What is Island Fox Density?

 

Density is a term used in biology to express the number of individual living things in a given area. Biologists use a defined area size–ie. a square meter, square kilometer or square hectare–to quantify density. 

A square kilometer is equivalent to the footprint of Disneyland in southern California and a square hectare is equivalent to a professional baseball field.

Understanding how many island foxes are living in a square kilometer is part of the calculation for estimating the size of an island-wide population.

At the recent Island Fox Conservation Working Group Meeting, land managers reported on the island fox density for their islands. There was a noticeable difference between northern and southern islands.

Northern islands are reporting considerably higher island fox density. The causes for this are not completely clear. Northern islands may benefit from less extreme weather–higher average rainfall and more moderate summer temperatures–which support diverse prey and plant food for island foxes. Northern islands also have fewer impacts from humans; Catalina Island and the two Navy islands, San Clemente and San Miguel, have roads and cars that cause the highest percentage of fatalities for island foxes. 

High density, however, can have a downside. Catalina Island has been stable for the past 10 years with a density around 9 foxes per sq km. The island with the highest density in 2023–14 foxes per sq km–was also the smallest, San Miguel Island. When density increases, individual island fox territory decreases. A smaller territory means a smaller area to find food. High density can push some individuals into habitats with fewer quality resources. San Miguel's resources are less diverse than the larger islands and there are fewer options for foxes when drought or other weather extremes occur.

 

When island foxes are living closer to each other, the possibility of disease moving rapidly through a population also becomes heightened. Parasites can spread more easily.

Understanding population density is important for calculating risks to populations and making informed conservation decisions. 

Counting Channel Island Foxes

How do you count a population of small animals across an island, when they are so well camouflaged?

How long did it take you to find the island fox?   photo courtesy of M. Jakubowski

Each July to December, across the six Channel Islands where island foxes live, biologists put out special capture cages to catch individual animals. These wire box-traps have a door that snaps shut when the fox steps on a trigger plate at the rear of the box.

The number of cages put out each night and the manner in which they are arranged differs between islands and land managers. Tim Coonan, senior biologist for Channel Islands National Park explains that on Santa Rosa and San Miguel "Traps are deployed in small grids." These grids typically are stretched over rolling hills. The grid pattern may be two traps by six traps wide or three traps by six traps wide. The capture-trap locations are spaced approximately 250 meters apart with the capture cages usually under bushes or scrub. The cage is covered with  burlap to further protect the fox from the elements.

Cages are baited with fragrant substances particularly interesting to island foxes. Each cage has food and water. The grid of cages is checked each morning. Foxes in the cages are first scanned for their microchip ID. Biologist Calvin Duncan explains why this is important.

Each individually identified island fox is counted. This year's pups are counted separately to determine an estimated fertility rate. Pups born in April are old enough to be on their own, but they usually are somewhat smaller than adults and will not yet have received an ID microchip. While in-hand, the island fox receives a health check, vaccinations and an ID microchip or radio tracking collar if needed.

On Catalina Island capture cages are put out along lines called "strings." On mountainous terrain, strings of cages are more practical than grids. On all of the islands, actual numbers of individual island foxes caught during the fall are entered into various population modeling software to determine the best estimate for the total population. After strenuous weeks of counting in the field, it takes months in the office to produce reliable numbers. That is why the number of island foxes counted in the fall becomes official in June of the following year.

These are the official island fox population numbers through fall of 2012.

• San Miguel Island - 540, up from a low of 15 in the year 2000
• Santa Rosa Island - 637, up from a low of 15 in the year 2000
• Santa Cruz Island - 1,354 , up from a low of ~62 in the year 2002
• Santa Catalina Island - 1,502 , up from a low of ~103 in 2000

Notice how Santa Catalina and San Miguel both showed slight population drops last year. Biologists believe this a sign the populations are stabilizing at their natural levels. Drought this year may impact populations as well. Notice too how Santa Rosa is finally showing robust population increase. We hope this population will continue to recover. Whether the numbers are up or down, counting island foxes is the only way we can have the best estimate of population recovery.

Pat Meyer Talks About the Island Fox

Friends of the Island Fox founder Pat Meyer recently was interviewed on a television program in Ventura County, California. 


Hear her talk about how the island fox became endangered, how she was motivated to start a public outreach and education non-profit organization, and how conservation efforts have helped take the island fox from critically endangered toward recovery. 

 

Friends of the Island Fox joined with the Channel Islands Park Foundation in January 2011.

Island Fox Population Update

courtesy of Anita Machlis

Every year biologists count island foxes across the California Channel Islands in the late summer and early fall. Island foxes are captured in safe traps so they can be given health checks, fitted with radio collars and given vaccinations. When the biologist from the six different islands meet for the annual Island Fox Conference in June, they share their official population numbers and discuss the successes and challenges from each island.


The official population estimates from 2010 are:

• San Miguel Island - 516
• Santa Rosa Island - 292
• Santa Cruz Island - 1302
• Santa Catalina Island - 1008
• San Nicolas Island - ~500
• San Clemente Island - 1127

graph generated by Friends of the Island Fox 2011

The numbers show that island fox recovery has been very robust on San Miguel, Santa Cruz and Santa Catalina Islands. The island foxes on Santa Rosa Island are recovering, but at a much slower rate. Each island fox population has its own challenges. For the current status of island foxes by island Download - Island Fox Update 2011 pdf

Stay tuned for updates on 2011 population numbers as they come in from the autumn field counts.