The Science of Counting Island Foxes

courtesy of National Park Service

In 2000 when there were only 15 island foxes surviving in captive breeding pens on San Miguel, you could count them easily on your fingers.

Today all Channel Island foxes across the six islands where they live are back in the wild. (A few rescued individuals from San Clemente Island can be seen in mainland zoos.) The recovery of the endangered island fox on San Miguel, Santa Rosa, Santa Cruz and Santa Catalina Islands has been record-breaking and the days of counting all of the physical island foxes are long gone.
 

courtesy of National Park Service

The process of counting foxes on each island begins in the fall with special capture cages.

These cages are set up either at the cross sections of grids or along a string depending on the island and its topography (More on strings and grids)

Cages in an area are set out over the course of a specific number of nights and checked each morning. On Catalina, each string is run for four consecutive nights; after that, typically the majority of individuals would be foxes that had already been captured on previous nights. Each island fox is identifiable because it has a Passive Identification Transponder (PIT) microchip. An island fox caught for the first time receives a PIT tag. From that point on the PIT tag allows it to be recognized as a specific individual. It can take weeks for biologists to evaluate each island.

Over the course of all the capture nights, the number of individual island foxes, their gender, ages and location caught are documented. Island foxes are quite territorial, therefore it is unlikely that a fox caught in one area of the island would be found on a different area of the island during the month of counting. (If it is, that is important too.)

The data is input into a modeling program for analysis. Different programs may be used on different islands and over the years land managers may change which program they use. The modeling program calculates the density of island foxes in specific habitats across the island and estimates a total island population.

Sometimes, however, it takes a human brain to see complexity in the numbers. On two islands where recovery has been robust over the past few years, San Miguel and Santa Catalina, the estimated population numbers appear to be higher in 2013 than they were in 2012.

But observations on both islands conclude that drought may have reduced births and did cause starvation in a number of pups last summer, prior to counting. How could the population number increase while the number of surviving pups decreased significantly?

Biologists Calvin Duncan and Julie King from the Catalina Island Conservancy explain that on Catalina they caught a greater number of adult island foxes that had not been counted for a number of years. Young animals are more likely to be curious and challenged with finding food; they venture into capture cages more readily. Once an animal is in the cage, no other can be caught that night. This year, on the fourth night of capturing, they were still finding a high percentage of animals that had not been caught on previous nights.

courtesy of Kevin Schafer

The increase in the population number for Catalina is warranted because of the number of unique individual animals that were counted. But rather than an increase in the population, the number represents a refinement of the estimate toward greater accuracy.

On both San Miguel and Santa Catalina Islands, island fox numbers have surpassed historic population figures. Both islands may have reached carrying capacity, or the number of individual animals that can find the territory and food resources necessary for a healthy life. As the drought continues this summer, there may be further impacts not only on pups, but also on older individuals.

Counting island foxes provides an important picture of island fox recovery, but observations throughout the year, health checks, blood tests, data collected from radio-collared island foxes and necropsies is vitally important to understanding the whole recovery picture.

400 Endangered Island Foxes Protected

photo courtesy of Melissa Baffa

These two island fox pups born on the Channel Islands this year have a glowing future because their human neighbors care about protecting them.

Friends of the Island Fox is thrilled to announce that the Fresno Chaffee Zoo has awarded us a Conservation Grant of $2,000 to vaccinate 200 endangered Channel Island foxes against canine distemper.

This is a doubly rewarding acknowledgement because:

1. These matching funds combine with private donations raised this summer to meet our goal of $4,000 to vaccinate 100 endangered island foxes against canine distemper on FOUR islands.
2. This is also the sixth year that the Fresno Chaffee Zoo has supported conservation efforts on behalf of the endangered Channel Island fox. In previous years they have funded radio tracking collars on individual island foxes.

It is heartening when a local California zoo sees the importance of supporting conservation of a local endangered species. The Fresno Chaffee Zoo has been a strong partner in helping to save the endangered island fox and we thank them for recognizing the continued success of island fox recovery depends on proactive vaccination against disease.

Thank you to everyone who supported our canine distemper vaccination drive this summer.

You helped protect 400 Channel Island foxes from canine distemper in 2012 !

Island Fox Pups

Across the Channel Islands, island fox pups are being born. For their first months of life they look more like little brown bears than their dramatically marked parents. 

Typically, island foxes become parents in their second year, but sometimes a male or female may become a first-time parent before they are a year old.


Most island fox pairs have 2-3 pups in a litter, but a young female like our Twitter fox, Tani, might have only one pup. 


This year rainfall on the Channel Islands has been low to normal. Weather can also affect the number of pups born because rainfall can impact food resources. Annual rainfall.

island fox bringing 3 deer mice home for family, photo courtesy NPS

While female island foxes are in their dens with their new born pups, the fathers play the role of providers, bringing food to their mate and eventually the pups. 


Over the last year Friends of the Island Fox has followed Tani, an island fox character, as she grew up and became a mother herself. Join us as we follow Tani's mate, Tiptu, and get a male fox's perspective over the next few months on Twitter and Facebook. Tiptu, is "big ears" in Chumash, the local native American language. Tiptu has befriended another rare animal only found on the Channel Islands - an island scrub-jay. 


Island scrub-jays have a family structure similar to the island fox, so Jay will have insight to help his friend the island fox through his first year as a father. The island scrub-jays are nesting as well. Spring brings all kinds of new life to the Channel Islands.