Rapid decline in the local population of lynx in Naliboki Forest, NW Belarus: density-dependent regulation or disease?

First, briefly about history of lynxes in Naliboki Forest during several last decades. In the early and mid-1990’s, after the Soviet Union crash, perhaps in conditions of relatively weak nature protection, the majority of lynxes were poached in Naliboki Forest. In the 1980’s there was a dense lynx population in the terrain, but by the late 1990’s lynxes occurred sporadically there. In the early 2000’s lynxes began recolonizing Naliboki Forest. The severe snow conditions in the late winter and early spring of 2013 seemed to impact lynxes negatively, and the local lynx population number dropped from 35 to 22 the next winter. Indeed, during the spring of 2013, as far as we learned, local forestry workers and antler searchers found at least three lynx carcasses. All of them seemed to be subadults or kittens, because they looked relatively small.

Anyway, afterwards the local lynx population grew further and in 2017-2019 the population number probably reached its maximal potential level. In the winter 2017-2018 we (Naust Eco Station and Wild Naliboki team) censused 84 individuals and the population density ranged between the localities as 2 to 6, averaged 4.4 lynxes per 100 km². In the last winter 2018-2019 there were about 100 lynxes in Naliboki Forest, and the population density ranged between the localities as 2 to 8, averaged 5.3 lynxes per 100 km². It looks like it was the highest density of the Eurasian lynx that was registered in Europe, at least (Sidorovich et el., 2018).

We counted lynxes by registering their distinctive track trails on snow cover and using about 70 camera-traps year-round that were put in known lynx spots within the census area of 800-1100 km² (see Sidorovich et al., 2018 for some details). By applying camera-traps we recognized individual lynxes with their spot patterns, too. All recognizable lynxes in the model study area of 800-1100 km² were given names to simplify our dealing with them.

Therefore we were looking forward to getting to know what would happen further with so dense lynx population in Naliboki Forest. This forest massif is not an isolated one (see Sidorovich, 2016 for the details), so, there are sufficient possibilities for lynxes to emigrate in the neighboring forests, where there are a few lynxes. On the other hand, prey supply for lynxes in Naliboki Forest is evidently sufficient and moreover markedly increases each year with growing of deer numbers.

One of the predicted scenarios could be a regular emigrating of lynxes and maintaining of the local population density of lynxes on the level 4-6 inds per 100 km². Another possible scenario could be a density-dependent decline in the reproduction (fewer adult females breed and getting small litter size). The third predictable way could be a whole population decline i.e. reproduction decrease as well as increased mortality in adults and subadults.

In summer 2019 the third scenario looked like to happen. It is merely a hypothesis that looks like the most plausible for the moment. You see the lynx distribution map in Naliboki Forest in January-February of 2019 (for the model area only).

Now you see the similar distribution map of lynxes in Naliboki Forest in December of 2019 and January 2020. It is not complete and so precise as one for January-February 2019. Anyway it is signaling a lot about the happened decline in lynxes during the summer of 2019.

It looks like that adult males S’tsiapan, Ksavery, Maxime, Bazyl’ and Kazimir, Jan and other adult male without name (7 out of 13 registered adult males) as well as adult female Jadz’viha (one out of 11) had disappeared. So, in adults mainly adult males died from the cause, we do not know. Six adult females are present without kittens; they are Bazylikha, Malanka, Jaryna, Pielahieja, Maximiliana and Pryhazhunja i.e. 6 out of 11 registered adult females. During the last winter all of them were registered that they mated; Bazylikha and Maximiliana were with sucked nipples in June. So, it is more a half of adult females lost kittens during the last summer or did not breed this biological year. Here it is worthwhile noticing that in Naliboki Forest normally only 14% of known adult females were registered without kittens in winters before (that is the multiannual average).

Furthermore, in December 2019 and January 2020 in Naliboki Forest four adult female lynxes were registered with one kitten only and only one adult female lynx was with two kittens. Before in Naliboki Forest we usually found adult female lynxes with 2 or 3 kittens mainly, and few times with 4 kittens and markedly less often with one kitten. Of course, in each third case there could be not only kittens of the biological year but joined subadult as we documented in Naliboki Forest. But anyway we are within the comparable situations, because the same (i.e. joining a subadult) could happen this winter, too.

Definitely there was no hunting on lynxes, nobody found any dead lynx (it is not easy of course), prey supply increased evidently. I do not think that the described features of the decline in lynxes in Naliboki Forest was a density-dependent regulation in the conditions of too high density of lynxes (too many prey and open area to emigrate); perhaps, that was a disease impact, but which one? This disease mainly affect adult males and kittens. It seems to be impossible to find out the disease that has happened. Definitely, that was not a rabies. It is essential to notice that the same factor seemingly affected breeder wolves – again a half of them had disappeared during the last summer in Naliboki Forest.

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