By: Åshild Ønvik Pedersen // Norwegian Polar Institute and Brage Bremset Hansen // Norwegian University of Science and Technology
The Svalbard reindeer (Rangifer tarandus platyrhynchus) is a key herbivore in the Svalbard tundra ecosystem. After years of monitoring the size, and the sex and age structure of reindeer populations in coastal and inland regions we now have the longest time-series on Svalbard tundra species (www.mosj.no; www.coat.no). Combined with local weather records, these data allow us to investigate the effects of climate change in regions with contrasting weather variability and ecological characteristics.
Climate regime shifts
Rapid climate change has taken place since the monitoring of the Svalbard reindeer started four decades ago (1978/79). During this period, the reindeer have lived through both periods of cold, stable winters in the 1980s, and more recent periods when the winters have often been mild and rainy. In addition, the reindeer have experienced considerable summer warming. These climate trends have accelerated in the current century.
Rainy winter weather often leads to formation of ground ice, encapsulating food plants in extensive ice sheets, which restrict forage access and result in increased mortality and reproductive failures. Recently, mild and rainy winters – and hence ice-locked pastures – have become the norm rather than the exception, causing a linked climate–cryosphere regime shift due to rapid winter warming around the turn of the century.
Warmer summers change the characteristics of the growing season and “greenness” of plants, and scientists use July temperatures to delineate bioclimatic subzones in the Arctic.
Changes in mean July temperature in Svalbard indicate that, climatically, parts of the Svalbard tundra have shifted an entire bioclimatic subzone, with implications for plant productivity (Figure 1). Given continued summer warming and sufficient moisture and nutrients, food for plant-eaters will likely become more abundant. In addition, if spring snowmelt continues to start earlier and the first snow of autumn falls later, the longer snow-free season will increase the overall carrying capacity of the Svalbard tundra for grazing reindeer.
After humans discovered Svalbard in the late 16th century, over-harvest gradually exterminated the Svalbard reindeer in parts of the archipelago. In 1925, the population was very small (up to a thousand) and the reindeer was protected by law. About 60 years later, scientists estimated the population to be around 11 000 individuals. Recently, a study based on extensive field sampling estimated 22 000 reindeer in Svalbard. The apparent doubling strongly indicates an overall positive population trend.
However, annual monitoring suggests that population trends vary locally. For instance, in two of the core reindeer monitoring regions (Brøggerhalvøya and Adventdalen), the populations show highly contrasting trends. After the re-introduction of 15 reindeer to Brøggerhalvøya in 1978, the population grew fast, numbering 360 individuals during the winter census of 1993. The population crashed to ~80 reindeer in the following winter due to a combination of heavy rain in early winter, overgrazed pastures, and very high population density (i.e. strong competition for food). The population has since fluctuated around low densities (~100 individuals), at least partly due to frequent rainy and icy winters. In contrast, the population in Adventdalen, which has a more “inland” climate regime, has increased more than three-fold since the census started in 1979 (459 reindeer), with a record-high number in 2018 (1701 reindeer).
Weather, climate, and population trends
Can climate and weather have the same effects on these two populations on an annual basis, while simultaneously causing long-term population trends to diverge? Annual fluctuations in winter rain essentially match the fluctuations in reindeer survival, reproduction, and population growth rates across Svalbard. The annual growth rates of the reindeer populations in Adventdalen and Brøggerhalvøya also correlate with each other.
However, some small but important differences in how climate trends affect these two populations are evident in figure 2. First, the increase in the annual amount of “rain-on-snow” (ROS) was stronger in Ny-Ålesund (Brøggerhalvøya) than at Svalbard Airport (Adventdalen). Second, summer temperature has increased over time at both stations, but the increase was greater at Svalbard Airport (ca 1.3°C versus 0.9°C).
Rainier and icier winters slow reindeer population growth over time by reducing survival and reproduction. Warmer summers have a positive effect on population growth most likely because of increased food abundance and extended grazing seasons. The slight differences in climate trends in our two monitoring sites may partly explain the local differences in population trends. In other words, in Adventdalen, the positive effect of climate change in summer overrides the negative effect in winter, whereas the opposite seems to be the case in Brøggerhalvøya.
Thus, local differences in the impact of seasonal climate change may translate to local differences in how well the reindeer population does. In some places, there will be losers, but in other places, there will be winners. Seen overall, the patchy nature of climate change impacts may help ensure long-term survival of the Svalbard reindeer.
Figure 2 (right). Time-series data (1978-2015) and population trends in the two of the core populations for long-term monitoring – Adventdalen (red) and Brøggerhalvøya (blue). Annual fluctuations in (a) rain-on-snow (ROS), (b) summer temperatures and (c) standardised reindeer population sizes, and the estimates of (d) population size trends (with varying trend-starting year; whiskers showing 95% CI). Meteorological data are obtained from the Ny-Ålesund weather station (blue) and Svalbard Airport weather station (red). Horizontal dashed lines in (a) and (b) denote detected regime shifts, i.e. change points in mean. The solid line in (c) shows the population size trends for the time-period after the irruptive population phase in Brøggerhalvøya (figure from Hansen et al 2019).
Svalbard reindeer status report
This report summarises research on the Svalbard reindeer and outlines important knowledge gaps. The recent extensive and diverse scientific activity now allows a much better understanding than previously of how Svalbard reindeer have adapted to, and interact with, their High Arctic environment. This knowledge is crucial to understand how the species and ecosystem respond to the large environmental changes associated with climate change. The report summarises knowledge from scientific papers, reports, theses, books and anecdotes, mainly from the early 1970s up to the present.
Hansen BB, Pedersen ÅØ, Peeters,B, et al. 2019. Spatial heterogeneity in climate change decouples the long-term dynamics of wild reindeer populations in the high Arctic. Global Change Biology https://doi.org/10.1111/gcb.14761
Le Moullec M, Pedersen ÅØ, Stien A, (2019) A century of recovery from overharvest in a warming High-Arctic: the successful conservation story of endemic Svalbard reindeer J Wildlife Manage 83:1676-1686 DOI: 10.1002/jwmg.21761
Peeters B, Pedersen ÅØ, Loe LE, et al. (2019) Spatiotemporal patterns of rain-on-snow and basal ice in high Arctic Svalbard: detection of a climate-cryosphere regime shift. Environ Res Lett 14
Vickers H, Hogda KA, Solbo S, et al (2016) Changes in greening in the high Arctic: insights from a 30 year AVHRR max NDVI dataset for Svalbard Environ Res Lett 11