In 2002, black rats were eradicated from Hawaii's Mokoliʻi islet; after just one shearwater chick survived from 1999 to 2001, 126 fledged in 2002 and 185 in 2003

From Oahu's shoreline, Mokoliʻi looks like little more than a rocky speck rising out of Kaneohe Baym, also known as Chinaman's Hat, the islet lies in Kāneʻohe Bay off Oahu. Few would guess that, until the early 2000s, one of Hawaii's smallest offshore islets had become the front line of a conservation battle fought not against storms or rising seas, but against black rats.

The rodents were not among the local species there. Nevertheless, they have been preying undetected on the eggs and chicks of wedge-tailed shearwaters for many years. By the time the conservationists took notice in 2002, it was almost too late for the colony. During the previous three breeding seasons, only one shearwater was reported to have survived long enough to leave the nest. A rapid turnaround followed soon afterward. A 2006 study published in Pacific Science documented that after only one wedge-tailed shearwater chick survived between 1999 and 2001, the number of surviving chicks rose to 126 in 2002 and 185 in 2003 after rats were removed from the island.

The predator that never belonged


Mokoliʻi is a small islet that occupies just 1.6 hectares (4 acres), approximately four acres, but still has great ecological importance of Mokoliʻi. Like many other small islands in Hawaii, Mokoliʻi provides suitable nesting ground for seabirds that fly over the Pacific Ocean and only return to land when it is breeding season. However, this only works if the area is not invaded by predators.

Historical records indicate that black rats have been present on Mokoliʻi since the 1950s. They fed on the chicks and eggs of nesting birds and ate local flora and fauna, such as plants, insects, and intertidal animals. As island fauna have evolved without mammalian predators, they have few natural defenses against them. Across the Pacific, invasive rats have become one of the leading drivers of seabird declines and island biodiversity loss. A broader review of island restoration efforts has shown that removing invasive mammals can produce rapid biodiversity gains, particularly for seabirds that nest on the ground.

Eighteen rats stood between survival and collapse

In contrast to other large-scale restoration programs that take decades to show results, the work on Mokoli’i was notably small. Conservation teams used snap traps, cage traps, and bait stations with diphacinone pellets, an anti-vitamin K rat poison. They set 354 bait stations and trapped 18 black rats in total. On May 27, 2002, the invasive rats that had previously inhabited the island disappeared.

The effort was small enough to be managed without a massive federal workforce. According to the study's authors, many participants were local volunteers and community members, suggesting that even relatively small conservation projects can achieve substantial ecological benefits with sustained public support.

Then the birds came back

The restoration of wildlife through the Mokoliʻi project was almost rapid. During the first breeding season after the rodent extermination, an impressive number of 126 wedge-tailed shearwater chicks fledged from their burrows. After one year, the number of successful fledglings increased to 185 chicks. The birds were not the only species to benefit.

Research teams also witnessed the restoration of native coastal vegetation, intertidal invertebrates, and Carter's panic grass, an endangered Hawaiian plant found on the island. Hence, removing a single invasive predator triggered changes across the ecosystem, and species are able to thrive after that.

Why islands become conservation laboratories

Island conservation is a successful method of ecological restoration due to contained ecosystems. Once an invasive species is removed, the ecological impact can often be determined much more easily than on vast land masses.

Hawaii, having many offshore islands, is becoming more and more dependent on eradication methods when working with the native birds, plants, and insects. Mokoliʻi demonstrates how removing invasive predators can help restore island ecosystems. The project made it possible to come up with successful methods of restoring other islands of Hawaii suffering from the same problems caused by invasive mammals. The Mokoliʻi project suggests that removing invasive species can be a crucial part of island restoration.

Restoration can be surprisingly simple

Conservation projects are often expensive and can take years to show results. On a small volcanic island, the project showed that removing invasive species can be an effective way to restore an ecosystem. Mokoliʻi is one of those rare exceptions. No new plants were introduced to create the colony of shearwaters. No birds were moved from other places. The island was not redesigned or reconstructed. The conservationists simply removed 18 rats from the breeding colony. They had kept the colony from breeding for years.