Seabird-burrowed soils

Middle Island in the Mercury Group (Peter Bellingham)

These areas are where soils have been disturbed and enriched by seabirds burrowing for nesting, track formation, and guano (excrement). Bird colonies may be in the open with almost no vegetation and only a few nitrophilous plants (e.g. Disphyma australe), beneath low-growing scrub (e.g. Muehlenbeckia complexa) or tussock grasses, or beneath a low forest where the birds simply drop through the canopy onto the forest floor where the cover of vegetation is proportional to the amount of bird activity.

Notable flora and fauna

Threatened plants include the nationally critical native hibiscus (Hibiscus richardsonii), and coastal peppercress (Lepidium banksii); the nationally vulnerable coastal cress (Lepidium flexicaule), Lepidium naufragorum, nau (Lepidium oleraceum) and New Zealand water cress (Rorippa divaricata). Declining species include Cyperus insularis. Relict species includes orache (Atriplex australasica), rengarenga lily (Arthropodium bifurcatum), William’s broom (Carmichaelia williamsii), Chatham Island button daisy (Leptinella featherstonii) and large-leaved milk tree (Streblus banksii). Naturally uncommon species include Buchanan’s orache (Atriplex buchananii), sand brome (Bromus arenarius), Senecio carnosulus, Senecio marotiri, Mokohinau groundsel (Senecio repangae subsp. pokohinuensis) and Senecio repangae subsp. repangae. Vagrant species include Senecio australis and the coloniser species include devils horsewhip (Achyranthes velutina) and mawhai (Sicyos australis).

Many petrel species create burrows, including the range restricted Pycrofts petrel (Pterodroma pycrofti) and Westland black petrel (Procellaria westlandica), and gradually declining Cook’s petrel (Pterodroma cookii), and black petrel (Procellaria parkinsoni).

Tuatara (Sphenodon spp.) are present, sometime co-habiting with birds, on some islands. There are several forms: nationally endangered Brothers island tuatara (S. guntheri), range restricted Cook Strait tuatara (S. punctatus 'Cook Strait') and sparse northern tuatara (S. punctatus subsp. punctatus).

Other threatened vertebrates associated with seabird burrows include Whitaker’s skink (Oligosoma whitakeri) and the robust skink (Oligosoma alani).

Threat status

Critically endangered (Holdaway et al. 2012)

Threats

High-fertility sites are invaded by weeds such as barley grass (Hordeum spp.) and annual exotic herbs spreading from surrounding areas, including via bird dispersal. These ecosystems are vulnerable to serious depletions of burrowing birds and other native fauna due to predation by rats, cats, and stoats. Sites with sparse and low vegetation cover pose little risk of fire, but forested sites may burn. The seabirds themselves are also at risk from being caught in fishing trawlers and from declining fish stocks on which they feed. Invasive mammals like rats also predate the chicks.

Where do they occur?

These systems may not have been naturally uncommon, because they would once have been widely distributed on the New Zealand coast, as far inland as the mountains of Te Urewera, and as high as the slopes of the Seaward Kaikoura Range, where they still exist. Now they are largely confined to islands, from the far northern New Zealand to the subantarctic. There are at least 22 in the Auckland Region alone.