Recent Replacement of Native Pili Grass (Heteropogon contortus) by Invasive African Grasses in the Hawaiian Islands

Abstract

We surveyed 41 sites from throughout O'ahu that had been dominated by native pili grass (Heteropogon contortus) in the late 1960s. Pili grass was absent from 14 (35%) of those sites in 1997 and had declined in abundance in most of the 27 remaining sites, relative to the late 1960s. The pili grass communities have been replaced by communities dominated by one of three African grasses: Cenchrus ciliaris (buffel grass), Pennisetum setaceum (fountain grass), or Panicum maximum (Guinea grass). Panicum maximum was often associated with the shrub Leucaena leucocephala, and Cenchrus ciliaris and Pennisetum setaceum communities showed little evidence of succession toward woody vegetation. Communities dominated by the African grasses were significantly less diverse, in terms of number of plant species, than the native pili grass-dominated communities. Observations made on other Hawaiian islands suggest that this rapid pili grass decline and replacement with alien grasses has not been limited to O'ahu. Research is needed to determine how higher-diversity native pili grass communities can be maintained in the Hawaiian Islands as a valuable natural and cultural resource.