Confidence level of the estimates

In document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 192-195)

The assessment of the true costs of INNS is difficult in the absence of empirical evidence (Huxley 2003). This report contains one of the most detailed assessments of the economic cost of INNS on a country’s economy, partially as a result of investigations into sector costs.

The methods used were dissimilar to previous studies of the same kind, and this study may add to the variability of methods used for the assessment of INNS costs to the economy (Born et al. 2004). However, the combination that we used, of research of the grey and

scientific literature, coupled with a questionnaire and follow-up interviews, has proved to be hugely valuable for gaining an insight of the market costs for a variety of sectors, species and stakeholders. Consequently, the direct cost estimate is based on a wide range of sources and different information for the many species examined. Most of the estimates are based on data obtained from literature, and where possible, the figures are based on information obtained from those involved, i.e. land managers, councils, scientists and other specialists. In other words, the estimates are based on real data and when compared with other studies, these estimates appear to be well-founded. On a number of occasions, data from other countries have been used in our estimates, or comparisons have been made with data from other countries to help verify the accuracy of these estimates. While the transfer of data from other situations adds variation to the estimates due to ecological and methodological differences in data collection (Hanley et al. 2006), this was still more accurate than using values obtained through educated guesses or assumptions.

The level of confidence in our estimates for market costs is very dependent on the species and sectors. For example, there is a high level of confidence in the cost estimate for the aquaculture sector, because those people who were contacted confirmed each other’s assessments that INNS are not a distinct issue currently. Nevertheless, they were aware of the presence of INNS and the potential problems they could cause. By contrast, despite considerable efforts to obtain figures, information availability was limited in various cases because of its commercially sensitive nature, for example in the utilities and pest control industries, and consequently, a number of calculations had to depend on assumptions about incurred costs. In addition, many costs were difficult to estimate since, for example, the population size of the species causing the effect was unknown, or because the control effort is not monitored consistently across the country (e.g. grey squirrel and mink). In this work, where solid evidence was not available, assumptions based on the biology and ecology of the species involved were used to extrapolate costs. These assumptions were checked with experts in the field, corrected where necessary, or remained as assumptions when no expert was able to provide a better estimate to use. When assumptions did have to be used, the figures that were used were intentionally conservative and it has been explicitly stated that they were assumptions. In the anonymous peer review process the calculations and assumptions were challenged, corrected or accepted. This has added greatly to the confidence in the estimates.

Previous studies have estimated the impact of alien plants, vertebrates, arthropods, plant pathogens and freshwater organisms in the UK. The estimate of arthropod pest damage presented here (approximately £255 million) is considerably lower than the UK figures

presented by Pimentel (2002), in which he estimated that mites alone caused $960 million (£591.65 million) of crop damage and the cost of forest insects was $2 million (£1.23 million).

In total, Pimentel estimated that insects and plant pathogens do $5 billion (£3.08 billion) of damage to crops and forests every year, whereas we have estimated that insects and plant pathogens cost just over £658 million per year. However, due to the fact that the estimates in this study were extrapolated from known costs, we are confident that the figure presented here is a fairly accurate assessment of the costs.

Oreska (2009) estimated the current cost of aquatic INNS on Great Britain using a modelling approach. His estimate, £19 million spent annually on control, was based on eight plant species. Although these estimates are not based on the same species, the estimates for the species that the two studies have in common are in general agreement. For example, we estimated a current cost of Ludwigia spp. of £24,000 p.a. and Oreska's estimate was

£10,263. Williamson (2002) estimated the economic impact of alien plants on the British Isles at £200-300 million. This is lower than our estimate of approximately £493 million for Great Britain alone. The economic impact of vertebrates was estimated at over £239 million by White and Harris (2002), with about 20% of that spent on control. Our estimate is £402 million. It is difficult to identify the cause for these differences, but it is possible that the considerable effort that went into our data collection using a wide variety of sources has resulted in the retrieval of more costs.

Non-market costs are notoriously difficult to estimate in any study (Perrings et al. 2000), although they compose probably the largest part of the economic impact of INNS (Colautti et al. 2006). This study captured some non-market costs, such as the estimated costs to the native water vole due to mink, but a key issue is that no estimates have been made for the majority of non-market costs, due to the lack of available data on which to base any calculations. In most cases, little or no research has been carried out to quantify these consequential effects. It was not the scope of this project to carry out additional research to quantify the costs of the ecosystem effects of INNS. Instead, we focused on capturing the costs of ecosystem effects that had already been quantified. The analysis of how the inclusion of non-market costs has affected the total estimate in previous studies of the impact of INNS to various countries' economies is shown in Box 1 in the introduction.

Studies that included non-market costs had estimates that were on average 57 times higher than studies that did not. This indicates that, as expected (Colautti et al. 2006), market costs represent only a small portion of the total cost of INNS. The estimate provided in this report consists almost entirely of market costs and the actual, total cost of INNS to the British economy is likely to be much higher, possibly as much as £96 billion per year. No attempt

was made to verify how realistic this is, as this would involve the valuation of non-market costs which was not possible in this study. However, it does provide an indication of what the true costs of INNS to the British economy could be.

In document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 192-195)