Controlling Rhododendron ponticum in the British Isles: an economic analysis

Abstract

What resources should be committed to the control of invasive species? This study is based on a survey of nature conservation and forestry authorities, wildlife trusts and private landowners which investigated the extent of the ecological and economic impacts of the invasive non-native plant Rhododendron ponticum in the British Isles. There are data on 52,000 ha of land affected by R. ponticum, more than 30,000 ha of it in nature reserves. For nearly all nature reserves, displacement of native species and habitat changes were both reported. In 2001, respondents controlled 1275 ha of R. ponticum at a cost of £670,924. To test the optimality of this, we apply a model of social expenditure. The external costs of R. ponticum control are estimated from the probability that it will spread to contiguous sites and the damage done on invaded sites. These are then used to calculate the socially optimal level of expenditure on R. ponticum control, and the funding gap it identified by comparing the result with current levels of expenditure. The results suggest that a socially optimal level of control effort requires a significant increase in social funding for R. ponticum control, although the size of the increase varies between landholders.

Introduction

Increasing attention has been paid recently to the economic consequences of invasive species (US OTA, 1993, Perrings et al., 2000, Pimentel et al., 2000, Pimentel, 2002). The economic consequences of invasive species include damage costs, such as biodiversity loss or habitat change, plus the costs of control or eradication net of any possible benefits. Whereas damage costs are not directly calculable, the costs of control and eradication are. The costs of control are a measure of the effort committed to the eradication or reduction of an invasive species. Since individual landowners will increase control effort up to the point where marginal benefits and costs are equal, this can be seen as an indicator of the marginal private damage due to the invasive species. Depending on the species, the private benefits of control may not be a good measure of the social benefits. Benefits can be widely dispersed across the different stakeholders concerned, and aggregate costs can be difficult to estimate.

The spread of any invasive species depends on the control efforts of all those affected. The control costs faced by each person accordingly depend partly on conditions on the invaded site, and partly on conditions in neighbouring sites. Put another way, the private damage costs of harmful invasive species may be expected to be strictly less than the social damage costs.

In this paper, we are interested in the determinants of private expenditure on the control of a particular invasive species (Rhododendron ponticum), and in the options open to induce private landowners to undertake socially optimal level of control. To do this we investigate the extent of the problem of the invasive non-native plant R. ponticum in the British Isles, both from an ecological and economic perspective. We consider the species in its entire synanthropic area thus including a wide spectrum of different natural and socio-economic conditions. We use a survey of potentially affected groups to generate data on control costs, the reasons why people control R. ponticum, and what control options they consider. Surveys are a standard way of generating such data in the social sciences. They have also been used by biologists. Perrins et al., 1992, Kowarik and Schepker, 1998, Williamson, 1998 have used questionnaire surveys before to get results on the perception of non-native species in general and their control in a certain areas. The taxonomy in the paper follows Stace (1997).

R. ponticum was introduced into Britain as an ornamental garden plant about 1763 (Curtis, 1803). Its native area is in the region of the Black Sea (Turkey, Georgia, Bulgaria) with disjunct occurrences in Lebanon, Spain and Portugal. The plants occurring in the British Isles today originate from Spain mostly (Milne and Abbott, 2000). First reports on its self-sowing abilities in Britain date back to the year 1849 (Hooker, 1849). Seeding may be one reason for its success as an ornamental plant in gardens and parks as well as for the use in extensive plantings for game cover, especially pheasants, in woodlands, particularly in the 19th Century (Elliott, 1996). Where conditions favoured establishment, it subsequently spread from these sites. R. ponticum was also used almost exclusively as the stock for grafted rhododendrons (Royal Horticulture Society, 1951), but it is so vigorous that it constantly sends up suckers which, if not removed, gradually take all the sap from the plant, eventually causing the grafted variety to die off (Cox, 1998). It is not known how far occurrences of R. ponticum go back to plantings of these grafted rootstocks. Today R. ponticum is widespread over the whole British Isles occurring in 2238 out of 3844 grid cells of (10 km)² (Preston et al., 2002).

Naturalisation has taken place in natural and near natural vegetation like acid oak woods, heaths and bogs. R. ponticum causes ecological modifications such as changes in the species composition, light reduction, prevention of regeneration of native shrubs and trees, thus endangering native species (e.g. Cross, 1975, Fuller and Boorman, 1977, Shaw, 1984, Thomson et al., 1993, Compton et al., 2002). However, forestry plantations, both conifer and broadleaf, are also invaded causing problems for forest management (Brown, 1953, Robinson, 1980, Tabbush and Williamson, 1987, Edwards et al., 2000). Consequently, the first experiments on best eradication techniques took place in 1949. They were carried out by the Forestry Commission (Brown, 1953, Miller, 1954) later followed by control measures in nature reserves and on private estates ([15], Gritten, 1987, Becker, 1988, Searle, 1999, Singleton and Rawlins, 1999).

Today R. ponticum is probably the major alien environmental weed in the British Isles (Williamson, 2002).

The economic problem investigated is the following. Since each landowner incurs direct costs from the presence of R. ponticum they will control the species up to the point where the private marginal benefits of control (the private marginal damage avoided) is equal to the private marginal costs of control. But control of R. ponticum in any one site also reduces the probability that it will spread in other sites. So the social marginal benefit of control (the social damage avoided) includes the damage avoided on contiguous sites. Since landowners in general have no incentive to increase control effort up to the point where their private control costs equal the social damage avoided, if landowners are left to choose the level of control they exercise, there will be too little control from the perspective of society.

To see the problem consider the following simple model. Let output on the ith of n contiguous sites, Qⁱ, depend on a vector of m inputs, $\text{X}{}^{\mathrm{i}}\text{,}$ and on the extent of R. ponticum on that site, Rⁱ. Further, let the extent of R. ponticum on the ith site depend on the level of control undertaken in all sites, $\text{∑}\text{j=1}\text{n}\text{E}{}^{\mathrm{j}}\text{.}$

That is

$$\text{Q}{}^{\mathrm{i}}\text{=Q}{}^{\mathrm{i}}\text{x}{}_1{}^{\mathrm{i}}\text{…x}{}_{\mathrm{m}}{}^{\mathrm{i}}\text{,R}{}^{\mathrm{i}}\text{∑}\text{j=1}\text{n}\text{E}{}^{\mathrm{j}}$$

We can identify two problems: a private problem and a social problem.

The private decision problem takes the form:

subject to Eq. (1), where

Πⁱ

profit on the ith site

p

the price of output

cⁱ

$\text{c(}\text{X}{}^{\mathrm{i}}\text{,E}{}^{\mathrm{i}}\text{),}$ costs on the ith site.

The first order necessary conditions for the solution of this problem include the following:

implying that private landowners will equate the private marginal benefits and costs of their control effort. The social decision problem takes the form

$$\text{Max}{}_{\mathrm{S}}\text{Π=}\text{∑}\text{i}\text{n}\text{[pQ}{}^{\mathrm{i}}\text{−c(}\text{X}{}^{\mathrm{i}}\text{,E}{}^{\mathrm{i}}\text{(S))]}$$

subject to Eq. (1) and the way in which private effort depends on the policy instruments, S. The first order necessary condition for the solution of this problem includes the following:

In which is the impact of control on the ith site on the value of output on all other contiguous sites. In the private decision problem the decision-maker has no incentive to take into account the effects of their actions on others. Solution of the social decision problem requires that they do. In the absence of intervention, there will be too little control. The implication of this is that it would be socially desirable to induce people, through choice of the policy instruments, S, to undertake the socially optimal level of control.

We have two problems to solve. The first is the estimation of the marginal external damage costs of R. ponticum in any one site—i.e. the value of the term The second is the specification and estimation of a control effort function in order to calculate the optimal value of S. We take the general form of such a control effort function to be $\text{E}{}^{\mathrm{i}}\text{=E}{}^{\mathrm{i}}\text{(}\text{Z}{}^{\mathrm{i}}\text{,S)}$ where Z is a vector of control inputs.

In other words we wish to calculate (a) how far current control efforts may fall short of the socially optimal control effort, and (b) what policy, S, would induce the socially optimal level of control.