Coastal rock stacks
Coastal rock stack, western Southland (Susan Wiser)
A coastal rock stack is an isolated pinnacle of rock off the coast, generally with steep to vertical sides (Priestly 1990). The bases of stacks occur below the high tide line. Some are always surrounded by water, others are surrounded by sand at low tide. Their summits are well above the high tide line and above the storm surge line (over 10m in height). In big storms, however, waves may crest over the tops.
Where do they occur?
Coastal rock stacks are widespread in New Zealand wherever elevated topography abuts the coast, and absent only from long, sandy or shingly stretches like the central Bay of Plenty and South Canterbury Bight.
Notable flora and fauna
Threatened plants include the nationally vulnerable Cook’s scurvy grass (Lepidium oleraceum), coastal cress (Lepidium flexicaule), Lepidium naufragorum, Chatham Island forget-me-not (Myosotidium hortensia), mutton bird plant (Leptinella featherstonii) and Pimelea tomentosa. Relict species include guano groundsel (Senecio sterquilinus). Nationally uncommon species include Buchanan’s orache (Atriplex buchananii), Cox’s fescue (Festuca coxii), Senecio carnosulus, Three Kings rangiora (Brachyglottis arborescens), coastal tussock (Chionochloa bromoides) and Macquarie Island saltgrass (Puccinellia macquariensis). A declining species is shore spurge (Euphorbia glauca).
Threatened fauna include the Mokohinau skink (Cyclodina mokohinau) and a stag beetle (Geodorcus ithaginis). Coastal rock stacks may support breeding sites for threatened seabirds, such as the light-mantled sooty albatross (Phoebetria palpebrata), Salvin’s albatross (Thalassarche salvini) and the Chatham albatross (Thalassarche eremita), and less threatened seabirds such as the common diving petrel (Pelecanoides urinatrix), grey-faced petrel (Pterodroma macroptera), red-billed gull (Larus novaehollandiae), white-fronted tern (Sterna striata), and the spotted shag (Stictocarbo punctatus).
Threat status
Not threatened (Holdaway et al. 2012)Threats
Most coastal rock stacks are protected by their inaccessibility to humans. Weed invasion can occur, however, promoted by instability of cliffs and seed transport by wind and birds. Native plants, however, tend to dominate.
Further reading
Clarkson BD, Bylsma RJ, Efford JT, Clarkson BR, Kirby CL 2014. Vegetation change (1968-2013) on a small rock stack adjoining Slipper Island (Whakahau), Coromandel, New Zealand. New Zealand Journal of Botany 52: 453-459.
Esler AE 1978. Botanical features of islands near the west coast of the Coromandel Peninsula, New Zealand. New Zealand Journal of Botany 16: 25-44.
Priestly R 1990. New Zealand landform inventory: second approximation. Research School of Earth Sciences Occasional Paper No. 4. Victoria University of Wellington.
Taylor GA, Lovegrove TG, Miskelly CM, McFadden I, Whitaker AH 1990. An ecological survey of small islands in the Mercury Group. Tane 32: 151-167.