When foxes declined, their prey became more – and less – abundant

Research projects are often short, spanning only a few field seasons, but different animal species have been monitored at Grimsö Wildlife Research Area for a long time. A great deal has happened over those more than 50 years. At first, foxes were abundant, then sarcoptic mange struck. The Eurasian lynx returned, followed later by the wolf – both of which can kill foxes. Meanwhile, researchers continued to count fox litters, trap voles and survey droppings from hares, capercaillie, hazel grouse and black grouse. Together, these changes create a natural experiment in which interactions in the ecosystem can be studied.

Fox reproduction

Fox reproduction increased when voles were abundant. The number of fox litters decreased during the sarcoptic mange outbreak in the 1980s and was also lower after both lynx and wolves had re-established in the research area – lynx in the 1990s and wolves in the 2000s. The densities of the fox's prey increased during the mange outbreak, but declined following the return of the apex predators, even though the number of fox litters had also declined. In addition, the relationship between vole abundance and the densities of the fox's prey weakened after the return of the apex predators.

During the mange period, the results fit the intuitive picture: fewer foxes coincided with more hares and woodland grouse. But when the apex predators returned, this simple explanation no longer held. Both fox litters and the secondary prey – hares and woodland grouse – was then less abundant.

'One explanation may be that the apex predators do not only affect foxes, but also take some of the same prey. When lynx and wolves returned, several relationships in the food web changed at the same time. The pattern was clearer for hares than for woodland grouse, probably because hares are more important prey for both lynx and wolves,' says Camilla Wikenros, researcher at  Grimsö Wildlife Research Station at the Swedish University of Agricultural Sciences (SLU).

Both food and predators are important

In ecology, populations are often described as being regulated from the bottom up through food availability or from the top down through factors such as predation and disease. Both processes appear to operate simultaneously for foxes in the Grimsö area.

Sarcoptic mange and the period when both lynx and wolves were present were associated with fewer fox litters. At the same time, there was a stable relationship in the other direction: more voles coincided with more fox litters throughout the study period, although at different levels. However, the researchers found no evidence that variation in the abundance of hares and woodland grouse explained the number of fox litters.

Long time series reveal relationships between species

The study shows why it is difficult to understand changes in a single wildlife population by looking at just one species. Species are not part of a simple chain, but of a food web in which a change can have effects in several directions.

'With long time series, we can follow the same ecosystem before, during and after both disease outbreaks and changes in the species community, such as the return of apex predators. This allows us to detect relationships that cannot be documented over a short period of time,' says Camilla Wikenros.

The study was recently published in Ecography.

Scientific article

The dual influence of top-down and bottom-up processes on a mesopredator and the consequences for prey