The hardy vegetation found at Heard Island and McDonald Islands (HIMI) has overcome the island’s extreme isolation, harsh climate and limited ice-free ground available for colonisation.
The islands provide a rare terrestrial home in the Southern Ocean for plants to grow. Vegetation covers almost 20 square kilometres of Heard Island, mostly in coastal areas at low elevations.
The diversity of plants is low, and HIMI has the smallest number of vascular (flowering) plant species of any major subantarctic island group – 12. One of these species, Leptinella plumosa, was only discovered for the first time during the 2003/04 Australian Antarctic program expedition.
Heard Island is also the largest subantarctic island with no known human-introduced plants, and there is little human activity on the island, so its terrestrial ecosystems are close to pristine. Some other islands have been significantly impacts as a result of both introduced plants and animals.
A vascular plant is one which has a system of vessels which transport water and nutrients between different parts of the plant (e.g. from the roots to the leaves).
At Heard Island and McDonald Islands (HIMI), there are no trees or ferns and the most advanced vascular plants are low-growing herbaceous flowering plants.
The vascular flora on Heard Island is the smallest of any major subantarctic island group, reflecting the island’s isolation, limited ice-free area and severe climate.
Twelve vascular species are known from Heard Island, of which five have also been recorded on McDonald Island. A single specimen of one of these species, Leptinella plumosa, was recorded for the first time during the 2003/04 Australian Antarctic program expedition to Heard Island.
The vascular vegetation covers a wide range of environments and, although only six species are currently widespread, glacial retreat and the consequent connection of previously separate ice-free areas is providing opportunities for further distribution of vegetation into adjacent areas.
None of the vascular species is endemic to the HIMI.
- Acaena magellanica
- Azorella selago
- Callitriche antarctica
- Colobanthus kerguelensis
- Deschampsia antarctica
- Leptinella plumosa
- Montia fontana
- Poa annua
- Poa cookii
- Poa kerguelensis
- Pringlea antiscorbutica
- Ranunculus crassipes
Non-vascular plants are those plants that do not use a system of vessels to transport water and nutrients between different parts of the plant.
Bryophytes (mosses and liverworts) are one kind of non-vascular plant and comprise a substantial component of the Heard Island flora, in terms of number of species, cover and biomass.
Bryophytes are found in most of the major vegetation communities and often occupy habitats unsuitable for vascular plants, such as cliff faces. A total of 44 moss and 12 liverwort species have been recorded, but more are likely to be added to these totals once recent collections have been identified.
Lichens are another kind of non-vascular plant and are common at Heard Island in all habitats with exposed rock. In fact, lichens dominate the vegetation in some areas. Thirty four species have been recorded on Heard Island, including several soil and moss-inhabiting species.
Six vegetation communities are currently recognised at Heard Island:
- Open cushion carpet
- Mossy feldmark
- Wet mixed herbfield
- Coastal biotic
- Salt spray
- Closed cushion carpet
Numerous other vascular plant species and vegetation communities and species found on other subantarctic islands north of the Antarctic Polar Frontal Zone are missing from Heard Island, although it is possible that some may colonise the island if climate change produces more conducive conditions.
Open cushion carpet vegetation
This is the most widespread and abundant vegetation community found on Heard Island. It is characterised by Azorella selago cushions interspersed with bryophytes, small vascular species and bare ground (20–75% cover). It is mostly found at altitudes of 30-70 metres above sea level.
Mossy feldmark vegetation
Species richness is high in this community, which consists of bryophytes and small Azorella selago cushions and is found at medium to high altitudes (30–150 metres above sea level) in areas with intermediate exposure.
Wet mixed herbfield vegetation
Wet mixed herbfield vegetation occurs on moist substrate, mostly on moraines and moist lee slopes (often in association with burrowing petrels colonies) at low altitude (<40 metres above sea level) where the water table is at or close to the surface. Species richness is the highest of all the communities, with dominant species being Poa cookii, Azorella selago, Pringlea antiscorbutica, Acaena magellanica, and Deschampsia antarctica.
Coastal biotic vegetation
Coastal biotic vegetation is dominated by Poa cookii and Azorella selago, and occurs mainly on coastal sites of moderate exposure and in areas subject to significant influence from seals and seabirds.
Salt spray vegetation
This vegetation community is dominated by the salt-tolerant moss Muelleriella crassifolia and is limited in extent, being found at low elevations (less than 5 metres above sea level) on lavas in exposed coastal sites.
Closed cushion carpet vegetation
Closed cushion carpet vegetation is found on moraines and sand at altitudes mostly below 60 metres above sea level. It can be subject to some burrowing by seabirds, and is dominated almost entirely by Azorella selago cushions, which often grow together to form continuous carpets.
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.
There is considerable evidence that the climate of Heard Island and McDonald Islands (HIMI) is changing.
The warmer trend observed at HIMI mirrors similar changes at the nearby Îles Kerguelen and elsewhere in the southern Indian Ocean.
These increased temperatures are having a significant effect on the terrestrial environment, with glacial retreat leading to the formation of lagoons and freshwater lakes, and exposing new land for colonisation by plants and animals.
Consequently, Heard Island has one of the most rapidly changing physical settings in the subantarctic.
The increase in available habitat for plant colonisation, in conjunction with the combining of previously discrete ice-free areas, has lead to dramatic changes in the vegetation of Heard Island in the last 20 years or so.
Some plant species are spreading and the structure and composition of plant communities is being modified.
It is considered likely that further changes will occur, and possibly at an accelerated rate. Changes in population numbers of seal and seabird species are also expected – this may affect the vegetation through changes to the nutrient regime and level of physical disturbance through trampling.
Scientists are studying the terrestrial ecosystems at Heard Island in an attempt to understand how it is likely to change if the climate at HIMI continues to warm, as is currently predicted.