The Polish Scientific Institute WARSAW has classified domestic cats as an "invasive alien species".
Does this mean they came from outer space? No. From a strictly scientific point of view, the term "alien" is used because of the domestication of cats roughly 10,000 years ago in ancient Middle East, making cats "alien" to Europe.
So how are cats an invasive species? The Polish Academy Institute claims cats fit the criteria of "invasive" as "opposed to any cruelty towards animals". In particular birds. Biologist Wojciech Solarz explains that "domestic cats have a harmful impact on biodiversity given the number of birds and mammals they hunt and kill".
Read more into it at News4Jax.