Hairy leaves, vibrant flowers and a botanical mystery, megaherbs are ubiquitous on the Subantarctic islands. Flourishing in the alpine-like climate, these hardy plants can be found from sea level to the tops of cliff faces and summit ridge lines. Campbell Island boasts at least seven species from the four genera that have been documented and studied to date, including the vibrant purple Pleurophyllum speciosum, or Campbell Island daisy.
Megaherbs are through to be remnants of Antarctic flora, from before the last ice age when the southern polar regions were covered in temperate forests and grasslands. How they spread to the Subantarctic islands is still not fully understood, but they are certainly a robust and established part of island ecology. High humidity and precipitation, along with a relatively stable temperature range and a lack of grazing animals (at least before the early 1800’s), are thought to be the main drivers behind the megaherbs evolving to such epic proportions, with some species reaching over a meter high with enormous flowerheads, like the abundant Anisotome latifolia
Large, sometimes hairy and sometimes corrugated leaves are typical of megaherbs. Current theories suggest the leaves’ structure helps trap moisture in the often cloudy air, and the corrugations and ‘hairs’ trap warm air, keeping the immediate area under the leaves up to 15⁰C warmer than the prevailing temperature (Peat, 2003 p. 71). Brightly coloured flower heads are thought to catch more UV light and may attract the weevils and other small invertebrates that play a role in pollination.
Years of grazing by introduced sheep, rabbits, pigs and goats on Campbell, Auckland and Macquarie Islands impacted heavily on megaherbs including Stilbocarpa polaris, the Macquarie Island cabbage. Large, palatable leaves made this plant a choice fodder for grazing mammals and its fleshy rhizome was eaten by sailors and sealers as an antiscorbutic.
Not all megaherbs were attractive to the intruders. The thick strap-like leaves of Ross’s lily, Bulbinella rossii were unpalatable to the sheep on Campbell Island, leaving the distinct golden flower heads a dominant feature of the landscape during the time of the sheep runs.
In the cold, windy and isolated Subantarctic islands, megaherbs come as a sudden surprise. Vibrant, enormous flower heads scent salt-laden air with nectar and paint bright swathes through the fog. Evolved to thrive in acidic peaty soils that have defied all attempts at cultivation, these charismatic plants are bouncing back from 2 centuries of impacts to reclaim the islands and their place in the complex ecology of the Subantarctic.