Stephens Island Weevil

From isolated paradise to endangered habitat

Stephens Island, a small rocky island in the Cook Strait, is renowned for its unique wildlife. Despite covering only 1.5 square kilometers, the island boasts a remarkable ecosystem that was once home to numerous endemic species, including many land and seabirds. Among its notable inhabitants was the Stephens Island weevil, which – as its name suggests – was found only on the island. It also served as the last refuge for Lyall’s wren, a small flightless bird from the family Acanthisittidae.

Before humans arrived, Stephens Island was an isolated paradise, free from mammals and thus devoid of land-based predators, allowing many species to evolve without fear of being hunted. This created a rich variety of endemic species, including the tuatara (Sphenodon punctatus), which today has its largest population on the island, numbering around 50,000 individuals. These ancient reptiles, with no close relatives anywhere in the world, share their habitat with one of the world’s oldest frog species, the Hamilton’s frog (Leiopelma hamiltoni).

The situation on Stephens Island changed drastically when humans settled the island. Although the Māori, who named the land Takapourewa, never permanently inhabited the island, it was taken over by the New Zealand government in 1891 for the construction of a lighthouse. From that point, the once-pristine island and its wildlife were exposed to a multitude of threats.

With the lighthouse came not only the keepers but also livestock such as sheep and goats. However, the most significant threat came from the introduction of cats and other mammals. A cat, likely brought to the island while pregnant, escaped, and its offspring quickly became feral. Soon, a large population of feral cats formed, with devastating consequences for the island’s wildlife, particularly its bird species. The endemic animals had not evolved defenses against these newly introduced predators.

In addition to the introduction of mammals, intensive grazing by livestock led to the destruction of natural habitats. About 90 percent of the island’s original vegetation was lost to agricultural use before Stephens Island was declared a wildlife sanctuary in 1966. Today, the island is managed by the Department of Conservation (DOC) in partnership with the Māori Ngāti Koata tribe and serves as an important sanctuary for many rare and endangered species.

Stephens Island weevil – Fact sheet

Scientific name	Mecodema punctellum
Original range	Stephens Island (New Zealand)
Time of extinction	1931
Causes of extinction	habitat loss
IUCN status	extinct

What is known about the Stephens Island weevil

The Stephens Island weevil belongs to the genus Mecodema, which includes around 102 species and subspecies. These large, flightless ground beetles (Carabidae) are endemic to New Zealand and surrounding islands. However, the Stephens Island weevil was only found on the small island of Stephens Island.

Reports from the Transactions and Proceedings of the Royal Society of New Zealand 1948-49 describe the Stephens Island weevil as a fairly large beetle, reaching 38.5 millimeters in length and 11.7 millimeters in width. Its body was black, shiny, broad, and flat.

Since the species was only scientifically described in 1921 by Scottish entomologist Thomas Broun and was last seen in 1931, little is known about its behavior and ecology. It is believed that the Stephens Island weevil lived in moist forests and sought shelter and food under large, rotting logs. Like many other members of its genus, it was likely a predatory insect that fed primarily on snails. Due to its inability to fly, it was probably a slow but effective ground hunter.

The extinction of the Stephens Island weevil

The last confirmed sighting of the Stephens Island weevil was in 1931. Afterward, scientists made numerous attempts to rediscover the species – in 1961, 1971, 1974-1976, 1981, 1990, and 1996 on Stephens Island, and even in 1997 on D’Urville Island, which is three to four kilometers away. However, all these efforts were unsuccessful, and today it is generally believed that the species is extinct. In 1994, the International Union for Conservation of Nature (IUCN) officially declared the Stephens Island weevil extinct.

The primary reason for its extinction was likely habitat destruction. After humans settled on Stephens Island, large portions of the forest were cleared, and the rotting logs that served as shelters for the weevil disappeared. Without these essential refuges, the Stephens Island weevil could not cope with the changing environment and was unable to reproduce sufficiently.

Ground beetles in New Zealand

Ground beetles make up 9.6 percent of New Zealand’s described native beetle fauna and form the largest group of flightless beetles in the country. A 2012 study, The Conservation Status of New Zealand Coleoptera, found that ground beetles are among the most threatened beetle species in New Zealand. Of the 45 beetle species classified as threatened, 42.2 percent are ground beetles. In addition to the extinct Stephens Island weevil, a subspecies of Mecodema costellum (spelles) is also considered extinct, known only from remains found in two caves in northwest Nelson; scientists have only found individual body parts from several specimens.

The threat to ground beetles is closely tied to their biological characteristics: many of these beetles are large and flightless, making them particularly vulnerable to habitat changes. Habitat destruction through deforestation and agricultural activities, as well as the introduction of invasive herbivores that destroy their host plants, are central causes of their decline. Additionally, their low reproductive rate and the long lifespan of some large species hinder their recovery after environmental changes.

Ground beetle species that rely on specialized habitats, such as caves, are particularly at risk. The dependence on highly restricted environments and their destruction due to human activity are major factors contributing to the extinction of these beetles. An example of this is the genus Kettlotrechus, whose species are found only in caves and are highly endangered.