29.01.2018 - 16:04

A new population estimate for The Arctic fox in Iceland

Figure 1. The estimated minimum population size of the Icelandic Arctic fox during autumn. The vertical lines show 95% confidential limits, which are larger in the last years, due to the high proportion of cohorts still unknown. The population estimate was conducted by Professor Páll Hersteinsson from 1979-2007 but after that by his successor, Ester Rut Unnsteinsdóttir. The new estimates are based on data from 2003-2015, as 12 years is the maximum life span of the species in Iceland.
Figure 1. The estimated minimum population size of the Icelandic Arctic fox during autumn. The vertical lines show 95% confidential limits, which are larger in the last years, due to the high proportion of cohorts still unknown. The population estimate was conducted by Professor Páll Hersteinsson from 1979-2007 but after that by his successor, Ester Rut Unnsteinsdóttir. The new estimates are based on data from 2003-2015, as 12 years is the maximum life span of the species in Iceland.
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An estimate of the population size of the native Arctic fox is propublished every 3-4 years, as a part of a long-term monitoring program that is based on the collaboration between scientists and foxhunters from across Iceland. The latter voluntarily send fox carcasses for measurements and age analysis. The population size is estimated by a method called age cohort analysis that allows for estimates on the proportion of each generation that is alive each year. As the method is based on past knowledge, it is not possible to estimate the population size reliably in present time and therefore the most recent estimate goes back 3-5 years. The cohort analysis is well suited for modelling the Icelandic Arctic fox population, as the hunting pressuse is relatively stable and a reliable sample is available for measuring age, fertility and body condition each year.

The last population estimate showed that the population had grown tenfold from 1980-2008 (E. Unnsteinsdóttir 2014) but then declined by 30% from 2008-2010. This was the first decline recorded since monitoring of the species started.. The estimateintroduced here, further suggests that the population continued to decline until 2012, in total by 40%. The population then remained stable around an average 6.500 individuals,whereas the population is now estimated to have been 7.000 individuals in the autumn of 2015. The confidential limits are large in the most recent years, as the unknown proportion of the population is still high.

Several attempts have been made to identify the factors behind population limitation and regulation of the Icelandic Arctic fox. Recent papers dealing with these questions show that the positive population growth rate from 1980 was best explained with positive growth in geese and wader populations following a period of mild climate (S. Pálsson et al. 2015). Furthermore, the expansion of the Arctic fox population was not explained by increase in fertility (litter size) as is the case in many other areas of the species´ range. An increased proportion of mature individuals taking part in breeding explained, in large parts, the tenfold increase in the population. This was possible by increase in carrying capacity through favourable conditions in weather and resources (E.R. Unnsteinsdóttir et al. 2016).

Monitoring of Arctic foxes has taken place on the Hornstrandir nature reserve since 1998, including monitoring of known denning sites for occupancy and cub survival. This area is the most important sanctuary for the native population in Iceland as the Arctic foxes have been protected there since 1995. Comparison of data from old foxhunting statistics and monitoring show that the population in Hornstrandir grew simultaneously with populations in other parts of the countryafter 1980. After it gained  protection status in 1995, the Arctic fox population in Hornstrandir remained considerably stable in regards to den occupancy, but fluctuations have occurred in mortality rate. There was a severe collapse in the summer of 2014 with many foxes found dead in the spring and few pairs breeding successfully.

The reasons behind this unexplained crash in Hornstrandir nature reserve in 2014, as well as the recent decline nationwide, are unknown.  No fatal diseases are known to occur in the Icelandic Arctic fox population but a severe mercury pollution has been found in Arctic foxes living in coastal habitats of Westfjords (Bocharova et al. 2013; G. Treu et al. 2018). This makes it even more important to continue monitoring the population health of the Icelandic Arctic fox. After all, it is the only native terrestrial species and an important apex predator in Icelandic ecosystems.

The population monitoring of the Icelandic Arctic fox began in 1979 when Prof. Páll Hersteinsson (1951-2011), then a PhD student at OxfordUniversity, asked foxhunters to collaborate with him by sending him the lower jaw of every fox they killed. With the insend jaws, the hunters gave a detailed report of the location and date of the kill, sex and social status (breeding or non-breeding). By extracting the canine from the jaw, Páll could age each individual by counting annuli in the root of the teeth. Páll studied the Arctic fox and other mammals all his scientific life until he sadly passed away in October 2011.  Ester Rut Unnsteinsdóttir, Páll´s former student, has since continued his long-term study of the Icelandic Arctic fox population. First as the founder and director of the Arctic Fox Centre and, since 2013, as a mammologist at the Icelandic Institute of National History.
The Environmental agency of Iceland (UST) is responsible for wildlife management but the procedure of the foxhunting mostly takes place on the behalf of local municipalities in each region.

For further information you are welcome to contact Dr. Ester Rut Unnsteinsdóttir, Chairman of The Arctic Fox Centre and Mammal ecologist at The Icelandic Institute of Nature History

Phone: +354 5900 500, E-mail: ester@ni.is

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