Why Did the St. Helena Olive Tree Go Extinct?

The remote South Atlantic island of St. Helena was once home to the St. Helena Olive (Nesiota elliptica), a unique tree woven into the island’s natural history. Its story of discovery, decline, and disappearance offers a compelling look at the fragility of island ecosystems and serves as a significant chapter in the island’s history.

The Unique Identity of the St. Helena Olive

Despite its common name, the St. Helena Olive was not a true olive. It belonged to the buckthorn family, Rhamnaceae, and was the sole species in its genus, Nesiota. Its closest relatives are found in Southern Africa, highlighting a distant evolutionary connection.

The tree was a stout, shrubby plant that grew as a small, spreading tree up to six meters high. It had a gnarled appearance with dark brown to black bark and distinctive deep grey-green foliage. Its leaves, about 50-70mm long, were thick and oblong with curled edges and undersides covered in fine white hairs.

From June to October, the tree produced clusters of small, pale pink flowers, believed to be pollinated by a native hoverfly, Sphaerophoria beattiei. After pollination, the tree developed hard, woody fruit capsules that contained triangular black seeds. It was a fixture of the high-altitude cloud forests, thriving in the cool, moist conditions on the northern slopes of the island’s central ridge.

Factors Leading to Decline

The decline of the St. Helena Olive began with changes to its habitat following human colonization. The island’s native forests were cleared for timber, fuel, and to make way for plantations. The introduction of non-native grazing animals, especially goats, compounded the problem by consuming young tree shoots and preventing natural regeneration.

Beyond habitat loss, the tree possessed a biological trait that made recovery difficult. Nesiota elliptica was highly self-incompatible, meaning it required pollen from a different tree to produce viable seeds. While this maintains genetic diversity in a healthy population, it becomes a liability when numbers crash. With the population dwindling to a handful of individuals by the 19th century, the chances of successful cross-pollination became improbable, creating an extinction vortex.

Conservation and Propagation Efforts

An effort to save the St. Helena Olive began in 1977 with the discovery of a single wild specimen by naturalist George Benjamin. This tree, found near Diana’s Peak, was a remnant of a population that had numbered only 12 to 15 trees a century earlier. Its discovery launched a race to propagate the species.

Conservationists faced difficulties with propagation. Cuttings from the wild tree were challenging to root using conventional techniques. Because of the tree’s self-incompatibility, very few collected seeds were viable, and those that formed were often susceptible to fungal infections that prevented germination.

Despite these setbacks, there was some success. A few cuttings were propagated and some seeds were germinated with help from institutions like the Royal Botanic Gardens, Kew. One cutting was planted in an endemic nursery on St. Helena in 1988, and a few seedlings were later raised from it.

The Final Extinction Events

The last known wild tree succumbed to a combination of wind, rain, and fungal infections in October 1994. Its death marked the extinction of the species in its natural habitat. The only remaining hope were the few cultivated specimens.

This hope was short-lived, as the cultivated plants proved to be just as vulnerable. In 1999, the last two saplings in a nursery died from a fungal infection. The final known individual, a cutting at a nursery on St. Helena, died from a fungal and termite infestation in November 2003, and Nesiota elliptica was declared fully extinct.

Legacy of the Lost Olive

The story of the St. Helena Olive serves as a lesson on the fragility of island ecosystems. Islands are often home to unique species that evolved in isolation, making them vulnerable to disturbances like habitat destruction and the introduction of non-native species. The tree’s extinction shows how quickly human activities can unravel these specialized environments.

Its loss also illustrates the concept of an “extinction debt,” where a species is functionally doomed long before its last individual dies. The St. Helena Olive’s fate was likely sealed once its population dropped below the threshold needed for reproduction. The last-minute conservation efforts, though intense, were too late to reverse the accumulated damage.

All that remains of Nesiota elliptica is genetic material stored in a DNA bank at Kew Gardens. This absence is a reminder of the permanent void left by extinction. The tale of the lost olive highlights the importance of proactive conservation to protect vulnerable species before they reach a point of no return.