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If you have a pet cat, it's probably hard to believe it could do any harm other than giving you the odd scratch or damaging the furniture.

But in Poland cats have been causing major problems for the ecosystem, due to the damage they do to birds and other wildlife.

The Polish Academy of Sciences has now named domesticated cats as an "invasive alien species".

An invasive species is something that causes harm to a country's natural environment, by damaging or attacking plants or animals where it is not native.

Biologist Wojciech Solarz said there is now more and more agreement within the scientific community that domestic cats have a harmful impact on biodiversity given the number of birds and mammals they hunt and kill.

Mr Solarz said that cats kill about 140 million birds in Poland each year.

Something being classified as an alien species means that the animals wouldn't have been naturally found in that area or environment.

Often alien species can enter a country by accident, for example a small insect being brought back in someone's luggage. This is why airports in some countries like Australia and New Zealand check carefully that food or even mud on someone's shoe doesn't enter another ecosystem accidentally as this can cause major problems for the species that already live there.

But sometimes alien species of plants and animals can be brought or introduced deliberately by humans.

The scientists say that cats were domesticated probably around 10,000 years ago by the great civilisations of the ancient Middle East - which from a scientific point of view makes the species alien to Europe.

The institute said that people should not be cruel to or blame cats, but is instead recommend that cat owners limit the time their pets spend outdoors during bird breeding season.

Many people in Poland weren't happy about cats being placed on this list, something Mr Solarz said he didn't expect.

There are 1,787 other species also on the list but it is cats that people's attention seems to have been drawn to the most.

Some experts have argued that it's unfair to just blame cats for the decline in birds and other wildlife

Dorota Suminska, a vet and the author of a book titled The Happy Cat, pointed to other causes of shrinking biodiversity, including a polluted environment and buildings that cause dangers to birds in flight.

She thinks cats have been unfairly blamed, arguing that if cats are on the list, humans should be too considering the negative impact they also have on local wildlife.