Spatial dynamics in the Scandinavian wolf population

Animal populations commonly show changes in individual behaviour and demography as they change in size and density. For wolves, this process is strongly affected by their territorial behaviour, making them defending exclusive areas (territories) towards conspecifics. This means that wolf distribution may be better defined by a presence-absence of territories rather than a continuously changing distribution of individuals. This territorial behaviour also has important consequences for the conservation and management of wolf populations because culling or other mortality of adult territorial individuals may result in complex spatial dynamics similar to a chess-board of areas successively changing between being vacant or occupied.The Scandinavian wolf population has been intensively monitored in terms of size and distribution of wolf territories since the early 1980’s. The monitoring has been based on extensive tracking and registration of scent-marking individuals on snow and later combined with DNA-analyses of non-invasive samples (scats, urine) collected during the monitoring season. This monitoring procedure has resulted in an almost complete pedigree of the wolf population, where the individual genetic profiles of reproducing individuals have been registered and linked to a specific geographic area (wolf territory) each year. This allowed a detailed analysis of dispersal distances and relatedness among breeding individuals in established wolf territories during 1999-2020. In addition, the identity of parents of non-breeding individuals that died during the study period was also determined, which allowed an analysis of the location of mortality in relation to their natal territory and to population demographic characteristics, such as population size and local density.The objective of this report was to answer four specific research questions formulated by the Swedish Environmental Protection Agency, by investigating data on population development combined with information at the individual level as received from the extensive wolf monitoring program in Scandinavia. First, we investigated the origin and natal dispersal of all territorial wolves in the population from 1999 to 2020. The second concerned how fast territories were re-occupied after being terminated by culling or due to other reasons, and what factors that may affect this. Third, we examined the probability of re-occupation of terminated territories by dispersing individuals from neighbouring versus non-neighbouring territories. Finally, we examined how wolf population size and density affected the proportion of young dispersing wolves to survive and establish a territory within the main wolf distribution area.During the study period we recorded territory establishment of 468 different wolf pairs of which 259 (56%) cases showed both partners to have a distant origin, 185 (39%) cases with one of the partners being local (widow/widower or offspring from the same territory), and 24 (5%) cases where both adults were locally recruited, i.e. with different forms of incestuous mating. It was more common for female offspring to take over the parental territory together with a new male (10%) as compared to a male offspring taking over together with a new female (3%). Excluding locally recruited territorial wolves (widow/widowers and offspring), average dispersal distance from natal territory to established new territory was 131 (10-553, min-max) km for males and 90 (6-424 min-max) km for females. Dispersal distances from the natal territory decreased with an increase in local density of breeding territories for both sexes, but the percentage of the total variation in individual dispersal distances explained by local density was low (During 1999-2017 a total of 159 wolf territories were terminated and of these, 21 (13%) had not been re-occupied by 2020. Of the remaining 138 territories that were re-occupied, the number of years from termination to re-occupation was on average 1.1 years and ranged from 0 to 11 years. The average number of years from termination to re-occupation decreased with both time and total population size ranging between one and three years during the 2002-2009 period but was reduced to less than one year for the 2015-2017 period. Territories with packs was on average re-occupied 1,2 years faster than territories inhabited by a territorial pair. The cause of termination of territories also showed a tendency to affect the time to re-occupation, with faster re-occupation after legal culling of both individuals as compared to territories where both individuals disappeared for unknown reasons.In 133 wolf territories located within the main wolf distribution area, both adult breeding wolves disappeared during the same year and the territory was therefore terminated. Of these, 37 (28%) were later re-occupied with at least one individual from a neighbouring territory. Eighty-three (62%) were later re-occupied by individuals from a non-neighbouring territory, whereas thirteen (10%) were never re-occupied during the study period. For 25 of the wolf territories, culling of both breeding adults was the reason for the termination of the territory and of these all were re-occupied during the study period. Territories that were terminated by culling or other known causes, except known illegal killing, had a higher (41%) chance to be re-occupied by a neighbour as compared to territories that were terminated due to the disappearance of both adults for unknown reason (23%).Excluding wolves that died from license culling, the total number of recorded dead wolves in Scandinavia during the study period was 634. On average, the number of dead wolves recorded per year corresponded to 11% of the estimated annual population size. There was no trend over time in the proportion of wolves that died relative to population size. The proportion wolves registered dead outside the main wolf distribution area increased in the population both over time and with total population size from approximately 1% to 5%. In contrast, the proportion of dead wolves found within the main wolf distribution area decreased both with time and population size from approximately 10% to 5%. During the same period, 718 individuals succeeded to establish as a territorial wolf, which was equal to an annual average of 13% of the total population size in the previous year, and this proportion did not change significantly over time, nor with increased population size. However, the ratio “dead wolves found outside the wolf area/new wolves establishing territories inside the wolf area” increased significantly with population size, which could indicate that the population is approaching saturation within its present distribution area.This study shows that, similar to other wolf populations, dispersal of young wolves in the Scandinavian population is extensive with large variation in distances, and that the population now has reached a size where most terminated territories in the main wolf distribution area rapidly are re-occupied. This provides conditions for high rates of population growth or resilience of population size in years with high rates of mortality. Actively terminating territories, i.e. through directed culling, to remove “problem wolves” will in most cases only have a temporary effect, as new occupants will rapidly re-colonize the area. There are indications that the wolf population now is approaching saturation with an increasing proportion of young wolves dispersing out of the main distribution area despite that a large portion of the current main wolf distribution area is still un-occupied by wolves. Counter to this, young dispersing wolves seem to have the same probability for establishing a territory within the main distribution area during more recent years.