High survival rates explain 20 years of rapid expansion of wolves in Germany

Young wolves in Germany had an annual survival probability of 75 percent during the first two decades since the beginning of the recolonisation of Germany; for adult wolves it was as high as 88 percent. For young wolves up to the age of two years, the variation in survival rate depended on the suitability of the habitat - the less suitable it was for wolves, the lower their survival; for adult wolves, such a relationship could not be established by the scientific team led by the Department of Ecological Dynamics at the Leibniz-IZW. "Using survival analysis, we were able to determine the median survival time of a wolf in Germany to be 146 weeks, which is around three years," says Prof Dr Stephanie Kramer-Schadt, head of the Leibniz-IZW department and professor at the Technische Universität Berlin. The highest recorded longevity of a wolf in the study dataset was almost 13 years.

"The survival rates of the German wolf population were very high compared to other regions, in fact they were among the highest in the world," Kramer-Schadt continued. "This indicates that the wolves settled in habitats that were very suitable for them during the 20 years which we analysed in this paper. Strict legal protection has also contributed to high survival rates." Landscapes that qualify as suitable are those that offer sufficient cover -- for example by forest cover -- and areas that are as far away from roads as possible. These areas allow wolves to avoid humans and can serve as refuge for the animals. If wolves settle in less suitable habitats, this reduces their survival and reproduction. "While adult animals can still survive and establish territories in less suitable areas, the lower survival rate of young and sub-adult wolves and the lower number of young per litter slows down population growth and thus the expansion of the species." As soon as the optimal areas are occupied, the growth of the population will slow down, according to the scientists.

The scientific investigation was based on a long-term dataset provided by the Federal Documentation and Consultation Centre on Wolves in Germany (DBBW) and included the surveys and findings of the comprehensive wolf monitoring in the German Federal States. It furthermore considered the results from tens of thousands of DNA analyses by the Senckenberg Center for Wildlife Genetics, which made this scientific investigation possible in the first place. The team also determined the reproductive performance of a total of 201 breeding females from 165 territories which were part of the analysed long-term dataset from 2000 to 2020. "We were able to analyse data from those female wolves over the years, that had offspring -- at on average for 2.8 years," says IZW scientist and first author of the paper Dr Aimara Planillo. "The analyses also show higher reproductive success of females with more experience and in more suitable habitats, with up to five reproductive years." Summarising, the models show that a well-suited habitat and the reproduction experience of the female have a positive effect also on litter size, which in Germany averages at least four offspring.

Similar scientific analyses in other countries and regions illustrate how high the survival rate for adult wolves in Germany actually is. Other non-hunted populations also have high adult wolf survival rates of 78 per cent in the USA or 82 per cent in the Alpine regions in central Europe, although these do not come close to the 88 per cent in Germany. The reason for this is that the German population is still expanding.

The Department of Ecological Dynamics at the Leibniz-IZW investigates (among other things) the population development of large carnivores such as the wolf or Eurasian lynx using individual-based, spatial models, ranging from analysing past processes -- such as the recolonisation of Germany by the wolf -- to predicting future developments. For the paper recently published in "Wildlife Biology," the scientists used demographic data on the wolf population (age, sex, year and place of birth, dates and places of resighting as well as cause of death) from the years 2000 to 2020 and related them to environmental variables such as habitat suitability (accounts for the different land use types and human disturbance effects, e.g. forested areas, distance to roads or human density), wolf population density (annual density of wolf territories around the focal territory) and the season of the year.

A significant proportion of the data comes from the ongoing molecular genetic analysis of samples collected in the field, which are sent to Senckenberg as part of the wolf monitoring programme of the German Federal States. Based on the genetic profiles created, numerous wolf individuals are collected several times over the years and can be assigned to wolf packs by analysing their relatedness. The Leibniz-IZW team then developed spatio-statistical models to determine the influence of environmental variables on the probability of survival of the population, the annual survival rates of different age classes, the probability of reproduction and reproductive performance. This revealed how well a variable -- such as high habitat quality -- can predict each population parameter and how substantial its influence is on the number and spatial distribution of wolves in Germany.

Previous scientific investigations by the Leibniz-IZW showed that

• the recolonisation of Germany by wolves is not a homogeneous, continuous process, but characterised by changing conditions. This means, for example, that wolves show different behaviours during different phases with regard to the suitability of habitats: In early phases, wolves liked "cherry-picking" when establishing new territories, while they were much less selective during later phases when the population was close to habitat saturation. The lower survival rate of young animals identified in the current paper and the lower number of offspring in less suitable areas provide an explanation for these preferences of wolves.
• the wolf population in Germany is essentially healthy and that human-caused deaths such as traffic accidents and illegal killings are responsible for the overwhelming majority of wolves found dead. Data from the 1,000 wolves dissected at the Leibniz-IZW in Germany show that around three quarters of dead wolves die in traffic collisions -- mostly with cars on country roads or motorways. In 13.5 per cent of all wolves examined, evidence of a criminal offence such as illegal gun wounds was found, although the animals did not always die as a result.

The scientific investigation was funded by the Federal Agency for Nature Conservation (BfN) with funds from the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) under grant number 3521 83 1300.