Harvest and decimation affect genetic drift and the effective population size in wild reindeer

Harvesting and culling are methods used to monitor and manage wildlife diseases. An important consequence of these practices is a change in the genetic dynamics of affected populations that may threaten their long-term viability. The effective population size (N e) is a fundamental parameter for describing such changes as it determines the amount of genetic drift in a population. Here, we estimate N e of a harvested wild reindeer population in Norway. Then we use simulations to investigate the genetic consequences of management efforts for handling a recent spread of chronic wasting disease, including increased adult male harvest and population decimation. The N e/N ratio in this population was found to be 0.124 at the end of the study period, compared to 0.239 in the preceding 14 years period. The difference was caused by increased harvest rates with a high proportion of adult males (older than 2.5 years) being shot (15.2% in 2005-2018 and 44.8% in 2021). Increased harvest rates decreased N e in the simulations, but less sex biased harvest strategies had a lower negative impact. For harvest strategies that yield stable population dynamics, shifting the harvest from calves to adult males and females increased N e. Population decimation always resulted in decreased genetic variation in the population, with higher loss of heterozygosity and rare alleles with more severe decimation or longer periods of low population size. A very high proportion of males in the harvest had the most severe consequences for the loss of genetic variation. This study clearly shows how the effects of harvest strategies and changes in population size interact to determine the genetic drift of a managed population. The long-term genetic viability of wildlife populations subject to a disease will also depend on population impacts of the disease and how these interact with management actions.

Biological and environmental influences on parturition date and birth mass of a seasonal breeder

Natal features (e.g. Julian birth date and birth mass) often have fitness consequences and can be influenced by endogenous responses by the mother to seasonal fluctuations in nutritional quality and photoperiodic cues. We sought to further understand the biological and environmental factors that influence the natal features of a polytocous species in an environment with constant nutritional resources and limited seasonal variation. During a 36-year study we assessed the influence of biological factors (maternal age and litter type [i.e., litter size and sexual composition]) and environmental factors (total precipitation and mean maximum temperature during months encompassing conception, the last trimester of gestation, and the entire length of gestation) on Julian birth date and birth mass using linear-mixed effects models. Linear and quadratic functions of maternal age influenced both natal features with earliest Julian birth dates and heaviest birth masses occurring at prime-age and older individuals, which ranged from 5-9 years of age. Litter type influenced Julian birth date and birth mass. Interestingly, environmental factors affected Julian birth date and birth mass even though mothers were continuously allowed access to a high-quality diet. Random effects revealed considerable variation among mothers and years. This study demonstrates that, in long-lived polytocous species, environmental factors may have a greater influence on natal features than previously supposed and the influence from biological factors is also complex. The documented responses to environmental influences provide unique insights into how mammalian seasonal reproductive dynamics may respond to current changes in climate.

The demographic consequences of the cost of reproduction in ungulates

The cost of reproduction can generate covariation between demographic rates that can potentially influence demography and population dynamics in long-lived iteroparous species. However, there has been relatively little work linking the survival cost of reproduction and population dynamics. The apparent scarcity of information on this important link is potentially due to covariation between vital rates, which can substantially influence fluctuations in population size. In this paper we examine the opportunity for survival costs of reproduction to leave a dynamic signature using a simulation model based broadly on an ungulate life history. We find that an increase in the cost delays the onset of reproduction and reduces reproductive rates of young, but not of prime-age, females. Accordingly, the number of offspring produced declines and the interval between reproductive events increases among young females experiencing high cost. These effects are translated to an age structure skewed toward young ages and reduced population density. These results suggest that, by delaying reproduction when conditions deteriorate, females protect their survival during the critical first three years of life, after which the negative effect of reproduction on survival declines. Unless conditions for reproduction are severe, it is not profitable to delay reproduction beyond age 3 years due to the high risk of death before having a chance to reproduce. We also demonstrate that lack of adjustment of reproductive strategies to elevated levels of the cost of reproduction, for example due to rapid changes in environmental conditions, results in lower average density and longevity compared to females that have sufficient time to adjust to changes in the cost. This suggests that even moderate costs of reproduction may have a major negative effect on population dynamics of ungulates.

Annual variation in maternal age and calving date generate cohort effects in moose (Alces alces) body mass

A general feature of the demography of large ungulates is that many demographic traits are dependent on female body mass at early ages. Thus, identifying the factors affecting body mass variation can give important mechanistic understanding of demographic processes. Here we relate individual variation in autumn and winter body mass of moose calves living at low density on an island in northern Norway to characteristics of their mother, and examine how these relationships are affected by annual variation in population density and climate. Body mass increased with increasing age of their mother, was lower for calves born late in the spring, decreased with litter size and was larger for males than for female calves. No residual effects of variation in density and climate were present after controlling for annual variation in mother age and calving date. The annual variation in adult female age structure and calving date explained a large part (71-75%) of the temporal variation in calf body mass. These results support the hypotheses that (a) body mass of moose calves are affected by qualities associated with mother age (e.g. body condition, calving date); and (b) populations living at low densities are partly buffered against temporal fluctuations in the environment.