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. 

FIF Research Grant to Investigate Island Fox Territory Size

Friends of the Island Fox is happy to announce Kathleen "Katie" Elder is the recipient of the FIF 2021 Island Fox Research Grant. 

island fox with GPS collar

In conjunction with the Biological Sciences Department of California Polytechnic University (CA Poly), San Luis Obispo, and Channel Islands National Park, Elder will be investigating territory size for individual male island foxes on Santa Rosa Island. This project will replicate and expand on a Santa Rosa study done in 2009–2010 using specially adapted GPS radio-tracking collars. Twelve years ago, Santa Rosa Island foxes were just starting to recover, with an estimated population size of 389 individuals. The GPS-collared male island foxes were calculated to have a territory size on Santa Rosa of 3.39 square kilometers (Drake et al. 2015).

Two island fox populations, which have not encountered recent population declines–San Nicolas and San Clemente Islands–have been found to have vastly different territorial sizes: 

• San Nicolas Island (2005–2006) 1.81 square kilometers (Powers 2009)
• San Clemente (2010–2011) .42 square kilometers (Sanchez 2012)
  

In the fall of 2020, Santa Rosa Island was home to an estimated 2,657 island foxes. As the population has recovered fully, have territory sizes decreased? 

island fox on Santa Rosa Island, courtesy Tim Bean

Elder will be investigating whether the density of the island fox population impacts territory size and/or the extent of fox territory overlap. She'll also be looking at whether habitat quality is a factor in territory size or territory use during different parts of the day. Are there some habitat areas, like beaches, that are included in the territory of multiple foxes? How do these territorial animals manage areas of overlap? Do they access overlapping areas at different times of the day? Are individuals with inland territories venturing to beaches occasionally? Beach areas provide supplemental food resources, but also may be primary zones for contact with introduced species and disease vectors.

Understanding territory size, overlap, and use, impacts island fox management. Territory-size data plays a role in the algorithms for calculating overall island population estimates. If the data on territory size is outdated, population estimates could be over- or understated. If specific habitats or areas are more important to island fox success, identifying those areas will help secure targeted protections. Identifying areas where island foxes are more likely to encounter threats, may also help us find ways to reduce those negative impacts.

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References:

Drake, E.M., B.L. Cypher, K. Ralls, J. D. Perrine, R. White, and T. J. Coonan.  2015. The Southwestern Naturalist. 60:247-255

 Sanchez, J. N. 2012. Spatial ecology of disease spread in the island fox. M.S. thesis, Humboldt State University, Arcata, California.

Insight on Channel Island Fox Territory Size & Habitat Use

A newly published study on Home-Range Size and Habitat Selection by Male Island Foxes (Urocyon littoralis) in a Low-Density Population by Elizabeth M. Drake, et al. reveals the largest documented territory ranges for male island foxes and new information regarding use of habitat at specific times of day. 

Prior to the catastrophic decline of island foxes on Santa Rosa (1996-2000), the population numbered over 1,700 individuals with a density of ~4 adult foxes per square km. Recovery on Santa Rosa was a slower process than on the other islands and this provided an opportunity to study territory size as the population recovered. The new study was carried out on Santa Rosa Island from 2009-2010, when the population was under 400 individuals and the density of adult foxes was less than 1 per square km. 

Santa Rosa Island

Drake found that in this period of low-density, male Santa Rosa Island foxes averaged territories of 3.39 square km in size. This is considerably larger than the less than .5 square km territories documented both on San Clemente (Sanchez, 2012) and on Santa Cruz prior to the population decline (Roemer et al., 2001). It is also much larger than the ~1 square km territories found on Santa Catalina when the population was well into recovery (King et al., 2014). 

While it is not surprising that island foxes expand territory size when there is less population density, the need for resources does not appear to be the driving force. During this period of fewer foxes, resources were plentiful. Therefore less territory was actually needed to provide adequate food for an individual island fox.

Drake found little overlap of territory between the individual males. In most cases, only minor overlap occurred at territorial borders, suggesting vigorous protection of territory.

Since Drake's data collection the population on Santa Rosa has more than doubled. As the density increases will the lack of territory overlap remain constant and will territory size decrease? Additional study is needed to answer these and other questions.

The study also looked at use of specific habitat. Surprisingly, there did not seem to be habitat types that were more desirable. But they did find significant data on use of habitat type at different times. 

The researchers found island foxes avoided bare landscape and grasslands during the day, but spent a significant amount of time in these areas at night. This use of specific terrain at night may suggest nocturnal hunting for deer mice and insects. Daylight hours were more typically spent in scrub and woodland areas. 

Link to Full Paper
http://www.bioone.org/doi/abs/10.1894/SWNAT-D-14-00021.1

Referenced papers above:
King, J. L., C. L. Duncan, and D. K. Garcelon. 2014. Status of the Santa Catalina Island fox 13 years after its decline. Western North American Naturalist. 7:382–396.

Roemer, G. W., D. A. Smith, D. K. Garcelon, R. K. Wayne. 2001. The behavioural ecology of the island fox (Urocyon littoralis). Journal of Zoology London 255:1–14.

Sanchez, J. N. 2012. Spatial ecology of disease spread in the island fox. M.S. thesis, Humboldt State University, Arcata, California.  

Island Foxes Take a Holiday

It's that time of year when island foxes take a holiday, a holiday from family life.

courtesy of National Park Service

Channel Island foxes typically form monogamous pairs. A male and a female with adjoining or overlapping territories, mate for life. Their committed relationship helps them to successfully defend territory with important food resources and enables them to successfully raise offspring.

Pups are born helpless and require dutiful parental care to survive. While the female may not allow the male into the den to see the pups for a period of time, the male plays an important role in supplying the female and the pups with food.

After the pups have been raised to an age where they can fend for themselves, 6-8 months, they set out to find their own territories. Female pups tend to establish territories close to their parents, while male pups disperse to the opposite side of the island. Male dispersal may be a natural selection adaptation to avoid breeding with close relatives.

When the pups head out, the parents tend to take a holiday from each other as well. From November to early January, island foxes live a more solitary life. Perhaps this separation enables island foxes to find adequate food during the late fall and early winter. Perhaps it creates a greater opportunity for those who have lost a mate, to find a new one. Or perhaps, after a spring and summer filled with parenting, island foxes just need a break and a little solitude.

When the male and female reunite in late December - early January, they will be ready to start a new family in the spring.

As well as monitoring for unexpected fatalities, radio tracking collars help biologists to understand island fox movement and territory size.