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The Kāhuli and ʻŌhi‘a: The Interspecific Interaction that Kept Hawai’i Healthy

As conservationists fight to save Hawai’i’s native ʻŌhi‘a tree, they realize the important role native kāhuli snails play in maintaining a healthy ecosystem.

Hawai’i is currently facing two epidemics: the near extinction of the native kāhuli snails and the spread of Rapid Ohi’a Death decimating the iconic ʻŌhi‘a trees. While seemingly not connected, these two epidemics can largely be attributed to the introduction of invasive species and the imbalance they have caused across the Hawaiian Islands.

The ʻŌhi‘a

One of the most abundant trees throughout Hawai’i is the ʻŌhi‘a Lehua, which is endemic is found nowhere else in the world. ʻŌhi‘a span all elevations and are the first to colonize fresh lava substrate, making them a key factor in the soil development and ecological succession of the island.

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ʻŌhi‘a lehua trees overlooking the Kuli’ou’ou Ridge Trail. Credit: Alan Levine

ʻŌhi‘a trees make up the largest portion of the canopy in native wet forests. They provide shelter and food for numerous native birds as well as innumerable insects, snails and other invertebrates. Their trunks act as nurse logs, supplying nutrients, water and protection for native seedlings and epiphytes. Their canopies capture mists and rainwater that replenish our island aquifers, which provide drinking and irrigation water for Hawaiʻi’s communities and agricultural sector.”

University of Hawaiʻi at Mānoa College of Tropical Agriculture and Human Resources

In recent years, stands of ʻŌhi‘a trees across multiple islands had begun to die. After extensive research, they found what is known as Rapid ʻŌhi‘a Death which is caused by a fungal pathogen called Ceratocystis lukuohia.

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Closeup of ʻŌhi‘a Lehua flowers. Credit: Hawaiian Sea

Since the disease was identified on Hawai‘i Island in 2014, more than a million trees have died—with more than 90% of those testing positive for C. lukuohia

The Hawaii Department of Land and Natural Resources

There is no cure to this disease and the only solution is by actively preventing the spread through bio-sanitation techniques. However, new data indicates a relation between the death of the ʻŌhi‘a and the Kāhuli tree snails, another endemic suffering in Hawai’i.

The Kāhuli

The Earth’s diverse family of mollusks is silently suffering, and the native Hawai’ian tree snail, the Kāhuli, is taking the brunt of it.

Over 750 species of terrestrial snails were once described from the Hawaiian Islands, representing one of the most stunning examples of species radiations in the world.  Sadly it is estimated that over 90% of this diversity has been lost.”

Hawaii.gov

The first thing you notice about the Kāhuli are their shells. They are much smaller than your average continental garden snail, but their shells are so beautiful, they have been described as “jewels of the forest.”

Variety of the Kāhuli. Credit: David Sischo

When they arrived on the islands, the snails branched out and took on a variety of ecological roles. Some of these species came to function as decomposers—like earthworms, which are not native to the islands—and fulfill the essential ecological role of breaking down detritus.”

Christie Wilcox, National Geographic

Along with this, their most critical service is controlling the abundance of fungi growing on the native trees. Their consumption of fungal spores protects ʻŌhi‘a trees from infection and controls the spread and reproduction of more dangerous fungal strains.

A Kāhuli resting on a leaf. Credit: David Sischo

The presence of invasive species in Hawai’i are the primary cause of the Kāhuli’s near extinction. Animals imported to the island such as pigs, goats, and deer degrade forest vegetation and fragment the snail populations. The invasive species with the most alarming effect on native snails is the predatory rosy wolf snail (Euglandina rosea), which was introduced to control the invasive giant african snail.

Hawaii’s Department of Agriculture deliberately introduced Euglandina rosea to the islands in an ill-advised bid to control another, previously introduced snail.”

Ed Yong, The Atlantic

Instead, the rosy wolf snail solely consumed the Kāhuli, which has resulted in drastic repercussions. Now, the Kāhuli are so endangered that their entire population is kept in a controlled environment in the hopes that these species will make a comeback.

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A Ka’huli feeding on fungal spores of an ʻŌhi‘a tree. Credit: USFWS

Maintaining a Healthy Forest

Since the introduction of the invasive rosy wolf snail to the Hawaiian Island Chain, the abundance of the Kāhuli has been rapidly decreasing. Their numbers are so few that spotting one in the wild indicates an area that has been less impacted by invasive species and habitat destruction.

Without the Kāhuli, native ecosystems are increasingly susceptible to fungal outbreaks and other threats like the spread of Rapid ʻŌhi‘a Death. As new research emerges, the impact of invasive species becomes clearer and clearer as the Hawaiian Islands as species teeter on the brink of extinction.

Featured Image: ʻŌhi‘a Trees Above a Gulch. Credit: Forest and Kim Starr

Sources:
The Atlantic, “The Last of its Kind”
Hawaii.gov, Snail Extinction Prevention Program
Yeung, N. & Hayes, K., Biodiversity and Extinction of Hawaiian Land Snails: How Many Are Left Now and What Must We Do To Conserve Them—A Reply to Solem (1990)
National Geographic, Lonely George the tree snail dies, and a species goes extinct
University of Hawai’i, Manoa,
Big Island Now

About Nicholas Scott

Nick is an undergraduate Marine Biology student at the University of California, Santa Cruz. Having spent his life exploring the ocean on California’s coast, he developed a passion and respect for it, that demanded him to pursue his interest in conservation. Volunteering for the communications team allows him to further his interests and gain more insight as to how to resolve the state of the world around us. In his spare time, Nick enjoys surfing, swimming, and spending time with friends.

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