By Roberta Kwok
8 May 2013
(The Guardian) – In the 1970s, a population of Arctic foxes on an island in the Bering Sea began to mysteriously decline. The animals were thin and mangy, and nearly all the cubs died. Today, only about 100 foxes remain.
The animals were not felled by an infectious disease, a PLOS ONE study suggests. Instead, the foxes probably suffered from high mercury exposure as a result of eating seabirds and other marine animals.
The researchers studied Arctic fox fur samples from four sources: Mednyi Island, where the population crashed; museum specimens of foxes from the Commander Islands; and two populations in Iceland. Three of the groups, including the Mednyi Island population, ate marine animals, while the fourth group mostly preyed on land animals such as mice.
Mercury levels in foxes with marine diets were almost three times higher than in foxes with an inland diet, the team found. The Mednyi Island foxes rely exclusively on marine animals for food, whereas the other two coastal groups eat land animals as well.
The team also tested the Mednyi Island foxes' blood, skin, and feces for signs of dangerous infections. But most tests came back negative, suggesting that pathogens weren't responsible for the population's collapse.
ABSTRACT: Changes in concentration of pollutants and pathogen distribution can vary among ecotypes (e.g. marine versus terrestrial food resources). This may have important implications for the animals that reside within them. We examined 1) canid pathogen presence in an endangered arctic fox (Vulpes lagopus) population and 2) relative total mercury (THg) level as a function of ecotype (‘coastal’ or ‘inland’) for arctic foxes to test whether the presence of pathogens or heavy metal concentration correlate with population health. The Bering Sea populations on Bering and Mednyi Islands were compared to Icelandic arctic fox populations with respect to inland and coastal ecotypes. Serological and DNA based pathogen screening techniques were used to examine arctic foxes for pathogens. THg was measured by atomic absorption spectrometry from hair samples of historical and modern collected arctic foxes and samples from their prey species (hair and internal organs). Presence of pathogens did not correlate with population decline from Mednyi Island. However, THg concentration correlated strongly with ecotype and was reflected in the THg concentrations detected in available food sources in each ecotype. The highest concentration of THg was found in ecotypes where foxes depended on marine vertebrates for food. Exclusively inland ecotypes had low THg concentrations. The results suggest that absolute exposure to heavy metals may be less important than the feeding ecology and feeding opportunities of top predators such as arctic foxes which may in turn influence population health and stability. A higher risk to wildlife of heavy metal exposure correlates with feeding strategies that rely primarily on a marine based diet.