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Les espèces envahissantes dans l’archipel néo-calédonien

Marie-Laure Beauvais
Alain Coléno
Hervé Jourdan

Part one. Synthesis and recommendations

Risks and environmental impacts

Texte intégral

1In environmental matters as in other fields, managing a crisis is always more perilous and the results more uncertain than taking preventive measures beforehand. Prevention in environmental matters involves far more than border controls, although those are essential. In New Caledonia perhaps more than elsewhere, an appropriate strategy would also involve monitoring contamination within the territory, particularly island-to-island transfers. Some species may be inoffensive in one part of the territory and invasive in another, where ecological conditions are different. Monitoring is especially necessary because the percentage of introduced species varies considerably from island to island. For example, we know that 22% of vertebrates on Grande-Terre are aliens, but that that figure is zero in part of the Entrecasteaux reefs.


2The time lapse between the introduction of an alien species and invasion proper is very variable. There are three steps in the invasion process: introduction, establishment and invasion. Introduction may be deliberate (for business or leisure purposes etc.) or completely inadvertent (see Table 7). It is far easier to control species intentionally introduced for the purposes of a major economic activity, but even that problem must not be underestimated. In New Zealand, an estimated 75% of weeds that are environmental hazards were originally introduced for horticulture.

3The acclimatization (adaptation) and establishment of a species depends on many factors. The characteristics of the host environment? soil type, available host plants, degree of ecosystem fragmentation? naturally play a part. So does the type of human environment? whether there are inspections, population density etc. The characteristics of the species itself? growth rate, reproductive capacity etc.? and the genetic diversity among the individuals introduced are also important.

Table 7 – Examples of introduction pathways (source: S. Burgiel et al., 2005)

Table 7 – Examples of introduction pathways (source: S. Burgiel et al., 2005)

4The final stage, when the species proliferates, also depends on a combination of human and natural factors. Transport, infrastructure and land use play a major part; the building of new roads, housing developments or business estates and changes in the use of farmland all disturb the environment and encourage proliferation. There may be a latency phenomenon in biological invasion: a species introduced and established in a small area can remain on standby for quite a long time before suddenly proliferating and causing damage. The reasons for such sudden multiplication are not always known. This phenomenon makes it all the more difficult to identify how the species was introduced and to design a control policy in consequence.


5In this report “invasion risk” is defined as the probability of the entire process being completed, from the introduction of a species through naturalisation to proliferation with identified environmental damage. Assessment of this risk must incorporate the capacity of the host ecosystem to absorb the shock of an introduction, this capacity being also affected by local economic conditions. Economic factors as well as biological factors must therefore be taken into account. Most research towards evaluating probabilities of invasion focus on particularly representative species or species associated with major environmental or economic damage or damage to specific ecosystems.

6In this connection there are three types of model. One is based on geographical, climatic and/or biophysical criteria. A large-scale model of this type has been made for the red ant. Based on essentially climatic data it gives an assessment of invasion probability for every country in the world, including islands and archipelagos. Other models use “gravity equations” correlating the numerical populations of invasive species discovered with economic and trade data. These look only at the data on introduction (cumulative volume of goods imported, country size, population density etc.). The third type of model starts from actual inspection data. It often uses a finer scale and considers species introduced by very specific pathways such as ships’ hulls, wooden pallets, ships’ ballast etc.

7In general, invasion probability assessments have some criteria in common:

  • probability increases with the cumulative volume of imports of goods and passenger travel. This concerns the first step in the process;
  • it decreases over time with the probability of invasion of other similar environments or countries (i.e. a species having invaded comparable environments and not yet having invaded the environment in question is considered to have less chance of doing so);
  • it correlates positively with the degree of similarity between the main trading partners and the importing country. This criterion concerns the establishment and proliferation stages;
  • intermediate goods are more likely to encourage the propagation of invasive species than finished goods destined for rapid consumption. Examples are plant material such as shoots and bulbs imported for replanting.
  • the likelihood of propagation depends on the number of exogenous disturbances affecting the host environment (climatic and natural events, deforestation, mining, infrastructure building etc.).

8Various models have been used, especially in countries that have been particularly hard hit by invasive species (see paper by A. Thomas et al.).


9Insufficient or inefficient management of invasive species may lead to disastrous situations. The Hawaiian islands, which have an exceptionally rich ecosystem and an impressive number of protected areas, have learned this to their cost (see paper by H. Jourdan and L. Loope). The implications for Hawaii are all the greater since the tourist trade there, worth US$ 10bn a year, depends largely on environmental quality.

10One of the most formidable invasive plants is Miconia calvescens. In 1971, botanist F.R. Fosberg, a specialist of the Pacific region, warned the Hawaiian authorities of the danger that this tree would destroy indigenous forests. Its huge leaves cast a deep shade on any plants or seedlings growing under its canopy. In the 1980s, it colonised two-thirds of Tahiti, which has some ecological similarities with New Caledonia. But in Hawaii nothing was done until the early 1990s, despite warnings by botanists and voluntary bodies? for lack of coordination between the various responsible government bodies, lack of funds, lack of communication, etc. In 1992, the Hawaiian Department of Agriculture classed Miconia calvescens as a noxious weed. Today, efforts to control it are costing US$ 2 to 3bn a year, with limited success.

11If no precautions are taken in New Caledonia, a dissemination of miconia could have disastrous consequences. At any rate this is what is shown by a simulation based on the bio-ecological characteristics of miconia and geographical maps of the New Caledonian archipelago (see Figure 1): in the absence of adequate control measures, 25% of Grande-Terre could be colonised (especially the east coast and Northern Province).

Map 2. Potential invasion of New Caledonia by miconia (Miconia calvescens)
The black areas (isohyet > 2,000 mm a year) are more favourable to the formation of dense, single-species miconia forest; the grey areas (isohyet < 2,000 mm a year) may be invaded by miconia, especially the wetlands.

12Other examples, again from Hawaii, of species slipping through plant nursery inspections, are the little fire ant, the coqui frog, Caribbean tree frog or common coqui (Eleutherodactylus coqui) and the nettle caterpillar (Darna pallivitta). These species are not easy to detect, and anyway are not included on the Hawaiian Department of Agriculture’s pest lists. Moreover, inspections concern only approved plant nurseries. Typically, while there were 3 recorded populations of fire ant (12 hectares) in 1999, by 2004 there were 31 (76 hectares). There too, the reasons were lack of will, lack of coordination, lack of staff, weak public awareness etc. (see below for the problems posed by little fire ant in New Caledonia).

13These and other examples (see paper by Lloyd Loope) illustrate how devastating the consequences can be when a new species colonises an oceanic island. They should be taken into account in thinking about the protective measures to set up in New Caledonia, given the fragile nature of its ecosystems and its similarities with other islands and archipelagos.


14The colonisation of New Caledonia by the little fire ant, Wasmannia auropunctata, sheds light on the scale and multiplicity of the noxious effects an invasion can have, on the environment, animal health and human health, and also the way in which these effects impact on economic activity. Accidentally introduced to New Caledonia in the 1960s, it initially spread along with the Caribbean pine. A ubiquitous and opportunistic species with a fierce sting, it colonises agricultural land and urban areas as well as wilderness. Today, it is regarded as one of the greatest dangers to New Caledonian biodiversity.

15W. auropunctata is a monomorphous species, a member of the ant subfamily Myrmicinae and is native to a zone stretching from the Caribbean to Uruguay and northern Argentina. In recent decades it has spread to many parts of the tropical belt. Today, it can be found in West Africa (Cameroon and Gabon), North America (Florida, the Bahamas and Bermuda) and the Pacific (Galapagos, New Caledonia, Solomon Islands, Vanuatu, Wallis & Fortuna, Hawaii, French Polynesia and probably Tuvalu).

16The biological characteristics of the little fire ant facilitate its spread through human activity, in contaminated material. It is polygenic, nuptial flight is much reduced or zero, it is an opportunistic feeder (nectar, detritus, seeds etc.) and opportunistic in its choice of nesting site. It can move it nest quickly (and is therefore known as a “tramp species”). It uses its venom against other ant species. Even a natural disaster like a flood cannot discourage these ants: they allow themselves to be carried along by the flow. They can reproduce sexually or by cloning. However, as the queen is shortlived (11 to 15 months) and her replacement vital to the community, any control strategy should take advantage of this fact. Another feature that can be harnessed for control purposes is the ants’ dislike of excessive damp, heat and drought, which makes them vulnerable to sudden changes in local micro-climatic conditions.

17W. auropunctata causes nuisance in a number of ways. Its bite is painful and followed by itching, although to date the theory that it might be a vector for pathogens has not been confirmed. However, it does affect quality of life, particularly in traditional Melanesian homes where it can nest in straw roofs and walls. It can cause blindness in domestic animals by repeated biting. Many branches of business are disrupted by its presence: tree farming, market gardening and fruit production (W. auropunctata entertains mutually beneficial relations with several homopteran species), stock farming etc. but also tourism and electrical and electronic infrastructures (the ants gather in large numbers in transformers, telephone relay stations etc.). As regards the environment, these ants are one of the greatest threats to biodiversity and ecological balance.

18No means of biological control has been found so far for the little fire ant (see paper by H. Jourdan & L. Loope). Whether for emergency measures or long-term control, chemicals are the only solution so far: delayed effect poisons, growth inhibitors etc. In any case the first step is to draw up as exhaustive a list as possible of possible sources and pathways of invasion. Three methods can be used to manage the ant: eradication, control (with confinement to areas already colonised) and palliative measures once the invasion has spread widely. Both the chemicals and the logistical operations are costly. Moreover, the success of the strategies adopted partly depends on a collective sense of responsibility, and community involvement, which requires a media campaign using the newspapers, TV, leaflets distributed in sensitive areas or at points of entry to the territory, etc., and information and education drives.


19Environmental conditions in New Caledonia are relatively propitious to biological invasions. From the limited number of entry points to Grande-Terre (the ports and airport), it is fairly easy to describe the possible pathways. Typically, taking the example of intentional introduction of ornamental plants, these are carried by road to Nouméa and the medium-sized towns. Low population density appears to be an aggravating factor: where houses are thinly spread there is little to prevent a species from spreading from them into the wild. Exploitation of the last patches of primary rainforest, with an increasingly dense network of roads and tracks, is propitious for plants and animal invasions. Some invasions are directly linked to mining activities, as with the rehabilitation of denuded mining areas by two invasive species, gaiac (Acacia spirorbis) and casuarina (bois de fer).

20In 2004 Keith Budd analysed the strengths and weaknesses of the surveillance system in New Caledonia. Here we will only mention the weaknesses. Keith Budd points in particular to shortcomings in the monitoring of several specific pathways and lack of coordination between different authorities responsible for surveillance. Insufficient knowledge of local biodiversity makes it difficult to identify organisms that are potentially invasive. Keith Budd identifies three introduction vectors to be monitored as a priority (ships, shipping containers and imported machines), and recommends reinforcing checks on all access pathways (see Table 8)? especially as New Caledonian strategy so far largely transfers the burden of risk prevention from the point of entry to the exporting country. This strategy leaves the door wide open to invasions if the rules of certification and declaration are not complied with.

21Given the nature and volume of trade and business in New Caledonia (see p. 167), rigorous identification of control points is decisive for combating invasions. The HACCP (Hazard Analysis Critical Control Point) system provides a suitable framework for this. Based on evaluating the risks of introduction, establishment and proliferation, this approach incorporates seven principles (see paper by A. Thomas et al.).

22The two main principles concern the control points themselves, which can be classed into three groups:

  • Pre-border controls: these entail additional costs for the consumer (inspection / verification by exporting country) but compensate for the lack of equipment or customs staff in the importing country;
  • Border controls: these are the most frequently used. Some countries, e.g. Australia and New Zealand, adopt even stricter measures requiring an environmental impact study of any species introduced.
  • Post-introduction controls: two mains types of control point are generally proposed, those concerning infrastructure projects (roads, mines, etc.) and those intended to protect wildlife reserves, nature parks and other protected areas.

Table 8 management and control of entry pathways to New Caledonia (source: K. Budd, 2000)

Table 8 management and control of entry pathways to New Caledonia (source: K. Budd, 2000)

23It seems that the probability of people accidentally introducing a species is limited, and that the species introduced in this way are usually insects or seeds arriving in personal belongings or household goods in the luggage hold, which are less closely inspected. The probability of deliberate or accidental introduction of animal species or animal products also seems low, but not negligible (see p. 175, Table 4, non-native vertebrates). Plants are a different matter: there is no ex post quarantine, personal belongings are not checked, and the ornamental plants trade is growing fast (see p. 167). There is also a high probability of intentional introduction by post of seeds, bulbs and tubers. As to the probability of establishment and propagation (i.e. rate of success of an introduction), this is high for the reasons given above: environment, infrastructure, economic activity etc. The most worrying problem concerns the likelihood of other islands being invaded from Grande-Terre. It is clear that no official measures can entirely remove the risk of species being introduced.

Table des illustrations

Titre Table 7 – Examples of introduction pathways (source: S. Burgiel et al., 2005)
Fichier image/jpeg, 219k
Légende Map 2. Potential invasion of New Caledonia by miconia (Miconia calvescens)The black areas (isohyet > 2,000 mm a year) are more favourable to the formation of dense, single-species miconia forest; the grey areas (isohyet < 2,000 mm a year) may be invaded by miconia, especially the wetlands.
Fichier image/jpeg, 164k
Titre Table 8 management and control of entry pathways to New Caledonia (source: K. Budd, 2000)
Fichier image/jpeg, 648k

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