Frank, S. C., Blaalid, R., Mayer, M., Zedrosser, A., & Steyaert, S. M. J. G. (2020). Fear the reaper: ungulate carcasses may generate an ephemeral landscape of fear for rodents. Royal Society Open Science, 7(6), 191644. https://doi.org/10.1098/rsos.191644
On the 26th August 2016, as storm clouds gather above the alpine plateau of Hardangervidda in southcentral Norway, a herd of wild tundra reindeer grouped together for protection. A split second later, in a moment of miserable luck, the herd fell to the ground dead, having been struck by a bolt of lightning. Norwegian ecologists took this once-in-a-lifetime opportunity to study how the mass die-off of 323 reindeer has since impacted the local ecology and food web.
Reservoirs of nutrients
Through a lifetime of grazing, large animals accumulate important minerals such as phosphorus, calcium and sodium in their bodies. This can lead to nutrient concentrations in their flesh and bones of over 10,000 times higher than in the nearby soils and plants. Consequently, when an animal, such as a reindeer dies, they provide a sudden pulse of nutrients into the local landscape, creating islands of enriched fertility. These fertile islands in turn entice all manner of creatures, as the food web descends upon the dead.
A banquet for all?
In a recent study, Shane Frank and colleagues took a once-in-a-lifetime opportunity to study how the mass die off of 323 reindeer in southcentral Norway created a landscape of life and death for the animals attracted to graveyard. The unlucky reindeer herd was struck by lightning, instantly dying en masse over an area spanning 240 x 100m. Immediately, foxes, wolverines, crows, ravens and rodents concentrated to the site to feast upon the unexpected banquet.
However, there is a strict hierarchy for a place at the table. First choice goes to the larger scavengers such as foxes and ravens, whilst the smaller animals must wait their turn. Moreover, the most vulnerable animals such as voles and lemmings have to be careful that they in turn don’t become dinner for the larger guests.
For two years following the lightning strike, Frank and his team monitored their study site by collecting animal faeces as an indicator of how different groups of animals moved through the landscape around the reindeer carcasses. Interestingly, they found that scavenging birds such as ravens and crows struck the most fear into those at the reindeer banquet. It was only a full year after the fateful lightning strike that the rodents were able to approach the dinner table once the birds had left. This suggests that despite the great abundance of food available, smaller scavengers such as voles and lemmings lived in a landscape of fear, which prevented them accessing vital nutrition whilst predators were most numerous. Crows and ravens are diurnal animals, or animals that are active exclusively during the day, whilst foxes and wolverines are primarily nocturnal. This meant that at no point during the day or night could the smaller scavengers get close enough to feed. Ultimately, the landscape of fear created by predators at the site of the reindeer carcasses forces a top-down control on the nearby rodent populations. By preventing access to a readily available and nutritious food source, rodent reproductive rates are kept in check.
Integration into future management strategies
Integrating landscapes of fear as a tool for rodent management has recently gained attention within the ecological community. In this study, Frank and colleagues utilise a rare meteorological misfortune to investigate how arctic food webs self-regulate. This research holds important insights into how human societies currently dispose of animal carcasses following hunting and conservation practices. Carrion is a natural phenomenon, yet we understand very little of how this contributes to the composition and functioning of the landscapes around us.