An infectious disease outbreak and increased mortality in wild alpine reindeer

Abstract

Climate changes may lead to tipping points where the epidemiological characteristics of infectious wildlife diseases suddenly change, leading to outbreaks. However, empirically documented cases of the extent to which emerging infectious diseases (EIDs) affect populations of large vertebrates are few, partly because of the limited surveillance of remote wildlife populations. EIDs may potentially lead to higher mortality than mean mortality levels. Here, we quantified the likely impact of digital necrobacillosis on the survival and population dynamics of wild alpine reindeer (Rangifer tarandus) in Hardangervidda National Park (NP), Norway. The outbreak of digital necrobacillosis in 2019 contributed to a decrease in the summer survival rate of juveniles to 51% compared to an average of 88% in 2005-2018 and 77% in 2020.However, since the outbreak mainly reduced juvenile survival, the impact on short-term population dynamics was moderate, lowering the population growth rate (λ) before harvest from an average value of 1.18 (2005-2018) to 1.08 (2019) before harvest. In 2019, anomalous climate conditions with a high daily mean temperature combined with the highest recorded daily precipitation in late summer were observed. Further studies are required to document the pathological development of digital necrobacillosis across demographic groups and proximate and ultimate causes of mortality.

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

“…The population size of wild reindeer on Hardangervidda is estimated based on the four surveys in the annual monitoring of the population and is associated with some uncertainty (Mysterud et al, 2020(Mysterud et al, , 2023Nilsen & Strand, 2018). A harvesting strategy based on overestimation of the actual population size will result in an unintentionally increased risk of loss of genetic variation and population decline (Lee & Saether, 2022;Wiedenmann & Jensen, 2018).…”

“…The data from the four surveys enables us to estimate the annual population size (N), and age and sex structure just prior to the harvest (Kvalnes et al, 2024; see Supporting Information A, Figures A1-A3) using an integrated population model (Kéry & Schaub, 2012;Mysterud et al, 2020Mysterud et al, , 2023Nilsen & Strand, 2018). This is a hierarchical change-in-ratio model with parameters estimated using Bayesian inference, and accounts for the available surveys having different coverages of the population from year to year.…”

“…Increased mortality or the decimation of a subpopulation will inevitably reduce the effective population size and result in increased genetic drift (Kuparinen et al., 2016; Ryman et al., 1981; Sæther et al., 2009). However, such effects could also result from diseases (Mysterud et al., 2023). Chronic wasting disease (CWD) is a fatal prion disease affecting cervids (Haley & Hoover, 2015).…”

“…In the coming years, changes in the climate are expected to enhance the negative impacts of existing threats such as diseases and parasites to wild mammal populations (Jones et al, 2008;Mysterud et al, 2023). Thus, maintaining the genetic variation in wild populations is considered critical to ensure their long-term persistence under present and future environmental conditions.…”

“…However, two more CWD cases have since been discovered in wild reindeer, one in 2020 and another in 2022, both in the Hardangervidda population in Norway (Tranulis et al., 2021). In the coming years, changes in the climate are expected to enhance the negative impacts of existing threats such as diseases and parasites to wild mammal populations (Jones et al., 2008; Mysterud et al., 2023). Thus, maintaining the genetic variation in wild populations is considered critical to ensure their long‐term persistence under present and future environmental conditions.…”