http://www.niwascience.co.nz/pubs/wa/11-1/mangroves
Anne-Maree Schwarz
The spread
of mangrove forests � currently seen by some as a problem in some North Island
estuaries � is by no means a global trend.
The value of mangrove (mānawa) ecosystems
has recently been the topic of public debate in New Zealand, with discussions
stimulated by the expansion of mangrove growth in some North Island estuaries.
An inevitable question is asked when people are considering the rate of spread
of mangroves in New Zealand: �Is the same thing happening in other places
around the world?�
In this article we take a global perspective
and ask how changes in mangrove distribution and related values in New Zealand
compare to those elsewhere.
Trends:
New Zealand
As a first step, understanding the extent of
mangrove spread in North Island estuaries is not straightforward. The Land
Cover Database (Terralink, NZ Ministry of Agriculture and Forestry) estimated a
total area of mangroves of 22,200 ha for 1996/97. Various initiatives are
underway to update this estimate. Although changes in some estuaries have been
mapped from aerial photographs no accurate estimate of total change is
available.
The value placed on mangrove ecosystems in New
Zealand has had a chequered history. While Māori traditionally utilised
both mangrove trees and the fish and shellfish from within mangrove forests,
during the 20th Century mangroves were often not highly valued by an
expanding New Zealand population. The State of New Zealand�s Environment Report
(1997) describes how �...seagrass and mangrove ecosystems have declined this
century as a result of widespread modifications to estuaries caused by
activities such as infilling for agriculture, rubbish disposal and commercial
land development�.
Changing attitudes toward the end of the
century are reflected in the New Zealand Coastal Policy Statement (1994), which
described a national policy for protecting ecosystems unique to the coastal
environment and vulnerable to modification, including mangroves. Around that
time there was a move towards conservation and education initiatives and
investigations into means of replanting damaged mangrove forests. Now, less
than a decade later, local authorities are being presented with applications
for Resource Consents to remove mangroves from some areas.
In 1997 the �World Mangrove Atlas� estimated
the total global area of mangroves at ~18 million hectares. However, accurate
current estimates remain elusive, complicated by a 2002 estimate by the ITTO
(see panel) that, during the last decade, approximately 100,000 ha of mangroves
have been destroyed annually. Notably, a survey in 1994/95 to study the global
conservation status of mangroves found that �no mangrove areas were actively
expanding�. (New Zealand was not included in the survey.) However there are
examples from specific locations (for example, in Mozambique and Thailand)
where, despite an overall reduction in mangrove area in the region, some bays
have seen an increase in mangrove coverage. In both these cases, the expansion
has been related to increases in sediment loading from the catchment.
Mangroves occur at the interface between land
and sea and are encroached upon by competing land-use, as well as being
indirectly affected by catchment development. From this perspective, the same
general considerations for management of mangrove ecosystems apply globally. So
what are the values and how do they differ from place to place?
There are over 60 species of mangroves
worldwide, of which about a quarter are of the genus Avicennia. Avicennia marina (grey mangrove) has the widest
latitudinal distribution and is the only species of mangrove represented in New
Zealand. Like all mangroves, Avicennia
forests contribute to marine food webs through production of detritus, and
several marine organisms spend all or part of their lifecycles there.
Mangroves provide livelihoods for millions of
people throughout the world thereby providing tangible economic benefits. The
cases summarised below (with examples of relevant countries) include values
that, due to exploitation, can also contribute to mangrove decline:
As recently as the 1970s New Zealand�s
mangrove forests were being reclaimed for various types of land development
including roading, oxidation ponds and agriculture, and were also used as tip
sites. There is no mangrove timber industry here, but mangroves continue to
support ecological values, traditional Māori values and other community
values.
Of other human activities that have indirect
effects on these values, catchment management is probably of most relevance in
New Zealand.
Here, recent increases in mangrove area have
been attributed at least in part to accelerated erosion of catchment sediments
into our estuaries, providing more suitable conditions for mangrove
colonisation. Compare this with the situation in some other countries, where
catchment development has sometimes had the effect of reducing the area of mangroves. In
Micronesia, India, Pakistan and Brazil, for example, damming of rivers has
reduced sediment load, starving mangrove forests of sediment. Elsewhere in
India, extreme siltation has changed the path of freshwater inflows so that
mangroves no longer receive sufficient tidal flushing.
As in New Zealand, attitudes to exploitation
of mangrove forests have changed in other parts of the world. Partly because of
the global decline in mangroves (noted above), in some places there is now a
strong emphasis on conservation, sustainable management and restoration. The
ITTO Mangrove Workplan (2002) stated:
�it is widely
believed that after total felling a [mangrove] forest will regenerate
spontaneously. On the contrary this happens under very special conditions and
usually with human assistance.�
Because of this, attempts to establish
nurseries and re-plant mangroves are the focus of a projects
in many regions, including the USA, Thailand, and Latin America (e.g.,
Colombia) as well as closer to home in Fiji.
It seems there is no hard and fast rule about
spread or decline that can be applied to all mangrove ecosystems. There are
places where mangrove forests are relatively untouched, are in dramatic
decline, have been completely destroyed, are spreading seaward into estuarine
areas, or, in places that are affected by sea-level rise, are spreading
landward onto adjacent river plains. Nevertheless, it is generally accepted
that on a global scale the area of mangroves is in decline and that mangrove
ecosystems are under threat.
Teachers: this article
can be used for Biology L7 A.O. 7.3a, and NCEA AS 2.4, 2.5 and 2.9. See other curriculum
connections at www.niwa.co.nz/pubs/wa/resources
Thus, with few exceptions, the trend for
spread of mangroves that has been recently observed in some of our estuaries
does appear to be a relatively local phenomenon at present. This is mostly a
result of different values and different degrees of catchment and coastline
development, all of which are likely to continue to change in the future, not
only in New Zealand but also elsewhere in the world.
NB: the PDF of this article includes
additional illustrations.
International
Tropical Timber Organisation (ITTO)
Many different research and interest groups around the world are working on mangrove management and conservation. One of these is the International Tropical Timber Organization, which counts mangrove timber amongst its global products. In 2002 ITTO organised a workshop in Colombia, South America, providing for a New Zealand representative (from NIWA) to attend. The objective of the workshop was to draft a mangrove work plan to guide ITTO members in the development of projects for sustainably managing and conserving their mangrove resources. While New Zealand is a consumer of tropical timber rather than a provider, the presentations made at that meeting provided a useful background for a general discussion of the �global� status of mangroves.
Further reading
Bradstock, M. (1989). Between the tides. David Bateman Ltd, Auckland. 158 p.
Green, M.; Ellis, J.; Schwarz, A.; Green, N.; Lind, D.; Bluck, B. (2003). For and against mangrove control. NIWA Information Series No. 31. 8 p.
Nicholls, P.; Ellis, J. (2002). Fringing habitats in estuaries: the sediment�mangrove connection. Water & Atmosphere 10(4): 24�25.
Walsby, J.; Torckler, D. (1992). Forests in the sea. New Zealand Geographic 15: 40�64.