Invasive Weeds


Non-native plant species are often introduced to new areas by human intervention.  Many of these enhance  
the quality of life (for example the introduction of food plants).  Some are visually appealing and are  
introduced for aesthetic reasons.  Yet others are accidentally introduced through human commerce.  In most  
cases, these newly introduced species are not especially damaging and cause no noticeable impacts.  Some  
newly introduced species, released from the forces that regulate them in their home environment, multiply  
rapidly and displace native species that occupy the same habitats.  They threaten all ecosystems with wide-
ranging consequences: agriculture becomes more difficult and costly, native biodiversity is lost, hydrological  
cycles are altered affecting water supplies and land stability, lagoons and reefs become degraded by  
herbicide run-off and sedimentation. These undesirable plants are often referred to as “invasive weeds”.   
Species such as Mile-a-Minute (Mikania micrantha) and Miconia (Miconia calvescens) are two examples  
found on many Pacific islands.

Once a weed has become established, eradication is extremely difficult and expensive.  One solution is to find  
a biological control for the weed.  Scientists identify a natural enemy or predator in the weed’s native range  
and introduce that enemy to where the weed has invaded.  This is a difficult process because extensive  
testing is needed to make sure that the natural enemy does not affect native plants also.   However, once the  
biological control agent is released, it attacks the target weed, sometimes to the point of virtual extinction,  
but more usually controls the weed and reduces its impacts to below threshold levels.  Additionally, once a  
biocontrol agent has been identified, only a small amount of additional testing is usually needed to  
implement the same process in another Pacific island, saving both time and resources.  

Case study:  Invasive weeds in the Cook Islands
In the short period of time since the first Europeans arrived in the Cook Islands, many new plant species  
have been introduced and some have escaped into natural areas and agriculture.  These new plant species  
(333) already outnumber native ones (287).  Some of these have become serious invasive weeds.  For  
example, the Te Kou land snail (Tekoulina pricei) is now critically endangered, if not already extinct due to  
invasive weeds that have destroyed its habitat. Moreover, on some islands, including Rarotonga, invasive  
vines such as Balloon Vine (Cardiospermum grandiflorum) and Mile-a-Minute (Mikania micrantha) are  
smothering trees.  The native trees die, causing massive deforestation leaving only impenetrable vine  
thickets.  Rarotonga is totally dependent upon surface water and if nothing is done to stop these vines, the  
native vegetation will be devastated and fresh water supplies could dwindle.  This will have a massive  
impact on the economy and quality of life for the people of Rarotonga.  

The Pacific Biocontrol Strategy   
The Pacific Biocontrol Strategy was formulated by experts and practitioners from throughout the Pacific  
region.  The strategy recognizes that many invasive weeds have a broad distribution throughout the region  
and a regional approach to identifying and testing biocontrol agents is the most cost-effective solution to  
invasive weeds.  It’s inception included:  
1) identifying common target weeds within the three primary epicenters for Pacific wide biological control  
of weeds, New Zealand, Australia, and Hawaii, in the Pacific;  
2) identifying other Pacific Island Countries and Territories that may have these weeds and identifying the  
status of those weeds; and  
3) leveraging funding in one or more areas to implement appropriate programmes.  

This strategy served as a template for a smaller scale plan, the Cook Islands Weed Biocontrol Programme.   
Developed in 2012, it included a prioritisation exercise conducted at a regional workshop attended by  
technical experts and representatives from the Cook Islands.  This resulted in identification of the 15 highest  
priority invasive weeds and a 5-year plan was prepared to address seven of these species.  The plan was  
funded by MFAT (New Zealand Partnerships for International Development Fund).  The goal of the  
programme is to use imported biocontrol agents to sustainably control/suppress the priority invasive weeds  
in the Cook Islands archipelago. Suppression of these invasive weeds will increase the availability of  
productive land for agriculture, lead to reduced use of chemical herbicides, and reduce potential threats to  
Rarotonga’s water supply.  

The Cook Islands Weed Biocontrol Programme serves as a case study and demonstrates the effectiveness of  
a regional approach to these issues.  Presently, the Pacific Biocontrol Strategy is not funded.


additional information
on this disc

the state of invasive species in the Pacific