Rats or cats? Which is the worst for killing island species?

Every scientific paper that you read regarding the extinction of native species on small islands in archipelagoes always refer to cats and rats as the top two culprits. One scientific study starts, “Invasive rats are one of the world’s most successful animal groups that cause native species extinctions and ecosystem change, particularly on islands”. And another commences, “Cats are among the most successful and damaging invaders on islands and a significant driver of extinction and endangerment”.

Archipelagos are a cluster of islands and the species living there are vulnerable to introduced species like rats and cats
Archipelagos are a cluster of islands and the species living there are vulnerable to introduced species like rats and cats. Photo: National Photographic.
Two useful tags. Click either to see the articles:- Toxic to cats | Dangers to cats

Cats and rats vie for the top spot. Scientific American state that “cats may take the top killer slot, but collectively rats and mice are even worse”. They state that rodents have been responsible for 75 extinctions and 30% of all modern extinctions for birds, mammals and reptiles. They also state that feral cats can be blamed for 63 modern-day extinction accounting for 26% of extinctions of bird species, mammal species and reptiles.

We should remind ourselves that in every case the underlying cause of these extinctions is the behaviour of humans. Humans imported rats and cats to these islands in one way or another. But for human carelessness these extinctions would probably not have taken place.

Rats, however, have been found to also affect marine wildlife living in the seas around islands where they have fed on birds’ eggs and chicks. Black rats were introduced to scores of remote archipelagoes in the 18th and early 19th centuries. This often occurred because of shipwrecks.

The rodents would kill off seabird populations by pillaging eggs and chicks of seabirds. The droppings of seabirds became fertiliser to the land. This fertiliser called ‘guano’ leaches into the sea around the island. A study has found that “guano-derived nitrogen in marine algae and sponges was more than 30% higher around islands on which the rats had been eradicated”.

Damselfish feed on algae. So eradicating rats from these islands improved the health and welfare of some fish species living in the seas around the islands. The study (Invasive predators and global biodiversity loss) has been published in Current Biology. It looked at 20 islands in the central and western Indian Ocean. They found that populations of certain species such as shearwaters and terns had increased dramatically on these islands where rats had been exterminated.

A separate team of researchers found that fish living off the coast of rat-free islands grow four times quicker than those that live on reefs starved of guano by the rodents.

Rats have been introduced inadvertently to about 90% of the oceans’ archipelagoes. That information comes from Professor Nick Graham of Lancaster University and the co-author of the study referred to. Rats had been removed from more than 500 islands.

The mass extermination of any species of animal is controversial, however. We don’t like rats but in an absolute sense they are another species of animal trying to survive like any other. It’s a known fact that humans are the biggest exterminators of animal species, by far. It seems we have to exterminate one to save another.

Those who dislike feral cats always quote extermination of species on islands by feral cats. Animals living on small islands are highly vulnerable to predators like rats and cats because they have no defences against them. These efficient predators are introduced species and natural selection never had a chance to provide native animals with a defence against them. This is why they are wiped out relatively quickly. But it comes down to mankind’s intervention. Without humankind nature would have found a natural balance. It is humankind’s artificial interventions which lead to these crises.

P.S. the preferred method of eradication of rats, and sometimes cats is poisoning. The poison is dropped by aircraft or drone which is the cheapest way to kill these animals. It must endanger other animals but the scientists weigh up the pros and cons and they say that the poison causes little damage to other species. Does some of the poison find its way into the seas around the islands? Is the eradication of rats having a detrimental effect upon the health and welfare of fish?

Source: Studies and The Times.

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