Poor Management Practices Accelerating Mangroves’ Growth

Resource management practices intended to protect waterways from invading mangroves are in fact leading to their total demise, according to a comprehensive report released this week in Auckland by the Centre for Resource Management Studies.

The report – Mangroves and Estuarine Ecologies – measures the impact of the explosive growth of mangroves on the harbours and estuaries of the northern half of the North Island.

Its conclusions challenge conventional wisdom regarding the growth and management of mangroves in New Zealand – including the assumption that increased sediment flows are a major cause of mangroves’ abundance. Mangroves are thriving and expanding most where sediment run-off is being reduced by planting and improved catchment management.

Rather, the report identifies the causes of the current explosive expansion of mangroves to be:

- continued warming of estuarine waters in the second half of the 20th century when temperatures have favoured the mangrove;

- removal of cattle and sheep, which graze on young mangroves, from the edges of harbours, tidal rivers and estuaries;

- increased supply of nutrients to estuarine waters from aerial topdressing since the 1950s; and

- reduced flushing flows caused by construction of causeways, railway lines, bridges, and dams.

“Those who administer the RMA now face a dilemma,” says Owen McShane, Director of the Centre for Resource Management Studies. “Decision makers must accept that current programmes to protect waterways are in fact generating a greater need to manage mangroves in those same waterways. Otherwise whole estuarine habitats will be lost to mangroves as massive reclamations of water to land occur as mangroves die and their beds turn into dry land,” he said.

While reducing nutrient flows may slow mangrove growth, the removal of cattle, sheep and other predators means that mangroves will continue their current explosive expansion, unless something replaces the impact of browsing livestock, the report found.

The report is based on studies of the Kaipara Harbour, which has environmental features common to many other areas in New Zealand - low population density, scattered residential areas round rivers, harbours, estuaries and inlets. The report rejects the effectiveness of “community based management plans” in such locations, as proposed by many Councils and organizations, because of the general absence of large communities and the consequent lack of resources to implement such comprehensive plans in these more remote areas.

Environmental regulation should be based on good science combined with local knowledge and observation – not regulatory regimes promoted by people with no experience in management “on the ground” and who have made no attempt to “ground proof” their theories.

The Centre commissioned the report to promote debate and encourage similar assessments of the impact of mangroves on other local environments.

- ENDS -

Copies of the full report are available from the Centre on request.

Contact

Owen McShane

Director, Centre for Resource Management Studies

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