Ecological and archaeological consequences of ongoing, largely human-mediated colonisation of endemic soft, shelly habitats by mangroves in a northern New Zealand embayment

ABSTRACT

Areal extent of native mangroves, Avicennia marina, has doubled in Bay of Islands since 1951. This expansion is widely attributed to deforestation and poor land-use practices beginning in the late 1800s to early 1900s, and led to greatly transformed intertidal and supratidal habitats. One example has been the ecological isolation and crowding (sometimes, colonisation) by mangroves of shelly beaches, together with their associated ‘chenier-like’ spits – themselves now among the rarest of New Zealand’s inshore marine habitats, with only six still relatively intact ecologically out of at least 44 in the Bay of Islands. These shores were also where pre Contact Māori often gathered to harvest intertidal cockles Austrovenus stutchburyi and to fish, archaeological signatures typically becoming obscured once mangroves colonised upper shores. Although recent seaward intrusion by mangroves into the cockle habitat of the Parekura Bay study site appears to have been proportionately small, there may be potential for more as elevated levels of sedimentation persist. With wholesale mangrove removals demonstrably ineffective in re-establishing original communities, the ongoing ecological and archaeological disruption of endemic soft, shelly contexts through largely human-mediated mangrove colonisation might be curbed in certain situations through strategic, proactive removal of new mangrove recruits, particularly on expanding edges.

KEYWORDS:

• Bay of Islands
• chenier
• cockle
• intertidal flat
• Māori
• midden
• Parekura Bay
• sedimentation
• shelly shore
• shorebird‌

Acknowledgments

Thanks Carolyn Lundquist (NIWA), and Paul Maxwell (NRC) for comments, advice and recommendations on earlier versions of this paper. We are grateful for insightful korero with shorebird experts Drs John Dowding and Murray Williams. We appreciated access to local museum collections – the Booth Whanau Collection (Museum of Waitangi), the Rewha Whanau Collection (Rawhiti), and the Waller Family Collection (Te Uenga). Dr Webber Booth kindly sorted many of the cockle samples. Two anonymous referees – one from overseas – made suggestions that led to improvements to the manuscript. Of particular significance was their help in our sorting fact from (albeit qualified) speculation, and in getting us to better clarify and articulate what it was that made the process of mangrove areal expansion in the Bay of Islands different to that reported for more-southerly locations.

Disclosure statement

No potential conflict of interest was reported by the author(s).