A review of invasive plant management in Special Ecological Areas, Hawai‘i Volcanoes National Park, 1984-2007

Abstract

In Hawai'i Volcanoes National Park (134,852 ha), management units called Special Ecological Areas (SEAs) were established to control 20+ highly disruptive invasive plant species perceived to be too widespread for parkwide eradication to be feasible. Instead control efforts were focused on excluding target weeds from high value areas. SEAs were prioritized for intensive weed management based largely on their 1) representativeness of a particular ecological zone or rarity of vegetation type, 2) manageability, areas are accessible, relatively intact and the potential for native species recovery is high, 3) species diversity and rare species, and 4) value for research and interpretation. Also important to the SEA concept was its flexibility. So while initial weed control focused on only a handful of the best areas, the number and size of units were expanded as additional resources were made available.Between 1984 and 1986 the first six SEAs and a buffer unit (total area >5,000 ha) were established in wet 'ōhi'a/hapu'u (Metrosideros polymorpha/Cibotium glaucum) forest, mesic koa (Acacia koa) forest, and seasonally dry 'ōhi'a communities (Tunison and Stone 1992). After initial treatment of weeds, crews revisited sites at one to five year return intervals to remove any new weeds that reestablished.Over the next two decades, additional funding was made available to increase the number and size of SEAs. By 2007, 27 SEAs and buffer units covering over 26,720 ha were managed to exclude target weeds. These included several more degraded areas that connected isolated units and served as buffer areas to reduce seed dispersal of weeds into adjacent SEAs that were more intact. Control data of 10 SEAs for which we had the longest data sets were evaluated. Typically, initial control of large numbers of weeds (knockdown phase) was followed by subsequent revisits to keep infestations at low or manageable levels (maintenance phase) in SEAs. This was accompanied by a drop in labor cost as fewer worker days were spent searching and removing individuals from a management area. At the maintenance phase, wet forest SEAs still required intensive foot sweeps to search for plants, and were the most expensive to retreat ($385/ha in 2007). Seasonally dry 'ōhi'a communities that employed a combination of aerial and foot sweeps were the least expensive (<$2/ ha including the cost of helicopter rental). From 1985 to 2007, control efforts in SEAs expanded from 5,045 ha to 26,687 ha, a 500% increase in area. In contrast, annual labor cost spent in the field increased only about 50% (adjusted to 2007 dollars). This translated to a three-fold decrease in labor costs per hectare ($28.96/ ha in 1985 to $8.61/ha in 2007) across all units. Additional cost savings were made by improving the efficiency in search and treatment methods (e.g. aerial spray rig, chemical treatment of kāhili ginger).In summary, long-term maintenance of SEAs was possible when initial weed infestations could be reduced to low levels; subsequent recruitment of new alien weeds was low; and work loads dropped significantly after initial control efforts. Weaknesses of the SEA approach were that follow-up treatment was required indefinitely, weed infestations could increase in surrounding unmanaged areas and reinvasion into units could become unmanageable especially for small SEAs. In the future, managers will be challenged to secure funding to address ongoing weed maintenance; and maximize program effectiveness (e.g. optimizing intervals between follow-up treatments, applying new search and control technology). Developing effective partnerships with the community and adjacent landowners to expand management areas, creating buffer zones that will reduce seed dispersal into SEAs, and reconfiguring or abandoning small or ineffective SEAs are among some of strategies that could assist the long-term sustainability of the SEA approach.