Most of the permanent residents at Heard Island and McDonald Islands (HIMI) go unnoticed when the occasional human visitors arrive.
Although many people would consider the seals, flying birds and penguins more photogenic, they are greatly surpassed in terms number of species and total numbers by the terrestrial, freshwater and coastal invertebrates.
Even so, there are fewer terrestrial invertebrate species on Heard Island compared to other Southern Ocean islands. This is largely due to the same reasons as the low floral diversity – the island’s isolation and limited ice-free areas because of extensive glaciation.
HIMI also has few, if any, (successful) human-introduced species, although two species of invertebrate – one thrip and one mite – are thought to be recent, possibly natural, introductions.
The recorded arthropods at Heard Island comprise 54 mite and tick species, one spider and eight springtails. The non-arthropod invertebrate fauna are poorly known.
Beetles and flies dominate the known insect fauna, which comprises up to 21 species of ectoparasitic insects (associated with birds and seals) and 12 or 13 free-living insects.
Approximately half of the free-living insects are habitat-specific, while the remainder are generalists found in a variety of habitat types, being associated with either supralittoral or intertidal zones, stands of particular species of plant (e.g. Poa cookii and Pringlea antiscorbutica), bryophytes, lichen-covered rocks, exposed rock faces or under rocks.
There is a pronounced seasonality in the insect fauna, with densities in winter months dropping to a very low percentage (less than 5%) of the summer maximum.
Distinct differences in relative abundances of species among habitats has also been shown, including a negative relationship between altitude and body size for Heard Island weevils.