Reinforcement of the populations of critically endangered endemic fern Diellia pallida, Kaua’i, Hawaiian Islands, U.S.A.

Summary

Diellia pallida, a Critically Endangered fern endemic to Hawaii's island of Kauai with a natural population of just 13 mature individuals across three sites, only eight of them reproducing, is threatened by habitat trampling and erosion from introduced goats, pigs, deer and jungle fowl that kill young gametophytes and sporelings before they can mature. Tallinn Botanic Garden researcher Ruth Aguraiuja began emergency protection in the early 2000s, caging the last mature individuals to shield them from browsing and stabilize surrounding soil, and replanting rhizomes uprooted by feral animals into a fenced exclosure; favorable weather from 2003 to 2006 produced a dramatic increase in gametophytes and sporelings, though survival of these young stages remained very low due to ongoing trampling and erosion. In 2006, working with the National Tropical Botanical Garden, the team began laboratory propagation from mixed-individual spore crosses, since the fern is an obligate out-crosser, discovering gametophytes can persist for three to four years and that germination to first sporeling takes about a year. In 2007, seven experimental population patches were established within fenced exclosures across the species' historical range, each planted with 30 individuals per recovery-plan guidelines, with survival highest at two sites suggesting habitat there remains suitable. Major difficulties included uncontrolled feral animal disturbance and erosion outside fenced areas, insect and fungal damage, and the fern's inherently difficult propagation. The project rates itself Partially Successful, noting workable methods have been developed but that reinforcement success so far is confined to a minority of sites.