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Invasive Species in Hawai‘i

Invasive species removal in the Ko‘olau Mountains, Hawai‘i

This November, East-West Center Fellows and Pacific RISA researchers Victoria Keener (third from top right) and Laura Brewington (bottom, right) worked with the Ko‘olau Mountains Watershed Partnership on Oahu Island to remove invasive species and replant endemic kawelu grass at Konahuanui, the highest region of the Ko‘olau mountain range.

In Hawai‘i, freshwater and forest resources are inextricably intertwined. Rainfall and fog drip captured by forests supply surface stream water and groundwater from the ridge to the reef, along natural geographic boundaries. For centuries, native Hawaiians practiced a system of governance and land tenure that was based on fundamental watershed units, known as the ahupua‘a. Abolition of the ahupua‘a system in favor of privately and publicly-owned lands disenfranchised many of the islands’ original inhabitants and facilitated extensive livestock grazing that would denude native forests, directly impacting water supply. By the late 19th century, cattle, goats, and deer had almost totally destroyed large areas of forest on every inhabited island. Alarmed by dwindling water supplies, well-meaning foresters attempted to bring groundcover species from elsewhere in the world that would thrive in the eroded, hardened soils, unintentionally leading to widespread invasion of weeds, insects, and disease. Their efforts were compounded by horticulturists who populated nurseries and botanical gardens with introduced species that outcompete the specialized native Hawaiian flora. It’s not only plants and animals that threaten the islands: introduced invertebrates and diseases, like the rosy wolfsnail (Euglandina rosea, pictured below), have decimated endemic highland species archipelago-wide. Over the past two centuries, entire Hawaiian ecosystems have been replaced by invasive species that are once again putting the islands’ freshwater supply at risk. When plants like strawberry guava (Psidium catleianum) and miconia (Miconia calvescens) invade, they convert the complex forest into a simpler ecosystem structure with little to no water-absorbing understory, while using up to 25% more water than native forest plants.[1]

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The rosy wolfsnail is a predatory snail that was introduced intentionally to Hawai‘i to control growing populations of the giant African snail, but also attacked the islands’ endemic tree snails, some to extinction. This shell was found at Konahuanui, in the Ko‘olau Mountains Watershed.

Hawai‘i’s changing climate exacerbates these concerns with declining rainfall, higher rates of evapotranspiration, and average annual temperature increases in the last 30 years – especially at elevations half a mile above sea level and more. Higher elevation ecosystems are already under pressure from development and land use changes that force upward range migrations by native species.[2] Currently only 10% of the watershed areas in Hawai‘i are officially protected, but land owners and members of public and private institutions have formed watershed partnerships, like the one pictured above, to preserve freshwater availability in native forests and facilitate adaptation to climate change. Because of the feedbacks present between climate change, species introductions and spread, and freshwater, the 2010 Hawai‘i Department of Land and Natural Resources (DLNR) considers upland forests and watersheds to have the greatest potential to mitigate the effects of regional climate change and promote island resilience. On Maui Island, Pacific RISA continues to coordinate with government agencies, the watershed partnerships, and the Maui Invasive Species Committee (MISC) to assist in mapping and modeling efforts at both the species level – to target areas for restoration; and the ecosystem level – to understand biodiversity patterns under future change scenarios for large-scale planning.

Find the Ko‘olau Mountains Watershed Partnership on Facebook https://www.facebook.com/pages/Koolau-Mountains-Watershed-Partnership-KMWP/114315495268556?ref=br_tf

For more information on Hawai‘i watershed management and invasive species, see:

  • Hawaii Department of Land and Natural Resources (DLNR): http://dlnr.hawaii.gov/
  • Hawai‘i Association of Watershed Partnerships (HAWP): http://hawp.org/
  • Hawai‘i Invasive Species Committees (ISCs): http://www.hawaiiinvasivespecies.org/iscs/

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