European sailors brought cats to Hawaii over two hundred years ago. Since then their numbers have exploded and there are now tens of thousands of cats in the Islands, many in loving homes and many ranging free.

 

While the domestic cat is the world's most popular household pet, the feral cat has gained a reputation as one of the worst invasive species impacting native wildlife. In Hawaii, they are found at all elevations and in all habitats where feral and free-ranging cats kill common, rare and endangered birds. Feral cats likely prey upon all native Hawaiian birds. Cats have been documented preying on nēnē (Hawaiian goose) and palila (Hawaiian honeycreeper), and have been captured on film killing female palila on their nests. On the Big Island, Lanai and Maui 'ua'u (Hawaiian petrel) chicks are particularly vulnerable to cats. This seabird lays a single egg in crevices or burrows and chicks take about 60 days to fledge. 

 

Cats also spread disease.

Toxoplasmosis can be fatal to humans and wildlife. The protozoan organism that causes toxoplasmosis was responsible for killing a number of 'alalā during the release of captive birds in the 1990s. Because it can complete an important part of its life cycle in seawater, this disease also poses a threat to marine mammals as was recently confirmed in an endangered Hawaiian monk seal, and may possibly threaten other marine mammals in Hawaii, such as the spinner dolphin. In addition to threatening wildlife, toxoplasmosis is zoonotic and poses a significant health risk to pregnant women and those with compromised immune systems.

 

Ethical and humane issues surrounding feral and free-ranging cats have resulted in heated debates among members of the Hawaii community. Some believe "managed colonies" provide safe havens where cats can be maintained. Sometimes, these colonies include the practice of trap/neuter/release, an effort meant to care for existing cats. Since they are not reproducing,  eventually the colony may disappear because of natural mortality. Unfortunately, these efforts are often undermined by abandonment of unwanted cats at sites where feral colonies are maintained. Thus colonies persist and often grow.