In case you weren't aware, outdoor cats are stone-cold killers

There are differing opinions on cats' personalities and behaviour: They're affectionate, they're aloof, they love you, they hate you.

Then there's the undeniable fact of what they become when left to roam free outside: indiscriminate, stone-cold killers.

A new study has found cats roaming free prey upon almost any animal, reptile, insect, and amphibian around the world – their hunting so prolific and so successful, the authors found, that it poses a legitimate threat to global biodiversity.

"What's shocking is just the indiscriminate nature of their hunting," said Christopher Lepczyk, the paper's lead author and an ecology professor at Alabama's Auburn University.

"If there is a food source that they can obtain, they will."

The team of scientists from Australia, New Zealand and the United States created the latest report by looking at more than 500 previous studies to better understand which animals free-ranging cats will eat globally.

The paper, published in Nature Communications, found domestic and feral roaming cats will hunt or scavenge more than 2,000 species from tiny hatchling turtles to hulking dairy cows.

​​Almost half of the victims were birds, followed by reptiles and mammals. Lepcyzk said they also found a surprising number of insects, including emperor dragonflies and endangered monarch butterflies.

The menu includes nearly 350 species that are threatened, vulnerable or endangered — including the little brown bat and green sea turtles. Cats proved especially lethal on islands that have evolved without as many natural predators.

"[Cats] don't have a big preference in terms of what they're eating, if it moves." said Lepczyk.

Most victims were small, weighing less than five kilograms, but cats scavenged the carcasses of animals much larger than they could kill themselves – like camels. The paper said the cats also ate spoiled and wasted food people left outside.

In all, the paper said the cat is among the most successful and "problematic" invasive predators in the world — a claim that's been made about cats before.

Last year, the Polish Academy of Sciences — a state-run scientific institute — classified domestic cats as an "invasive alien species,'' citing the damage they cause to birds and other wildlife.

Wojciech Solarz, a biologist at the centre, told the Associated Press there's a growing scientific consensus that domestic cats have a harmful impact on biodiversity given the number of birds and mammals they hunt and kill.

The criteria for including the cat among alien invasive species "are 100 per cent met by the cat,'' said Solarz.

Author recommends cats be kept inside

Lepczyk, 53, has spent decades researching the feline diet and the last five years compiling the database with his co-author, Jean E. Fantle-Lepczyk — who is also his wife. 

He said studying what cats eat globally made him see his own cats differently. He and his wife have two: Mochi, a long haired siamese, and Ahi, an orange tabby – both named after Japanese foods found in Hawaii, where the family lived for seven years.

"Even watching my cats indoors, you can tell they're predators," he said. "They're actively interested in things that move."

Despite being so lethal, a separate report from Lepczyk last year found there aren't many places in the world doing anything about roaming cats.

As an ecologist, Lepczyk keeps both his cats inside and recommends others do the same. It not only helps prolong the cat's life, he said, but it gives the animals a better fighting chance.

"It's going to be better for wildlife, but it's also going to be better for the cat," he said.