Fox Foto Friday - Surviving the Heat

Summer heat is impacting the Channel Islands and island foxes just as much as the mainland.

High temperatures and low rainfall increase the challenge for island foxes to find food. Just like with people, high temperatures put greater stress on the young and the old. 

Island fox numbers are good across the islands (See the Island Fox Update 2018), but fox biologists warn that 2018 may be a harder year for island foxes.

Thank you to Michael Lawshe for this great photo of an island fox searching in the dry grass.

Channel Island Fox Status Update 2015

photo courtesy of M. Solomon

The annual meeting of the Island Fox Conservation Working Group in 2015 reported that the general status of the six subspecies of California's Channel Island foxes is GOOD.

However, four of six islands saw dips or declines in population in 2014. The historic drought and its impacts on food resources is believed to be the cause. Biologists on Santa Catalina have documented a direct connection between rainfall levels and island fox reproduction. Years with drought-level precipitation result in reduced plant food and prey, thereby causing fewer island fox pups to be born or to survive. The extent of drought impact varies significantly by island.

Minimal drought impacts on Santa Cruz Island, 2015

 Despite the drought, recovery of the four endangered subspecies of island fox has been very successful and this spring (March 2015), the US Fish and Wildlife Service (USFWS) announced initiation of a status review of the four subspecies. 

Introduced disease continues to threaten all island foxes and a lack of canine distemper virus (CDV) vaccine safe for use in island foxes has been a major concern for two years. The Working Group took urgent steps, and Friends of the Island Fox participated, to determine the effectiveness of a newly available CDV vaccine.
 

courtesy of E. Gotthelf

All island fox subspecies are maintaining stable populations and there are no issues which put any Channel Island fox subspecies in eminent threat of extinction. The continuing drought, however, has more severely impacted island foxes on San Miguel and San Nicolas Islands. As seen in the past, each island faces its own concerns and challenges.


Island Fox Update 2015 pdf a specific island-by-island summary

• San Miguel Island - 520 (low of 15 in the year 2000). Population recovered with over 500 individuals since 2010. Slight population dip. Concern: Threat from new parasites continues, as do drought impacts (see below)
• Santa Rosa Island - 874 (low of 15 in the year 2000). Population recovery stalled by drought, but stable.
• Santa Cruz Island - 1,750* (low of 62 in the year 2002). Population recovered and robust with over 1,000 individuals since 2009. No discernible drought impacts.
• Santa Catalina Island - 1,717 (low of 103 in the year 2000). Population recovered and stable with over 1,000 individuals since 2010. Slight population dip. Concern: Drought impacts and human related impacts, including car strike, and high risk for introduced disease.
• San Clemente Island - 1,230* (not Endangered). Population stable. Concern: Continued fatalities to automobile strike.
• San Nicolas Island - 263* (not Endangered). Population has declined ~41% since 2012 because of drought impacts. Concern: Habitat destruction has left island foxes dependent on non-native plants and prey which have declined dramatically in the drought, see below.

The graph above shows population numbers on even years, therefore the dip from 2013 is not represented. For more detailed graphs see the Island Fox Update document. The numbers represented  are the official 2014 population figures reported at the Island Fox Conservation Working Group Meeting 6/16/15. *Population figures represent adults only.

Concerns:
Healthy plant and animal populations fluctuate normally with available resources. Recovered island fox populations naturally dip when there is not enough food, water, or territory. There is a natural limit to the number of island foxes an island can support, also known as carrying capacity.

Low rainfall has impacted food resources across the Channel Islands. However, islands have not been impacted equally. Some islands appear to have greater natural drought tolerance.

As recently discovered by analyzing island-fox diet, Santa Catalina and Santa Cruz Islands have greater native plant biodiversity than the other smaller islands. Native island vegetation evolved with periods of drought. Native plants are better able to survive and produce fruit vital for the survival of island animals. The greater the native plant biodiversity, the greater the survival options for island foxes.

island fox footprints among ice plant on San Nicolas

The island-fox-diet study pointed out that island foxes on San Miguel and San Nicolas Islands were dependent on plant and invertebrate species which had been introduced to their islands, replacing native plant habitat.  Unfortunately, these introduced plant species, especially, ice plant, and the non-native creatures that live in it–snails and earwigs, have declined dramatically during the last two years of extreme drought. The impact has been greatest on San Nicolas Island where average island-fox weight has declined and the population has dropped 41% since 2012.

Restoring native vegetation is the best long term solution for healthy island fox populations on these two islands. (for more on this topic, see the Island Fox Update 2015 pdf document above)

A secondary impact of the drought has been increased complications with parasites. Changes in diet and decreased general health leave island foxes more vulnerable to internal and external parasites. Here again, each island has its own specific parasite challenges. (See Island Fox Update 2015 pdf for details) (Spiny-headed worm on San Miguel Island first detected in 2013).

Introduced disease continues to pose a threat to all island foxes, especially on islands visited by people. Dogs, cats, and introduced species, like raccoons, are all avenues for disease to be transported to isolated and disease-naive island foxes. Efforts to verify the effectiveness of a new CDV vaccine for island foxes is currently underway.

Positive Notes:

courtesy of D. Mekonnen

There are early anecdotal signs that the small amount of rain this spring and summer may be improving resources on the islands. More island fox pups have been spotted this summer than during the last two summers. However, the official impact on the populations will not be known until counting begins in fall.
  
Monitoring with radio collars continues to provide important information to land managers. Radio collars have helped verify that no golden eagles have returned to eat island foxes on the northern islands. Radio collars were instrumental in determining the cause of island-fox decline on San Nicolas Island. And they continue to provide early warning of disease introduction. Radio monitoring collars will also enable land managers to ethically determine the effectiveness of the new CDV vaccine.

Annual island fox health checks, serology testing, and counting identify threats to island fox survival early, so that actions can be taken to protect island populations.

‘Fox-Saver’ bins on Santa Catalina and the Navy's education outreach on San Clemente are all helping to reduce the number of island foxes hit and killed by cars.

Thank you to the Island Fox Conservation Working Group and all of the important work that has helped island foxes recover from the brink of extinction. Thank you also to the many participants on our June Santa Cruz Island Trip that have allowed us to show you their photographs.

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.

Climate and Island Foxes

Santa Catalina Island

Across the six islands that are home to island foxes (Habitat), pairs like Tani and Tiptu are breeding and finding an appropriate den site. Island fox pups are typically born in late April.

Like all wild animals, island fox reproduction is impacted by the amount of local resources–food, water and territory. As endangered populations of island foxes have recovered, there have been several years where female foxes have had large litters of pups. Typically two pups are born in a litter, but when resources are abundant there can be as many as five pups.

Because the recovering endangered populations were small, there was little competition for territory and food. In the mid-2000s, it was not unusual for island foxes on San Miguel to have litters of three to five pups. With large litters of offspring the San Miguel Island population, that had nearly gone extinct in 2000 when there were only 15 surviving individuals, was able to exponentially increase each year.  Graph

But severe climate can greatly reduce the resources available to island foxes. 2007 was the driest year on record in southern California since National Weather Service records began in 1878; less than four inches of rain fell. The lack of rainfall reduced the plant foods on the islands which reduced the deer mice, insects and bird life. Without winter rains, the Catalina cherry, native currants, toyon and other fruit producing plants produced less fruit for the foxes to eat. Less food, meant fewer island fox pups were born or survived. The winter seasons of 2008 - 2011 averaged approximately 12 inches of rain, the low side of normal, but still enough that Channel Island wildlife flourished.
 

This winter season is showing early signs of drought. As mid-March approaches the Los Angeles area has received less than six inches of rain. This year if spring rains do not arrive, we may see fewer island foxes born and fewer that will survive. Climate fluctuations act to moderate animal and plant populations, but climate change means greater weather extremes. Drought in 2012 could make it more difficult for young island foxes like Tani to successfully reproduce and could slow down the recovery of endangered island foxes.