Quantifying the Effect of an Invader

The Raccoon Dog, an alien species, has made its way to Sweden recently. But what sort of effect does it have on the native fauna?

The Raccoon Dog, an alien species, has made its way to Sweden recently. But what sort of effect does it have on the native fauna? (Image Credit: Prue Simmons, CC BY 2.0, Image Cropped)

Nest predation by raccoon dog Nyctereutes procyonoides in the archipelago of Northern Sweden (2018) Dahl & Åhlen, Biological Invasions, https://doi.org/10.1007/s10530-018-1855-4

The Crux

We’ve spoken about biological invasions at length on EcolMass, and the detrimental effects that the arrival of a new species can have on native populations. Yet eradication is often impossible, and management expensive, so before taking extensive action, it’s always important to ensure that an alien species IS having a negative effect.

The raccoon dog is an Asian species, closely related to foxes, that was introduced to Europe in the early 20th century and has since spread into Scandinavia. Voracious predators that could spread further north due to climate change, our paper this week looks at the extent of their impact on the ecosystems they’ve spread to.

What They Did

Our researchers looked at a series of islands off the coast of Northern Sweden where raccoon dogs are present. Four dogs were captured, sterilised, GPS-tagged and then released onto the islands. Camera traps were set up near a selection of bird nests. To avoid increasing predation risk, artificial nests were also used. Several islands which the raccoon dogs did not have access to were used to establish a baseline for predation, as the islands are also inhabited by native predators like foxes and gulls.

It is not a given that the introduction of a new predator will increase predation, as an extra predator can mute the effects of others.  So the main goal was to see whether or not the presence of the raccoon dogs increased predation on the nests. If nests on islands with raccoon dogs were preyed on more often, then predation by the dogs would place further pressure on local populations.

Did You Know: Judas Animals

Traditionally used by farmers to coax their herds to slaughterhouses, Judas animals are members of a social animal species that will help humans in locating other members of their species. The technique has been used extensively for species which are highly social like goats, camels, and carp. Researchers will tag a member of an invasive species and then release them into the wild, then track them until they have met up with their herd, at which point they will cull the herd. This technique is being used in Sweden to locate and desex raccoon dogs, bringing down the breeding population.

What Did They Find Out?

There was much higher predation on islands where the raccoon dogs were present, confirming their additive effect on egg predation. The dogs also actively scared birds away from their nests in order to prey on the eggs, in contrast to the predatory birds, which do not scare larger prey species, or foxes and mink, which tend not to swim out to the islands. The raccoon dogs were also prolific swimmers, often moving between islands during the periods surveyed.


Researchers found that the raccoon dog has an additive effect on nest predation, increasing pressure on native bird populations (Image Credit: Karlakas, CC BY-SA 3.0)


The number of dogs used in this study was extremely low, and only one test site was used, giving fairly low statistical power. All dogs had been sterilised, which may have affected their behaviour, with foraging behaviour perhaps different in situations when procreation and care for young was possible.

So What?

The raccoon dogs have a clearly negative effect on local bird populations, and will be able to move further north as the climate warms, with island populations clearly not exempt from this threat. This is worrying, however their ability to move around at such speeds means that the current management protocol, that of capture, sterilisation and release, could be quite effective in the future.

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