Biology subjects

Willebrand, T.

Publications and source records attributed to Willebrand, T..

5 recordsLinked to original sources

bioRxiv2026

Evaluating threshold management for willow grouse harvest: tracking open and closed areas during 12 years.

Managers must make decisions in the face of uncertainty, especially when available resources are limiting. Identifying thresholds when certain conditions are met or exceeded enable the potential to mitigate risks. In 2005, sustainable harvest levels of willow ptarmigan were identified to avoid harvest efforts exceeding three hunter days km2. Here we evaluate these recommendations by analyzing line transect counts and harvest data from six areas forming three open/closed pairs in a region of state managed willow ptarmigan harvest. We developed three sets of Bayesian hierarchical models, one static distance model, and two dynamics models. One mechanistic hazard model and a Gompertz phenomenological model. Adult and juvenile density showed pronounced year-to-year variation that was largely synchronous across all six sites regardless of hunting status. The harvest effort parameter shows a striking difference between the two models. In the Hazard model, is positive, and excludes zero with near certainty, but in the Gompertz model, the parameter is highly uncertain. However, the two models do not contradict each other but answer complementary questions with different sensitivity to the harvest signal, harvest mortality is additive at the individual level, but this additive mortality is masked at the level of population abundance. The demographic cost of harvest is therefore real and quantifiable through the survival chain, but bounded in the long run by the stabilizing dynamics. A fixed limit anchored to monitored effort and bag is not a crude substitute for adaptive management but the appropriate design under the information commonly at hand. It will be a precautionary instrument grounded in the one relationship this study establishes firmly, the translation of hunter effort into harvest mortality.

ecology↗

bioRxiv2026

Mortality causes and annual survival rates of red foxes (Vulpes vulpes).

Mortality causes and survival rate estimates from individual follow-ups of GPS-collared animals provide reliable demographic data, but such estimates are rare for red foxes Vulpes vulpes. We used data from 126 GPS-collared red foxes tracked between 2011 and 2019 across a latitudinal gradient in central Sweden and Norway to quantify mortality causes and estimate annual survival probabilities using the Andersen-Gill extension of the Cox proportional hazards model. Hunting was the dominant mortality cause (63%), followed by vehicle collisions (14%), stress or malnutrition (11%), sarcoptic mange (9%), and predation (3%). Annual survival was 0.58 for adults and 0.32 for subadults. Subadults had approximately twice the hazard of adults, males had elevated hazard relative to females, and mortality risk was highest in autumn and early winter. These collar-based estimates are strikingly similar to mark-recovery estimates from the same region nearly five decades earlier, despite an intervening population collapse from sarcoptic mange, subsequent full recovery, and a major decline in harvest pressure. This convergence is consistent with density-dependent regulation at a food-determined and food-limited carrying capacity, suggesting that current harvest levels are not limiting the population. Furthermore, human-caused mortality was approximately four times greater than natural mortality, illustrating that living in close proximity to humans -- which often favours generalist predators such as red foxes -- may also come at a cost of increased mortality risk. Developing validated census methods for red foxes in boreal Scandinavia is identified as a key priority for quantitative population management.

ecology↗

bioRxiv2026

Latitudinal effects on weight and age-dependent survival of red fox Vulpes vulpes

Age-dependent survival is central to understanding population dynamics and life-history evolution. We analysed carcass weight and age-at-harvest data from 6 022 red foxes (Vulpes vulpes) collected across Sweden between 1967 and 1971 to evaluate latitudinal effects on body mass and age-dependent survival. Carcass weights decreased from south to north in both adults and sub-adults, contrary to Bergmanns rule, with southern foxes weighing approximately 1.27 times more than northern foxes. The latitudinal weight gradient exceeded the sex difference in both age classes, and no sex x region interaction was detected. The decrease in weight with latitude is consistent with reduced prey availability and harsher winter conditions in the north, which limit growth and body size during development. Using a Bayesian age-at-harvest model with region-specific population growth rates ({lambda}), we estimated age-dependent survival probabilities for four latitudinal regions and both sexes. Despite the strong latitudinal gradient in weight, survival did not show a corresponding pattern -- regional differences were uncertain, with all credible intervals spanning zero. Regional population growth rates were consistent with slight decline in the north and near-stability in the south-central region, which suggests that body condition and population dynamics are coupled at the regional scale despite no survival gradient. The decoupling of body condition and survival across regions suggests that mortality patterns are similar across the latitudinal gradient. We discuss these patterns in terms of latitudinal productivity gradients, prey availability, and life-history trade-offs in a widely distributed carnivore.

ecology↗

bioRxiv2026

Forty-four years of change: Capercaillie and Black grouse responses to changes in forestry, climate and vole decline in icentral Sweden

Human land use has greatly affected the natural landscapes in most parts of the world, and Scandinavian industrial forestry has substantially altered the age structure of the boreal forest over the last 75 years. In this study, we analyzed 44 years of line transect counts of Capercaillie and Black grouse collected in an industrial forest landscape in central Sweden between 1980 and 2024. The area was heavily logged in the 1960s and 1970s, followed by large-scale replanting. A hierarchical Gompertz state-space model to assess long-term population dynamics of both species. The adult state model included a brood production submodel, and forest age structure, snow depth, vole abundance, and spring frost were covariates. Our results reveal strong density dependence and temporally variable environmental effects. Capercaillie and Black grouse exhibited contrasting decadal trends: Capercaillie increased markedly during the early study years, whereas Black grouse declined steeply before recovering later in the series. Increasing proportions of forest aged 21-40 years had a clearly positive effect on Capercaillie, whereas snow depth in the previous winter negatively influenced Capercaillie but not Black grouse. Brood production exhibited substantial interannual unexplained variation, although late spring frost reduced brood size in Capercaillie. Toward the end of the study, the two species reached similar levels of latent adult abundance, demonstrating that both can persist at sustainable densities across a broader range of forest structures than suggested in earlier studies.

ecology↗

bioRxiv2023

Adding density-independent breeding success reduce process error in a state space model of the population dynamics of Willow grouse (Lagopus lagopus)

Willow ptarmigan Lagopus lagopus is abundant in Arctic and tundra regions, but rapid climate warming has raised concerns about possible declines as has been observed in several bird species. In this study, we used a hierarchical state-space model to analyze data from a 139 km line transects in mid-Sweden over 48 years. Adult numbers and breeding success were analyzed separately, and we included covariates on vole abundance, numbers of snow-free days in autumn and spring, and the last day of frost in May. We assessed long-term trends in the adult population and estimated the effects of breeding success and weather variation on short-term changes. The estimates of adult density did not show any trend for the period 1976 to 2023, and the dynamics were characterized by a strong direct negative density dependence indicating a stationary process. A number of possible mechanisms have been suggested for how a warmer climate affects willow ptarmigan population dynamics, but our results do not support the hypothesis that lack of snow in autumn and spring increases the vulnerability of willow ptarmigan to predation and leads to population decline. Breeding success is an important driver of changes in, but independent of, adult density. In addition to predation, we propose that climate conditions and emerging vegetation during egg formation and laying is important. We suggest that our results can be explained by a diverse predator assemblage that makes it difficult for the population to escape top-down control, resulting in short-term fluctuations at lower densities.

ecology↗