It’s a compelling question – could a single species of introduced predator cause the total extinction of a native bird species? This issue has been hotly debated among conservation biologists and ecologists, with the most well-known case being cats and songbirds in New Zealand. On some remote islands around New Zealand, populations of unique, flightless songbirds evolved in the absence of mammalian predators. When humans introduced cats, rats, and other predators to these islands, many of the endemic birds were devastated and some were driven completely extinct. But there is still disagreement around whether predation by cats alone could fully account for the extinction of some species. Teasing apart the roles of different factors in extinction – like habitat loss, disease, climate change, and predation – can be incredibly challenging. In this article, we’ll examine the evidence surrounding one particularly contentious case – the Stephens Island wren and the lighthouse keeper’s cat.
The Stephens Island Wren
The Stephens Island wren (Xenicus lyalli) was a small, drab brown songbird related to the New Zealand wrens. It was endemic to Stephens Island, a small, forested island off the northeast coast of New Zealand’s South Island. Stephens Island was an important habitat for many unique bird species, being the only breeding site for the rare Stephens Island wren.
Though flightless, the wren could move extremely quickly on the forest floor. Its wings had evolved to be tiny and useless for flying. Like many island birds that evolve without predators, it became terrestrial, adapted for ground living.
Early accounts of the island described the wren as extremely abundant. When a lighthouse was constructed on the island in 1894, the wren population was thriving. But within a year, the species was completely gone from the island. What happened?
The Lighthouse Keeper’s Cat
In 1894, a lighthouse keeper’s family moved to Stephens Island along with their pregnant cat named Tibbles. Soon after arrival, Tibbles gave birth to a litter of kittens. As the story goes, Tibbles and her kittens developed a taste for the abundant wrens on the island. The ground-nesting birds had no defenses against the hunting cats.
By 1895, it was reported that the wren population was decimated. The last known individual was collected by a naturalist in 1897. Just a few years after human settlement, Stephens Island wren was gone forever.
So it seemed clear that the lighthouse cat was solely responsible for driving the species to extinction. But is that truly the whole story?
Cats Evolved as Specialist Bird Hunters
As an introduced predator, Tibbles was unfortunately well equipped to hunt the naive birds. Domestic cats (Felis catus) share a common ancestor with the European and African wildcats. Unlike other small predators like foxes and coyotes, cats evolved specifically to hunt smaller prey like rodents, birds, reptiles, and insects.
Specializations that make cats such effective bird hunters include:
- Lightning quick reflexes and ability to silently stalk prey
- Keen long-distance vision adapted for detecting bird movement
- Excellent hearing to locate prey rustling in vegetation
- Powerful yet nimble forelimbs for grabbing and killing prey
- Rough tongue that can grip feathers and eat birds efficiently
These adaptations would have given Tibbles all the tools needed to readily catch and consume the wrens. And an adult cat can hunt very effectively – catching and killing over 10 birds per day. With a litter of kittens learning to hunt on the island, the wrens would have been exposed to extreme predation pressure.
But some scientists still doubted that cats alone could have driven the wren fully extinct. What other factors may have been at play?
Vulnerable Island Populations
Insular island species like the Stephens Island wren already exist precariously. Their tiny populations are vulnerable to inbreeding depression, disease outbreaks, habitat loss, and fluctuations in weather. Island species evolve in isolation, without predators or competition. This allows them to lose costly anti-predator defenses over time.
The Stephens Island wren population was likely already struggling from other pressures. Being flightless, it had no way to disperse to new habitat if local conditions deteriorated.
And the island had recently become more accessible to people. With the new lighthouse came more visitors and human disturbance.
So when cat predation was added to the mix, it may have pushed an already fragile species over the brink.
Disease and Habitat Loss
Around the same time cats were introduced, a disease outbreak devastated the island’s bird populations. Because island species have weaker immune defenses, an introduced pathogen could spread unchecked.
Deforestation was also occurring on Stephens Island by the 1890s. Loss of forest habitat made it easier for cats to catch the wrens. The cats may have dealt the final blow to an ecosystem already in collapse.
Overhunting by Early Explorers
Some scientists argue that humans caused the wren’s initial decline before the cats ever arrived. Stephens Island was frequently visited by early Polynesian explorers and sealers. The wren was said to be “good eating” and easy to catch. It’s possible these groups collected the wrens as a food source and depleted its numbers.
By the time the lighthouse brought permanent settlement, the wrens may have already been reduced to an unsustainably small population. The cats then finished off the declining remnants.
Unanswered Questions
The precise timeline of the wren’s extinction is uncertain. The island was not thoroughly surveyed by naturalists prior to the cat’s arrival. Telescopes in the lighthouse even allowed staff to observe the birds being caught by cats. But we don’t know if the species was already declining before regular cat predation began.
Determining population sizes and trends would require more complete historical data. Unfortunately, wildlife biology was still a developing science in the late 1800s.
So while cats were definitely a factor, claims that they alone drove the wren extinct may be an oversimplification.
Applying the Lessons
The lessons from Stephens Island remain relevant today. Worldwide, cats have contributed to at least 63 extinctions of birds, mammals, and reptiles. And domestic cats still threaten wildlife across the globe.
For island restoration projects, removing cats and preventing recolonization is a priority. Controlling populations of feral cats is also critical in protecting mainland species.
But the full context needs to be considered. Eradicating cats cannot undo habitat destruction, pollution, or climate change. To preserve fragile populations, all threats need to be addressed.
Ultimately, the Stephen Island wren extinction reminds us of how quickly human actions can irreversibly damage native ecosystems. Careful management of predation, habitat, and disease is needed to protect biodiversity.
Conclusion
The introduction of a single cat to Stephens Island in 1894 was closely followed by the extinction of the endemic wren. This rapid demise suggests cats were primarily responsible. But reconstructing the wren’s decline is difficult with limited historical data. Other factors like overhunting, habitat loss, and disease likely also played a role. While Tibbles the lighthouse cat certainly contributed to the species’ downfall, she was not necessarily solely to blame. The true cause was likely the combined effects of human disruption to an extremely fragile island species. The story serves as a cautionary tale about the impacts invasive predators can have on naive island fauna. Going forward, comprehensive conservation strategies are needed to manage cats while also addressing widespread habitat degradation. Only by alleviating multiple pressures can we prevent further extinctions.
