Magazine article Endangered Species Bulletin

Rescuing Island Castaways

Magazine article Endangered Species Bulletin

Rescuing Island Castaways

Article excerpt

The Laysan duck or teal (Anas layanensis) has the most restricted range of any cluck species and is among the world's most highly threatened birds. Before the arrival of humans to the Hawaiian Islands, this species occurred on most islands in the archipelago, and it apparently was well adapted to harsh environments and variable food resources. Today, however, it is restricted to Laysan Island, a single predator-free coraline island of only 990 acres (400 hectares) northwest of the main islands. Laysan has been protected as part of the Hawaiian Islands National Wildlife Refuge since 1909, and the Laysan duck was among the first species listed in the United States as endangered.

Like many isolated island species from Hawaii and New Zealand, the Laysan duck evolved in the absence of mammalian predators and is ill-suited to life where non-native predators have invaded. For example, when startled, Laysan ducks are more likely to freeze their motion rather than to flush or fly. This strategy is well suited as a cryptic defense against Hawaii's native flying predators but ineffective against predation by non-native mammals.

Small, isolated populations are extremely vulnerable to extinction from chance events and human related disturbance. The Laysan duck population on nearby Lisianski Island disappeared after successive shipwrecks on the island in the 1840's, probably due to direct human consumption. The species reached the brink of extinction when rabbits were introduced to Laysan Island in the early 1900's, but it recovered from fewer than 10 individuals after the rabbits were removed and the vegetation restored. Although rabbits no longer occur on Laysan, the duck population has gone through several severe bottlenecks. The most recent population crash occurred in 1993 during El Nino drought conditions. The population size (more than 500) was larger than the drought stricken island could support, and a die-off occurred from starvation and parasite infestation. Duck numbers appear to fluctuate with rainfall and population density.

Ecosystem restoration on Laysan Island National Wildlife Refuge and the establishment of additional wild populations on other islands are needed to reduce the risk of extinction. Control of a non-native plant, the sandbur weed (Cenchrus encinata) has been very effective in restoring the duck's nesting habitat, which includes native bunch grass (Eragrostis variabalis). Despite the bird's former distribution in forested areas of the main Hawaiian Islands, only remote, predator-free oceanic islands are being considered as sites for establishing new populations. Even the predator-free islands will likely need pest and weed control or reestablishment of freshwater seeps. …

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