The causes of long-term decline in the polecat population in Belarus

Population decline of carnivore species may happen suddenly, develop rather fast and proceed short-term. For instance, such demise character was known for the European mink in Belarus and Russia (Macdonald et al., 2002; Sidorovich, 2011). Population decline of other carnivore species may pass gradually and less evident, and a considerable decrease in population density may take many more years.

The situation that is characterized by gradual and long-term decline is fairly hard to notice, while manipulating short-term data only. It takes much time to establish a right hypothesis on the decline and to prove the hypothesis. Usually, in such a case, when a population decline appeared to be evident, it is already too late to get a complete dataset to analyze the declining process and to reveal the factors that impacted the population. The polecat Mustela putorius population decline in Belarus was characterized by the mentioned features of gradual and long-term decline (see Sidorovich, 2011).  Our results of the study on the polecat demise suggest the following negative factors that are responsible for the population decline.

Competition over prey with the naturalized predators

It was found that the naturalized American mink may be a strong competitor to the polecat (Sidorovich, 2011). Mean overlap by Morisita’s index (CH) of the two species diets that was calculated as a frequency of occurrences of food items in the diet (hereafter %FO) in river valley habitats was 0.83. The cold-season diet of the American mink was registered to overlap greatly with that of the polecat (CH=0.89-0.97). Competition over resources acts when the resources are limited. In valley ecosystems such a limited prey supply plausibly takes place in the harsh seasonal situations, especially during strong frost and summer drought. In these environmental conditions with limited aquatic prey, the American mink became mostly terrestrial predator shifting feeding to the polecat food niche. Actually, during strong frost both predator species appeared to be small mammal (rodents and shrew) eaters: American mink – 80.6%FO and polecat – 95.3%FO; while during summer drought they specialized in feeding on rodents and toads: American mink – 63.2%FO and polecat – 48.1%FO.

In Belarus, severe summer drought happened quite rarely – once per 4-6 years; but strong frost periods were recorded nearly every winter and quite often 3-5 times per winter. So, generally harsh environmental conditions were more or less common there. Thus, there is much support that the American mink became a strong competitor to the polecat in these harsh seasonal periods.

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To study on possible strong competition between the species over prey in the breeding season, I searched for polecat dens. It was found that in semi-natural terrain, a mixture of grassy marshes and black alder swamped woods in valleys of rivers and lakes was the main habitat type, where pregnant female polecats gave birth and then raised the litter (36 out of 37 findings). By investigating diets of polecat kits (n=357 scats analyzed and 69 prey remains identified), it was revealed a great importance of the riparian voles: the water vole – 12-83(on average 29)% biomass consumed, hereafter %BC. Also, Microtus voles were consumed frequently – 22-71 (on average 42)%BC. Among remains of Microtus voles root voles were found in 76% of cases. So, approximately root voles comprised 32%BC. On the other hand, these riparian vole (the water vole and root vole) populations were deteriorated by excessive predation of the extra predator – the American mink (Macdonald et al., 2002; Sidorovich, 2011). Thus, the decline in riparian voles, which were so beneficial for polecat breeding, may be one of the causes of the gradual disappearance of polecats because of weakened reproduction.

Also, there was revealed negative influence on the polecat population of strong competition for carrion with raccoon dogs in late winter and early spring (Sidorovich et al., 2000; Sidorovich, 2011). Carrion from wild ungulate carcasses was found the main food consumed by polecats in forest-bog mosaic during this harsh seasonal period (75.3%BC). Raccoon dogs were specialized in feeding on carrion (70.6%BC), too. Fast consuming of carrion by numerous raccoon dogs (the species population density may be up to 160 inds per 100 km2) in this harsh season could negatively affect the polecat population, especially in forest-bog mosaic.

In human settlements evidently non-limited food supply for the polecat and naturalised stone marten  (mainly rats, poultry and their eggs, young rabbits, toads and sparrows for both species; livestock food and fruits for stone martens) suggest a little support to the idea of a strong competition over food between these native and exotic species.

Aggressive interference with the naturalised predators

Radiotracking data (Sidorovich et al., 2000; Sidorovich, 2011) suggest the high plausibility of aggressive interference from male American minks towards both sexes of polecats. Such aggressive attitudes may have a detrimental effect on polecat family groups and, hence, negatively affect its reproduction.

Simultaneous radiotracking study on polecats and stone martens in human settlements and their surroundings also supported the idea of interference influence of stone marten on polecats. Again, stone marten’s aggressive behavior was mainly directed on female polecats and, hence, negatively affected the species reproduction.

Plausible impact of epidemic

Another essential threat to a rarefied population of the polecat that persisted locally was the epidemics happened in American minks after 2003 (Sidorovich, 2011). Afterwards we found that many local rarefied populations of the polecat had disappeared.

In conclusion, I would like to emphasize that the strong long-term decline observed in polecats up to the disappearance of the species in many areas in Belarus happened because of the direct and indirect impacts of the three naturalized predators, i.e. the American mink, raccoon dog and stone marten. Mostly American minks influenced negatively on polecats in valley habitats, raccoon dogs – in forest-bog mosaic, stone martens – in human settlements and man-made landscape.