Poa annua, a non-native species of plant at HIMI. Photo by K. Kiefer

Heard Island is the largest subantarctic island with no known human-introduced plants. Invasive introduced species (those which spread rapidly and displace existing vegetation) can have considerable consequences for the diversity of plants and invertebrates.

Presently, there is one plant species on Heard Island considered to be an 'alien' (non-native), Poa annua, a cosmopolitan grass native to Europe. The grass was initially recorded in 1987 in two recently deglaciated areas of Heard Island not previously exposed to human visitation, while at the same time being absent from known sites of past human habitation. Consequently, it is thought to have been naturally introduced, possibly from the Îles Kerguelen where it is widespread.

Since first recorded in 1987, the populations of Poa annua have increased markedly in density and abundance within the original areas and have expanded into new areas. The spread of the grass beyond the initial two deglaciated areas may be at least partially the result of expeditioners, but is probably mainly due to dispersal by wind and the movement of seabirds and seals facilitating further spread around the island.

The Management Plan and associated Environmental Code of Conduct for the HIMI Marine Reserve include strict quarantine measures to prevent the introduction and spread of alien species and disease to HIMI.