Macquarie Island’s total of 43 native vascular plant species is low by most standards – some Tasmanian National Parks of similar size have more than ten times this diversity.* But amongst the Subantarctic islands, none come close to Macca’s species richness. The Kerguelen islands (7,215 km2 of which 6,450 km2 is ice-free) in the southern Indian Ocean have a land area around 50 times the size of Macca (128 km2), yet only 30 native plants. Other Subantarctic islands support even fewer species – Australia’s Heard Island for example has just eight. Further south, in the Antarctic proper, only two vascular plants survive in the harsh climate.
The theory of island biogeography suggests that species richness on an island is a function of island size (land area) and isolation (distance from nearest major landmass). Amongst the Subantarctic islands, area does predict plant species richness, but isolation does not. However temperature is as important or more so – the plant species richness declines with temperature. A third factor that may also be influential is glacial history, because it seems that vascular plants were wiped out during glaciations on some of the more southerly islands, such as South Georgia, and therefore needed to recolonise those islands following each glacial period. Isolation is perhaps not so important in the Subantarctic where every island is very isolated and the typical plant species are not limited by dispersal ability.
Macquarie Island, having no history of glaciation and a slightly milder climate than other Subantarctic islands, therefore has a relatively high richness of vascular plants, despite its modest size.
Not surprisingly, definitions come into play here: if we are to generously include New Zealand’s so-called ‘Subantarctic’ islands (arguably not Subantarctic because they support trees), then the numbers rocket up to 188 for the Auckland Islands.
But that’s just vascular plants; like the temperate rainforests of Tasmania, Macca’s non-vascular flora dwarfs the angiosperms and ferns in species richness. There are at least 75 mosses, 60 liverworts and 55 lichens known from the island.
Macquarie Island’s native vascular plants – 22 dicots, 16 monocots and 5 pteridophytes – along with 3 introduced species are all illustrated in an online key.
* Tasmania’s Maria Island National Park has over 470 native vascular plants in an area slightly smaller than Macquarie Island; Douglas-Apsley National Park, a little larger than Macquarie Island at 160 km2, has over 270 native vascular plants, while the 169 km2 Freycinet NP has over 500 species.