Weed eradication - an economic perspective

There has been recent interest in determining the upper limits to the feasibility of weed eradication. Although a number of disparate factors determine the success of an eradication program, ultimately eradication feasibility must be viewed in the context of the amount of investment that can be made. The latter should reflect the hazard posed by an invasion, with greater investment justified by greater threats. In simplest terms, the effort (and hence investment) to achieve weed eradication comprises the detection effort required to delimit an invasion plus the search and control effort required to prevent reproduction until extirpation occurs over the entire infested area. The difficulty of estimating the required investment at the commencement of a weed eradication program (as well as during periodic reviews) is a serious problem. Bioeconomics show promise in determining the optimal approach to managing weed invasions, notwithstanding ongoing difficulties in estimating the costs and benefits of eradication and alternative invasion management strategies. A flexible approach to the management of weed invasions is needed, allowing for the adoption of another strategy when it becomes clear that the probability of eradication is low, owing to resourcing or intractable technical issues. Whether the considerable progress that has been achieved towards eradication of the once massive witchweed invasion can be duplicated for other weeds of agricultural systems will depend to a large extent upon investment (. $250 million over 50 yr in this instance). Weeds of natural ecosystems seem destined to remain more difficult eradication targets for a variety of reasons, including higher impedance to eradication, more difficulty in valuing the benefits arising from eradication, and possibly less willingness to pay from society at large

Shade Tolerance as Reflected in Population Structures of the Woody Weed, Groundsel Bush (Baccharis halimifolia L.)

Previously documented shade tolerance in establishing seedlings of groundsel bush (Baccharis halimifolia L.) was reflected in the similarity of the size structures of populations growing within slash pine (Pinus elliottii var. ellottii Engelm.) stands of differing ages. The accumulation of litter appeared to be the dominant factor leading to the cessation of seedling recruitment following canopy closure. Evidence for self-thinning obtained from the B. halimifolia population occuring within the younger pine stand provided an indication that the carrying capacity of the site was approached fairly early in the rotation

Evaluation of weed eradication programs: containment and extirpation

Weed eradication programs often require 10 years or more to achieve their objective. It is important that progress is evaluated on a regular basis so that programs that are 'on track' can be distinguished from those that are unlikely to succeed. Earlier research has addressed conformity of eradication programs to the delimitation criterion. In this paper evaluation in relation to the containment and extirpation criteria is considered. Because strong evidence of containment failure (i.e. spread from infestations targeted for eradication) is difficult to obtain, it generally will not be practicable to evaluate how effective eradication programs are at containing the target species. However, chronic failure of containment will be reflected in sustained increases in cumulative infested area and thus a failure to delimit a weed invasion. Evaluating the degree of conformity to the delimitation and extirpation criteria is therefore sufficient to give an appraisal of progress towards the eradication objective. A significant step towards eradication occurs when a weed is no longer readily detectable at an infested site, signalling entry to the monitoring phase. This transition will occur more quickly if reproduction is prevented consistently. Where an invasion consists of multiple infestations, the monitoring profile (frequency distribution of time since detection) provides a summary of the overall effectiveness of the eradication program in meeting the extirpation criterion. Eradication is generally claimed when the target species has not been detected for a period equal to or greater than its seed longevity, although there is often considerable uncertainty in estimates of the latter. Recently developed methods, which take into consideration the cost of continued monitoring vs. the potential cost of damage should a weed escape owing to premature cessation of an eradication program, can assist managers to decide when to terminate weed eradication programs

Eradication of invasive species: progress and emerging issues in the 21st century

At an international conference on the eradication of invasive species, held in 2001, Simberloff (2002) noted some past successes in eradication—from the global eradication of smallpox (Fenner et al. 1988) to the many successful eradications of populations (mostly mammals) from small islands (e.g. Veitch and Bell 1990; Burbidge and Morris 2002). However, he cautioned that we needed to be more ambitious and aim higher if we are to prevent and reverse the growing threat of the homogenization of global biodiversity. In this chapter we review how the management strategy of eradication—the permanent removal of entire discrete populations—has contributed to the stretch in goals advocated by Simberloff. We also discuss impediments to eradication success, and summarize how some of the lessons learnt during this process have contributed to the other strategies (prevention and sustained control) that are required to manage the wider threat posed by invasive alien species. We concentrate on terrestrial vertebrates and weeds (our areas of expertise), but touch on terrestrial invertebrates and marine and freshwater species in the discussion on emerging issues, to illustrate some of the different constraints these taxa and habitats impose on the feasibility of eradication

Evaluating progress in weed eradication programs

Evaluating progress towards eradication is critically important because weed eradication programs are very expensive and may take more than 10 years to complete. The degree of confidence that can be placed in any measure of eradication progress is a function of the effort that has been invested in finding new infestations and in monitoring known infestations. Determining eradication endpoints is particularly difficult, since plants may be extremely difficult to detect when at low densities and it is virtually impossible to demonstrate seed bank exhaustion. Recent work suggests that an economic approach to this problem should be adopted. They propose some rules of thumb to determine whether to continue an eradication program or switch to an alternative management strategy