Opinion: Dr Sharon De Luca on the value of mangroves

The values of mangroves in New Zealand are often misrepresented and misunderstood, and this has led, in some cases, to mismanagement and unanticipated adverse ecological outcomes. 

Public perceptions of mangroves are often polarised, with a strong and vocal contingent believing mangroves are an unwanted nuisance and that their spread is unacceptable. Some people complain that mangroves are an unnatural invader that must be removed.

Others understand the environment doesn’t stand still; and that mangroves are native, have important ecological values, and play critical roles in coastal protection and storage of carbon.

Last year, the Thames-Coromandel District Council (TCDC) and Hauraki District Council (HDC) proposed a bill that provides for a community-based process to enable mangrove removal, avoiding the Resource Management Act (RMA). The Bill passed its first reading in Parliament, opposed only by the Green Party. Submissions were sought, and heard by a Parliamentary Select Committee in mid-March.

The Bill was developed in response to the frustration that some residents and Councillors feel about the time and cost associated with applying for consent to remove mangroves under the RMA. This is primarily because the process of obtaining a resource consent is sometimes not quick or simple, especially when contentious on environmental, ecological, landscape, natural character and economic grounds.

While some might argue getting rid of mangroves would improve the look of the shoreline, in ecological terms the outcome is usually bad for our marine environment and birds. Arguments put forward in support of mangrove removal typically include emotive language and flawed ecological statements.

In the case of TCDC and HDC, loss of shorebird feeding habitat at the internationally significant Firth of Thames RAMSAR site is mentioned as a reason, with international migrant bird species, such as godwits, framed as being in grave danger of running out of food. However, published data on bird abundance and the extent and quality of available feeding habitat tells us that the estuaries and harbours where mangroves exist are nowhere near their bird population carrying capacity. This means there is far greater availability of food and habitat than there are birds to use them.

Not only are these migrants not at risk, but recent Waikato Regional Council publications indicate that a large variety of Threatened and At Risk birds actually use mangroves or habitats fringing mangroves for foraging and/or protection from disturbance; including New Zealand fairy tern, white heron, Australasian bittern, pied shag, North Island fernbird, pied oystercatcher, pied stilt, eastern bar-tailed godwit and banded rail. This is rarely mentioned in the debate.

There is no doubt that mangroves are spreading in some areas in response to a range of factors; and it seems that some people just don’t like looking at mangroves and resent the fact that mangroves have appeared in their harbours and estuaries, perhaps in places where historically they were not. However, the establishment and spread of mangroves is a natural phase in the life-cycle of many estuaries, which has been accelerated by unnaturally high sediment runoff. In contrast, sea level rise over time will cause mangrove areas to decrease.

A recent Environment Court decision regarding mangrove management provisions in the Bay of Plenty concluded that mangroves have ecological values and provide valuable ecological habitat. The decision also clearly states that removal of mangroves has no ecological benefit, and that a change from a habitat previously not inhabited by mangroves, to one inhabited by mangroves, is merely habitat change; and it does not follow that one habitat is better or worse than the other. The science research to date supports this view, and the Judge drew on that science to reach a decision, which also stated that a mere public dislike of mangroves is insufficient justification for removal.

The TCDC/HDC bill before Parliament supports a different view. If the Bill passes, it could very well lead to widespread clearance of mangroves and significant adverse effects on marine environments and birds in the Thames-Coromandel and Hauraki Districts.

Maybe if more waterfront residents took up bird-watching, our mangroves would be better appreciated.

For further information please contact Dr. Sharon De Luca

20 April 2018