Evaluating the feasibility of eradication for terrestrial weed incursions
Eradication, defined here as the elimination of every single individual of a species from an area in which recolonization is unlikely to occur is a strategy that is favoured wherever possible. Eradication programmes are usually very expensive. Before committing to such a strategy, it is preferable to be confident that eradication can be achieved. The aim of this evaluation process is to develop a means of determining whether a weed incursion is more feasible to eradicate than another.
Many eradication programmes have been conducted for animals, while very few are reported for plants because of major differences between plants and animals:
- Some seeds develop large, persistent seedbanks, extending considerably the eradication programme since eradication can only be considered completed when all plants and viable seeds are eliminated
- Some plants are extremely difficult to detect in the vegetative state or are undetectable as propagules
- Interference with the reproductive process effective for insects (e.g. sterile male release) is not available for weeds.
The first step consists in determining if a weed is a suitable target for eradication. Eradication may be undertaken when all the following questions are answered positively:
- Is there a suitable socio-political environment? Widely-cultivated plants are not suitable targets for eradication, and the more agencies involved in the eradication efforts, the higher the risk of failure
- Can immigration be prevented?
- Are effective control measures likely to be available for all situations? Estimate effort (resources) required to achieve eradication
- Does cost-benefit analysis favour eradication over other management strategies?
- Are resources sufficient to fund the programme to its conclusion?
Assessing the feasibility of weed eradication is done by taking into account the gross infestation area, logistical considerations, weed detectability, weed biological characteristics and control effectiveness.
- Gross infestation area (A): the area over which weeds are distributed and have to be surveyed. Based on eradication attempts in California, it has been showed that eradication of gross infestations areas of ;1 ha was almost always possible, that approximately 30% of the infestations between 1 and 100 ha were eradicated, and that about 25% of the infestations between 100 and 1000 ha were eradicated. The relationship between the infested area and the effort required to achieve eradication may vary according to the biology and ecology of the weed and the environmental context
- Logistical considerations (L): the number of infestations as well as the accessibility of infestations are assessed
- Weed detectability (D): weed detection is usually slow, labour-intensive and costly. The search rate (hours/ha) depends on the characteristics of both the target species and the habitats in which it is found. The lasting of the visible stage of the plant within the invaded vegetation is assessed (in months).
It is also critical that a plant is detected and controlled before it reproduces, detectability is therefore scored according to the visibility of the plant prior to reproduction (whether the vegetation is emergent or not or has distinctive features)
- Weed biological characteristics (B): the fact the species can reproduce through vegetative fragmentation is considered since these individuals may develop this capacity at a young age (e.g. Tradescantia spp., Opuntia spp.). For species reproducing by seeds or vegetative propagules, the minimum length of the pre-reproductive period as well as the maximum longevity of seeds or vegetative propagules are assessed
- Control effectiveness (C): the number of treatments required to kill the largest plants, taking into account any expensive procedures required for some infestations as well as consideration of the dispersal mechanisms (pathways involving humans or not).
This evaluation consists in a scoring system allowing the calculation of the eradication effort, defined as the measure of gross infestation area multiplied by the sum of the scores of the logistical considerations, the weed detectability, the weed’s biological characteristics and the control effectiveness (Ax(L+D+B+C)). The use of this scoring system has been tested for a few agricultural weeds. Documentation of eradication programme for environmental invasive plants would allow further testing of this evaluation.
Panetta DFD, Timmins SM (2004) Evaluating the feasibility of eradication for terrestrial weed incursions. Plant Protection Quarterly 19, 5-11.