Pacific Island Ecosystems Research Center
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Ant invasions of mamane-naio forest at high elevations on Mauna Kea. |
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John W. Slotterback, Peter T. Oboyski, and Paul C. Banko. USGS-Biological Resources Division, Palila Restoration Project, Hawaii Nat'l Park, HI |
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Introduction Ants are not native to Hawaii and are a significant threat to endemic insects which have not evolved in their presence. We study the palila, an endangered honeycreeper that inhabits the dry subalpine slopes of Mauna Kea. The ants are a threat because they are potential predators of insects upon which the Palila feed. Methods We sampled ants by baiting along transects that ran from the Saddle Road (1850-2000 m elev) to approximately 2400 m up the southwest slope of Mauna Kea (Fig. 1, 2).  
Transects were spaced 500 m apart near the Saddle Road and stations spaced every 200 m along each transect. Using a method similar to Wetterer et al. 1998, we placed three baited plastic disks (Fig. 3) within 10 m of each station, and placed approximately 2 g of oil-packed tuna and 2 g of honey on each disk. We revisited stations after 2-4 hours to record the species and number of ants visiting each station. We conducted three surveys: Summer 1999, Winter 1999, and Summer 2000. Results Four species of ants were detected at bait stations throughout the study area: Argentine ant (Linepithema humile), cardiocondyla ant (Cardiocondyla venustula), big-headed ant (Pheidole megacephala), and pharaoh ant (Monomorium pharaonis) (Fig. 4).  Argentine ants were most commonly found. Cardiocondyla ants were found at the highest elevations. Big-headed ants were found only on the western part of study area. Pharaoh ants were detected at four stations in the flats along Saddle Road. There were no ants found in a large section in the middle of the study area, except for a few cardiocondyla ants in Winter 1999. The ant distributions in the west were very similar in all three surveys. However, in the east the Argentine ants were detected at 33 additional stations during Winter 1999 than during Summer 1999 (Fig. 5).  Also, in the east part of the study area, there were no cardiocondyla ants detected in Winter 1999 but this increased to 10 stations in Summer 2000. Discussion The Argentine ant and cardiocondyla ant are of special concern for the dry, subalpine zone of Mauna Kea. Both species are able to invade the higher elevations and threaten insect communities. Argentine ant queens are wingless (Fig. 6) which causes colonies to advance along a broad front.  Cardiocondyla ant queens are winged, enabling this species to distribute more widely. Central areas along the Saddle Road which are currently under drought conditions (Fig. 7) were perhaps unable to support ant populations because of the absence of live vegetation and associated insects.  Future Direction We plan to continue biannual ant surveys to monitor changes in the distribution of ants. In addition, we will use pitfall traps and under-rock surveys to compare arthropod fauna inside and outside ant infested areas. A continuing question is: why are ants found in some areas and not in others? |
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://biology.usgs.gov/pierc/
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