This is the main conclusion in a risk assessment undertaken by the Norwegian Scientific Committee for Food and Environment (VKM) on their own initiative.

Due to lack of data, some of the assessments have considerable uncertainty.

Background

It is estimated there are between 690.000 and 870.000 domestic cats (Felis catus) in Norway, including bot owned and feral cats. Cats has historically been kept as a predator of vermin such as mice and rats. Today cats are kept mainly as pets.

A large proportion of owned cats roam freely, without limitations to where they can go. Allowing domestic cats to roam freely is controversial as it can have negative consequences for native fauna. It is well documented from Norway and other countries that domestic cats injure and kill many animals such as small rodents, birds, reptiles, amphibians, and various species of macroinvertebrates.

The keeping of domestic cats also raises questions regarding the animal welfare of their prey, as these are often not killed immediately but are used for hunting practice. In the breeding season for birds, predation on birds can lead to starvation and death of fledglings. Also, the cats themselves are exposed to hazards that pose a risk to their own welfare as they spend time outside unattended.

Methods

VKM has assessed the risks posed by domestic cats to biodiversity in Norway and has described the role they play in the spread of various pathogenic organisms like parasites, bacteria, and virus. VKM has also assessed animal welfare aspects both for domestic cats and their prey.

The risk assessment is based on publicly available information and on scientific literature, the basis from which VKM developed a model to estimate the number of animals killed by domestic cats each year. This was done for all major groups of prey.

The risk of negative effects on biodiversity were assessed separately for lethal and non-lethal effects of cat predation. The risk related to lethal consequences was assessed separately of birds, mammals, reptiles, and amphibians.

VKM has also assessed various risk-reducing measures regarding keeping domestic cats. The measures were grouped into four categories: 1) measures that restrict a cat’s access to prey, 2) measures that reduce a cat’s ability to hunt successfully, 3) measures to reduce a cat’s hunting behaviour, and 4) measures that reduce the number of cats.

Results and conclusions

VKM estimate that the roughly 780.000 domestic cats in Norway kill between 21 and 68 million prey each year.

“We have estimated that this accounts to between 4 and 14 million injured and killed birds, and between 13 and 43 million mammals. Around 75% of these fatalities are caused by owned cats,” says Erlend Nilsen, Scientific Leader of the project group.

“Our conclusion is that keeping cats presents a high risk of negative impact on vulnerable species of birds that live in areas with high density of cats. Regarding common mammals, we assess that the risk of negative consequences is low,” says Nilsen. “However, for some species of bats, even low levels of predation can have major consequences, as these have small and sparsely distributed populations in Norway.”

“We also assess that the risk from domestic cats on biodiversity, through competition with wild native predators, is low. However, the risk is assessed as medium for species with high dietary overlap with cats, in areas with high cat density.”

VKM concludes there is considerable risk associated with further spread of pathogenic organisms to native wildlife and other domestic animals through domestic cats. Some of these pathogens can also infect humans.

“For most pathogenic organisms, cats play a peripheral role as carrier. However, in some disease systems cats play a more prominent role in the spread of the disease-causing agents that harm a large number of individuals of both wild and domestic species, some of which also infect humans. This includes Toxoplasma gondii and oxocaria, the bacteria Franscisella tularensis, and bacteria in the Salmonella-genus. In addition, cats can play a role in the spread of new viruses that affect the respiratory system, especially influenza,” says Nilsen.

According to VKM it is highly likely that keeping domestic cats has a negative impact on the welfare of prey animals in areas with high cat density, especially in urban areas where there are few other predators.

Risk-reducing measures

VKM concludes that measures that reduce cats’ access to areas with populations of prey species is most effective in reducing the negative impact on biodiversity.

“The use of outdoor enclosures or walking a cat on a leash, is likely to have a profound effect on its ability to injure or kill prey and on reducing the spread of pathogens. These measures are likely to have little negative impact on cat welfare,” according to Nilsen.

Data gaps and uncertainty

VKM has identified several data gaps and uncertainties. This includes, among other things, the lack of studies on the direct effects of cat predation in Norway. These data gaps result in higher uncertainty in the assessment.

“Our assessment shows that domestic cats kill a large number of prey each year in Norway. However, it is uncertain how this strong impacts this has on their preys’ populations as we don’t know how much of this mortality that is additive to other mortality factors,” says Nilsen.

There is also high uncertainty regarding the specific role that cats play in spreading of parasites and pathogens to wild animals. Data on the effects of risk-reducing measures is also lacking, so there is uncertainty about the assessment pertaining to these.

The risk assessment was approved by VKM’s Panel on Biodiversity.