Porirua's ecological sites inventory
"We are made wise not by the recollection of our past, but the responsibility for our future."
- George Bernard Shaw.
View of Kakaho Estuary.
Photo by Keith Calder, 2009.
Background, objectives and scope
In May 2000 the Porirua City Council commissioned a survey of ecological sites in Porirua City and assessment of their management requirements. The Council wished to enhance its capacity to meet its repsonsibilities under the Resource Management Act (RMA) as set out in sections 5, 6 and 7, in particular s 6(c) "the protection of areas of significant indigenous vegetation and significant habitats of indigenous fauna." It also wished to be in a good position to implement the planned National Policy Statement on biodiversity and generally to contribute to the implementation of the New Zealand Biodiversity Strategy.
The objectives of the project were as follows:
- Identify and survey ecological sites within Porirua City.
- Identify the relative significance and importance of the sites.
- Identify the range of threats to health and viability of the sites.
- Identify tools for statutory and non-statutory management and protection of the sites.
- Review District Plan provisions as a result of the inventory.
- Develop a monitoring programme for ecological sites.
The project contributes to the Council's Strategic and District Planning objectives. In particular it is a major mechanism towards Strategic Goal 4: 'The natural and physical environment is clean, safe, sustainable and attractive'.
The inventory spanned the whole of Porirua City, both urban and rural. It is primarily centered on terrestrial sites but also includes freshwater sites contained within those terrestrial sites, key sites for wildlife, sites on margins in water, coastal and estuarine foreshore areas, etc. A simple working definition of an ecological site, adopted for the recognition of potential sites, was "an area that is either occupied by indigenous vegetation or which forms a habitat for indigenous fauna."
Ecological sites inventory - Methodology
"Man is a child of his environment."
- Shinichi Suzuki.
View of Porirua Harbour.
Photo by Keith Calder, 2009.
The programme of work
The programme of work developed by Porirua City Council and the consultants split the work into three stages and a number of tasks:
Stage One: (April - June 2000)
Task 1: A desktop survey to obtain and collate all existing data onto a new database.
Task 2: Identification and description of ecodomains (areas of similar environmental factors) and summary of recent history of the vegetation.
Task 3: Initial analysis of aerial photos and field reconnaissance.
Stage Two: (July - November 2000)
Task 4: Initial work on criteria for assessing significance, discussion of project to date with council and landowners: seeking permission for field survey from individual owners.
Stage Three: (September - July 2001)
Task 5: Detailed field survey of most ecological sites.
Task 6: Finalise boundaries and database information and assess the significance of individual sites.
Task 7: Discussion of current management and ecological issues, presenting options for management and monitoring programmes.
Principal assessment criteria
For assessing the significance of sites, the principal assessment criteria were:
- Vegetation and animal species present;
- Distinctiveness of site and species composition;
- Structure and height of vegetation/habitat;
- Variety and integrity of physical habitats and natural processes;
- Representativeness within Porirua City boundaries and ecodomain;
- Rarity of plant and animal species, and of habitat;
- Ecological context (relationship to other sites or ecosystems in the vicinity);
- Condition and threats;
Ecological sites inventory - The setting: ecodomains in Porirua City
"There are two ways of being creative. One can sing and dance. Or one can create an environment in which singers and dances flourish."
- Warren G. Bennis.
View of Pauatahanui Wildlife Reserve.
Photo by Keith Calder, 2009.
Four ecodomains in Porirua City were recognized and mapped. They span the severe coastal salt belt, the mild humid "nikau" belt, the inland hill country, and the cool hill tops. The ecodomain framework provides a reference point for evaluating the scarcity value of a natural community because it takes into account: processes as well as static patterns, successional pathways as well as mature communities.
For example, a reasonably "natural" vegetated gully is rare in the "nikau" belt. The fact that there are numerous vegetated gullies in the inland hill country is irrelevant - the conditions of loess cover, its subsequent style of erosion and flat bottomed, swampy valleys that are characteristic in the seasonal climate of the "nikau" belt provide a set of conditions that are not repeated in the steep, rocky, twisted gullies of the hill country which are prone to flash floods and cooler temperatures. The latter will not be habitat for marsh crake, mudfish and swamp flax. Yet, in both cases a mature, undisturbed vegetation would probably have supported pukatea, kahikatea, kiekie, nikau and tawa; it is the modern situation that amplifies the differences.
Ecological sites inventory - Summary of the database
"In the end, we conserve only what we love. We will love only what we understand. We will understand only what we are taught."
- Baba Dioum, Senegalese poet.
View of Taupo Wetland.
Photo by Keith Calder, 2009.
A total of 171 sites were surveyed and described. The sites ranged in size between less than 0.2ha and more than 300ha. The average overall size is 14.32ha. The total area of the sites is 2463ha, or just under 14% of the total area of the city. The six largest sites, each of more than 100ha in area, account for just under half of the total area of ecological sites. There was great variability in tenure, landforms, and vegetation of the ecological sites.
Threats to ecosites
The most significant threats are:
Weeds: the most pervasive threat, with several groups of weed species providing different types of threat;
Animal pests: also pervasive; principally possums, hares, rabbits, ferrets, stoats, and rats;
Grazing: a serious threat in many coastal areas and unfenced rural remnants;
Clearance: direct current threat of clearance for other land uses affects a significant number of both rural and urban ecological sites;
Exposure to wind: an insidious threat to many small sites, exacerbated by pest and grazing threats.
Sites were grouped into ten "management groups", reflecting the ecological character and landform of the site, as well as the type of management required to maintain their ecological values.
- Sizeable areas of mature forest (13 sites)
- Small forest remnants within farmland or other open spaces (64 sites)
- Small forest remnants within built-up areas (30 sites)
- Wetlands (13 sites)
- Coastal scarps (outer coast) (9 sites)
- Coastal wetlands (9 sites)
- Riparian areas within farmland (7 sites)
- Small areas (usually riparian) within forest plantations (8 sites)
- Reverting bush areas (15 sites)
- Other: diverse nature reserves and dune area (3 sites)
Ranks of significance
Of greatest ecological significance, and rare or scarce in Porirua City (34 sites);
Of greatest ecological significance, but not rare or scarce in Porirua City (8 sites);
Highly ecologically significant, and rare or scarce in Porirua City (38 sites);
Highly ecologically significant, but not rare or scarce in Porirua City (44 sites);
Of moderate ecological significance (mainly as seed source) (33 sites);
Not ecologically significant, but may have amenity or other values (14 sites).
There are a large number of ecological sites in Porirua City, most of which have some ecological significance. Together these areas cover nearly 2500ha, or 14% of the area of the City. Most ecodomains are reasonably well-represented and it is likely that there have not been large losses or entire key SES in the last 20-30 years. More than one third of sites have some legal protection, either as public reserves or as legal covenants on private land. There are a handful of sites (mainly forest blocks in which effective possum control has been maintained for some time) that are in very good condition. However, a large number of sites are in very poor condition, and have clearly declined in extent and condition in at least the last couple of decades.
The principal conclusion from this brief analysis is that the highest priority for management and protection of remaining natural areas in Porirua City is to improve the condition of the existing Significant Ecological Sites, both through addressing significant threats in order to arrest the decline in condition of as many sites as possible, and through more ambitious programmes to enhance the condition of some key sites.
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