying capacity. People who need cookies will have to leave the table and go to another habitat; there aren't enough resources to support the larger human population.

Seeing three island foxes in such close proximity, as in the image above, is unusual.  A unique weather situation provided unusual resources.

Because island foxes live on islands, they are definitively confined to the resources of their habitats. They can not migrate to another location when resources or climate become challenging. Carrying capacity, however, isn't a strict number. The Channel Islands are wild natural places and island resources vary from year-to-year depending on rainfall and weather. 

Island foxes on Santa Rosa Island (pink line, in graph) have been increasing as they recovered from near extinction twenty-two years ago. In 2021, biologists documented the first estimated decrease in the Santa Rosa Island fox population. (Official 2021 population numbers will be available later this month.) This could be signally that the population estimate of 2,657 in 2020 was a larger population than the island's resources could actually support.

How does the population adjust down? Older island foxes (8–10 years old) may have a more difficult time surviving. They may be pushed out of their established territory by younger foxes. 

In addition, reproduction can slow down, decreasing the natural population replacement of individuals. We know that very few pups were documented on Santa Rosa Island in 2021. 

This female fox on Santa Rosa Island (right), identified as F257, was captured in 2019 as pup and then again in 2020 and 2021. At two years old, F257 is in her breeding prime, but she has yet to produce pups. This most likely is in response to limited resources.

Santa Rosa Island was extremely dry in 2021. Canines are adapted to produce offspring in balance with the amount of available resources. A female island fox with lots of food resources can produce five pups in her annual litter, but if there aren't enough resources or available territorial space to keep pups safe, her body can reabsorb embryos and give birth to no pups. 

Carrying capacity can sound ominous, but it doesn't mean that island foxes will experience a die-off event because they have reached a specific population number. 

If we look at Catalina Island (red line, in graph above), we can see that the island foxes on Catalina reached carrying capacity around 2013–2014. A healthy population on this island appears to be between 1,800 and 2,200 island foxes. During years of normal rainfall, resources increase and so do island foxes. Drought years, like the winter of 2015–2016, can result in natural fox fatalities and less reproduction. The population gradually adjusts down. When normal rainfall returns, the population rebounds. This is the natural fluctuation expected in a wild population. 

Now that the Santa Rosa Island fox population is at a high and has potentially reached carrying capacity, 2022 is the perfect time to re-evaluate territory-size and fox movement to access various island resources. Collecting GPS tracking data this summer will provide valuable data on territory, the current interactions between individual foxes, and how they access resources in their habitat.

Your donations are helping fund two important GPS-tracking studies on Santa Rosa Island this year.

FIF is also taking applications for the FIF 2022 Research Grant.

Friday, April 15, 2022

FIF Taking Applications for 2022 Research Grant

This is a GPS radio collar being prepped to be worn by an island fox this summer. Katie Elder was FIF's 2021 Research Grant recipient and she will be investigating territory size on Santa Rosa Island. More on her work

Friends of the Island Fox is taking applications for our 2022 FIF Research Grant now through August 29, 2022.


What will you uncover about island foxes?

Research in island fox ecology is an investment in the future and island fox survival. Not only is this work important for island foxes and the Channel Islands, but also for other endangered species with small population sizes in confined geographic habitats. An island doesn't have to be surrounded by water, it can be a confined habitat surrounded by desert, a city, or some other physical barrier.

Population viability calculations used for island foxes have been applied to the Sierra Nevada red fox.

See more on Current FIF Funded Research Projects

Recently published island fox discovery 

Discussion on island foxes and island spotted skunks

Update on island fox microbiome research

Tuesday, April 05, 2022

Island Foxes and Beach Foods

In 2018 Juliann Schamel was the recipient of Friends of the Island Fox's first Research Grant. Her investigation of island fox diet through stable isotopes in whiskers has evolved and deepened to look for connections between diet and surviving drought conditions. 

In 2019 FIF continued to fund Schamel's research as she followed the foxes and the potential of "beach foods" in their diet. It is no coincidence that the island fox above is in a beach area. 

If you look closely at the pile of kelp washed up on the beach, or "beach wrack," you'll see island fox foot prints all around it.

photo courtesy of Nick Schooler, UCSB
Combining her stable isotope research with collaborators at the University of California, Santa Barbara, Schamel has revealed that some island foxes are making use of food resources connected to sandy beaches. Tiny arthropods called "beach hoppers" live in the sand and emerge to eat the kelp that washes up on the beach. Look closely and you will see a group of them feeding on the edge of the kelp blade pictured. 

Some island foxes are eating these tiny, quick moving creatures. If you've turned over a clump of beach wrack and seen little hopping critters, commonly referred to as sand fleas, you've seen beach hoppers. They are not fleas at all; they are more closely related to shrimp.

Juliann Schamel recently presented a poster at the 2022 Wildlife Society Conference. POSTER

And her paper Diet of a threatened endemic fox reveals variation in sandy beach resource use on California Channel Islands with Henry M. Page, Marine Science Institute, UCSB, et. al. was published in PLOS ONE. Read the Paper

Your support for FIF helped fund this important research. The island fox and the beach hopper demonstrate an intertwining of terrestrial and marine ecosystems on the Channel Islands.

Saturday, April 02, 2022

Island Foxes, the U.S. Navy, and Island Spotted Skunks

"Date with a Fox" February 2022

Island foxes on San Clemente Island live with an active U.S. Navy Base, while island foxes on Santa Cruz Island live with island spotted skunks.

FIF's February virtual program "Date with a Fox" featured:

Holly Gamblin
a biologist with the Institute for Wildlife Studies  on San Clemente Island. Gamblin provided a rare inside-look at the island foxes on the U.S. Navy island. Few civilians have access to San Clemente Island.


Research biologist Ellie Bolas shared her masters' thesis studies on the interactions between island foxes and island spotted skunks. Island spotted skunks live only on Santa Cruz and Santa Rosa Islands. Very little is known about the ecology and behavior of island spotted skunks. How do these two small predators co-exist on the islands?

If you missed the program you can watch the video here:

To participate in FIF's "Date with a Fox" in May - join our donor list or subscribe to our e-newsletter.

See past "Date with a Fox" programs

Wednesday, March 30, 2022

Island Fox Webinar Today

Find out more about island foxes TODAY!

Wednesday, March 30

3 - 4  pm Pacific Time, 6 - 7 pm Eastern Time

in conjunction with Wolf Park in Indiana

Friends of the Island Fox will be giving a webinar on island foxes, with an update on Dr. Alexandra DeCandia's microbiome research.

To register for this free on-line event: Wolf Park Webinars

Wolf Park is celebrating 50 years of conservation of canine species and has been a supporter of island fox research.

Monday, February 21, 2022

Friends of the Island Fox Presentation for CIC Speaker Series

When island foxes on four islands became threatened with extinction 22 years ago, little was known about their natural history. 

Join Friends of the Island Fox 
President Mike Watling and 
Ed. Director Keri Dearborn 
on Friday, Feb. 25 at 5:30 pm PT

We'll talk about what's been learned about these charismatic little predators and the important role they play in the island ecosystem as a keystone species. We'll touch on the conservation measures over the past two decades and the current research that is illuminating island fox diet, interaction with other species, and how dramatic population decline may have unexpected impacts on longterm island fox health. 

Register for this free Virtual Event at: Catalina Island Conservancy

We hope to see you there

Friday, February 04, 2022

Love is in the Air for Island Foxes, Too

February isn't just the month of romance for humans, it is also when island fox pairs find a den and mate.

Island foxes are more monogamous than their gray fox ancestors. Territory and resources are limited on the Channel Islands and a pair of island foxes works together to protect their territory from other island foxes. They also work together to provide food for their pups. 

For the first month or so of the pups' lives, the female island fox stays close to the den to feed the pups and help keep them warm. She needs a male partner she can depend on to bring her food. A male island fox may bring his lady love a dead deer mouse or a small bird, to show that he is a good provider.

He also will protect their territory to keep the pups safe (and to keep other male island foxes away from his mate). 

A mouse gift may not seem as romantic as chocolates to us, but to a female island fox the gift of a deer mouse means her mate will be dependable and committed to helping her raise healthy pups. 

Friday, January 28, 2022

Fox Foto Friday - Beginning of the Year Surprise!

What is it? 15 refurbished radio collars!

These radio-telemetry collars, still in their packaging, have just arrived for island foxes on Santa Cruz Island. They come packaged as pairs of collars set inside each other with their antennae wrapped around. 

The Fresno Chaffee Zoo Wildlife Conservation Fund provided FIF with a special year-end grant that funded these 15 refurbished radio collars and will fund serology testing for Catalina Island foxes this summer.

We are so thankful for our fox friends at the Fresno Chaffee Zoo.  

The Fresno Chaffee Zoo has been an important partner in island fox conservation since 2006 - funding radio collars, vaccinations, and island fox health investigations.

The Santa Barbara Zoo and the Fresno Chaffee Zoo have long been champions of island fox conservation. They are the perfect example of how zoos can make a real conservation difference.

After these radio collars are prepped and banded with color-coding, they will be ready to be fitted on island foxes. 

You are an important partner in island fox conservation, too. Donors like you are matching the Fresno Chaffee Zoo's grant! 

This winter donors have funded 14 refurbished collars and 2 new collars.

FIF hopes to fund 20 more radio collars this season!

Radio collars are the number one way to monitor island fox health and survival in the wild. 

Your donations help FIF provide important radio collars for island foxes!

Friday, January 21, 2022

Friends of the Island Fox is Now a Partner with 1% for the Planet


Friends of the Island Fox is pleased to announce that we have joined 1% for the Planet as a nonprofit partner! This partnership is intended to advance our impact on the Channel Islands as well as involve more businesses and individuals in the environmental movement.

1% for the Planet is committed to building support for the essential work of Friends of the Island Fox and other nonprofit partners within six core issue areas: climate, food, land, pollution, water and wildlife. 1% for the Planet approved nonprofit partners are carefully vetted environmental organizations that are eligible to receive certifiable funding from their members.


"The intent of 1% for the Planet is to help fund these diverse environmental organizations so that collectively they can be a more powerful source in solving the world's problems," writes Yvon Chouinard, co-founder of 1% for the Planet.

More about 1% for the Planet

Learn more by viewing Friends of the Island Fox's profile here: Friends of the Island Fox - 1% for the Planet

Tuesday, December 28, 2021

Not Every Fox on an Island is an Island Fox

Friends of the Island Fox educators had a great question from a student in the Oak Park School District in California.

Are the foxes on the San Juan Islands in Washington island foxes?

Monascherie for San Juan Visitors Bureau

San Juan Island in Washington does in deed have a fox population; it's the only island along the coast of Washington that does.

The foxes range in color from tawny red to silvery black, but if you know your foxes there is a distinct characteristic that tells you these foxes are very different from the Channel Island fox. The tail tip is white! 

How to ID North American Foxes

The foxes on San Juan Island are red foxes (Vulpes vulpes). 

The story of the San Juan Island foxes has some similar plot points to island foxes

  • introduction of non-native species to an island
  • damage to an island ecosystem
  • a search for ecosystem balance

In the 1800s, a lighthouse was built on San Juan Island to help ships avoid dangerous waters. (Just like the lighthouse on Anacapa Island). Transporting fresh food to the island was costly, so in the late 1800s, European rabbits were introduced to San Juan Island to provide a fresh meat source for lighthouse keepers.

Without any natural predators on the island, the rabbits quickly began devouring the native island plants. The island ecosystem was crashing. There were too many rabbits for the people to control. It's believed that some time around 1930, red foxes and/or red foxes bred to have black fur for the fur industry were introduced or set free on San Juan Island. It was hoped that the red foxes would control the rabbit population and also provide foxes for people to hunt.

The red fox population on San Juan Island is fairly small, but they have controlled the European rabbits and, fortunately, they have not had a negative impact on other native island species. The island has returned to a tenuous balance and the red foxes are now protected on San Juan Island. 

The red foxes have been isolated on San Juan Island for almost 100 years. 

Will they develop island adaptations? Will they become island dwarfs over time like the island fox? How long will it take for them to evolve into a new species? Only time and evolution can tell.

Island foxes on California's Channel Island remain the only island-endemic fox species in the United States.

If you have a good question about island foxes send FIF an email at