تجارت، آسیب گونه های بیگانه و اثرات حمایت از تولیدات تحت ساختارهای بازار جایگزین

We first construct three measures of the expected damage from the unintentional introduction of alien species into a country called Home. We then focus on four market structures. First, perfect competition prevails in both Home and Foreign, and Home is a small country. Second, the Home and the Foreign markets are both perfectly competitive, but Home is now a large country. Third, the exporter in Foreign is a monopolist and there are no import competing firms in Home. Finally, the Foreign exporter and the import competing firm in Home engage in Cournot competition. In all four scenarios, we analyze the impact of small and optimal Home tariffs on prices, exports, imports, the damage from alien species, and social welfare in Home. Inter alia, our analysis identifies conditions under which it makes sense to use trade policy (tariffs) to regulate invasive species and conditions under which it does not.

The fact that alien species (also known as invasive or non-native species) have been and continue to be introduced into one part of the world from another is not new. What is new is the realization that such introductions, particularly the unintentional ones, have often been very costly for the concerned nations. In this regard, consider the case of the United States. A report by the Office of Technology Assessment (OTA, 1993) declared that the annual monetary damage resulting from biological invasions is between $4.7 and $6.5 billion. More recent research by Pimentel et al. (2000) has concluded that the total annual monetary damage from invasive species is in fact over $100 billion. Researchers now recognize that maritime trade in goods comprises a sizeable proportion of the world's total international trade in goods. Ships are the primary vehicle in maritime trade, and consequently they are routinely used to carry goods of all kinds (often in containers) from one country to another. Now, international trade theorists have demonstrated that there are benefits to the nations involved in such voluntary trade. This notwithstanding, in recent times, natural resource and environmental economists have contended that these gains are likely to be smaller than what most researchers have believed thus far. Why? As Perrings et al. (2000), Costello and McAusland (2003), Batabyal (2004), Batabyal et al. (2005), and Margolis et al. (2005) have noted, this is because in addition to carrying goods between nations, ships have also managed to carry an assortment of deleterious non-native plant and animal species from one part of the world to another. As far as unintentional introductions, the primary focus of this paper, are concerned, there are two main ways in which alien species have been carried from one part of the world to another. First, many invasive species have been introduced into a country, often inadvertently, by ships discarding their ballast water. Cargo ships usually carry ballast water in order to increase vessel stability when they are not carrying full loads. When these ships come into a seaport, this ballast water must be jettisoned before cargo can be loaded. This manner of species introductions is important, and the problem of managing alien species that have been introduced into a particular nation by means of the discharge of ballast water has now received some attention in the economics literature (see Nunes and Van den Bergh, 2004, Yang and Perakis, 2004 and Batabyal and Beladi, 2006). The second way in which alien species have been introduced into a particular country is by means of contaminated goods (agricultural goods readily come to mind) that may or may not be carried in containers. In this regard, the reader should note that invasive species can remain concealed in containers for long periods of time. In addition, material such as wood, which is often used to pack the cargo in the containers, may itself contain alien species. In fact, as pointed out by Costello and McAusland, a joint report from the United States Department of Agriculture (USDA), the Animal and Plant Health Inspection Service (APHIS), and the United States Forest Service (USFS) has noted that nearly 51.8 percent of maritime shipments contain solid wood packing materials and that infection rates for solid wood packing materials are substantial (USDA, APHIS, and USFS, 2000, p. 25). For example, inspections of wooden spools from China revealed infection rates between 22 percent and 24 percent, and inspections of braces for granite blocks imported into Canada were found to hold live insects 32 percent of the time (USDA, APHIS, and USFS, pp. 27–28). Economists and ecologists are both very interested in managing invasive species. This is because (see the first paragraph of this section) biological invasions can and often have proven to be very costly from an economic standpoint. In addition to these economic costs, the work of Vitousek et al. (1996), Simberloff et al. (1997), Costello and McAusland (2003), and others reminds us that alien species can alter ecosystem processes, act as vectors of diseases, and diminish biological diversity. In this regard, Cox (1993) has observed that out of 256 vertebrate extinctions with a known cause, 109 are the outcome of biological invasions. This discussion tells us that non-native species have been and continue to be a great menace to society. It is only very recently that economists have begun to analyze questions pertaining to invasive species management. For instance, Eiswerth and Johnson (2002) have studied an intertemporal model of alien species stock management. They note that the optimal level of management effort is responsive to ecological factors that are not only species and site specific but also stochastic in nature. Second, Olson and Roy (2002) have used a stochastic framework to examine the circumstances under which it is optimal to wipe out an invasive species and the circumstances under which it is not optimal to do so. Third, Horan et al. (2002) have analyzed the properties of management approaches under full information and under uncertainty. Fourth, Batabyal et al. (2005) have observed that there is a tension between economic cost minimization and inspection stringency in invasive species management. Finally, Batabyal and Beladi have analyzed maximization problems stemming from the steady state analysis of two multi-person inspection regimes. Despite the known connection between goods trade between countries and the damage from alien species, with the exception of Jenkins (1996), ecologists in general have paid scant attention to the role of trade policy in mitigating the damage from alien species introductions. Jenkins has contended that it may be necessary to use trade policy (bans and restrictions) to protect biological diversity. Very recently, a small number of papers have begun to analyze issues at the interface of international trade and invasive species management. Barbier and Shogren (2004) have analyzed a growth model in which a biological invasion occurs as a spillover effect from the importation of capital goods. They show that when a biological invasion diminishes the productivity of all firms in the economy, the government ought to impose an output tax to equate the private and the social desires for consumption growth and capital accumulation. Costello and McAusland (2003) and McAusland and Costello (2004) have studied the impact that tariffs have on the damage from invasive species introductions. Costello and McAusland show that a tariff can either decrease or increase the damage from invasive species. McAusland and Costello show that although it is always optimal to use tariffs to control the damage from alien species, the same cannot be said about inspections. In particular, in their model, there are several circumstances in which it is optimal not to inspect imported goods at all. Prestemon et al. (2006) study international trade in forest products and show that trade liberalization will have a negligible effect on US imports of Siberian logs and on the risk of a biological invasion. Finally, using an integrated model with an international trade component, Zhao et al. (2006) demonstrate the consumer and the producer responses to livestock disease outbreaks and the welfare effects of alternate invasive species management policies. Although the papers cited in the previous paragraph have certainly advanced our understanding of the impacts of trade policy on the damage from invasive species, three outstanding questions concerning the desirability of using trade policy to manage invasive species remain. Therefore, the purpose of this paper is to analyze these three questions in detail. First, unlike the extant literature, we use a two country model to study the efficacy of tariffs as an invasive species management tool under four different market structures. 1 Second, we focus not just on small tariffs but on small and on optimal tariffs. Finally, our emphasis is less on the impact that tariffs have on the damage from invasive species per se and more on the impacts of tariffs on social welfare when social welfare depends in part on the damage from alien species. 2 The rest of this paper is organized as follows. Section 2.1 briefly describes our two country model. Sections 2.2, 2.3 and 2.4 construct three measures of the expected damage in Home from the introduction of alien species. Section 3.1 continues the description of our two country model. Then, this section derives a general expression for the change in Home social welfare as a function of a change in the Home tariff. Section 3.2 analyzes the impact of small and optimal tariffs imposed by Home on the damage from invasive species, on prices, on exports and imports, and on social welfare in Home for the case in which perfect competition prevails in both Home and Foreign and Home is a small country. Section 3.3 does the same for the case in which the Home and the Foreign markets are both perfectly competitive, but Home is now a large country. Section 3.4 conducts a similar analysis for the case in which the exporter in Foreign is a monopolist and there are no import competing firms in Home. Section 3.5 also conducts the same kind of analysis as the previous three sections, but now the Foreign exporter and the import competing firm in Home engage in Cournot competition. Section 3.6 first discusses the form of the dependence of all the tariff expressions on the expected total damage from alien species introductions in Home. Next, this section comments briefly on scenarios in which the above discussed form of dependence would be different. Section 4 concludes and offers suggestions for future research on the subject of this paper.

In this paper, we provided a theoretical perspective on the impacts of small and optimal specific tariffs when international trade in goods results in the stochastic introductions of alien species from one country to another as a byproduct. Conducting the analysis from the standpoint of the tariff imposing country (i.e., Home), we first derived three (two monetary and one physical) measures of the expected total damage from alien species introductions into Home. Next, we analyzed the effects of small and optimal tariffs under four alternate market structures. Our basic result is that there are several circumstances in which it makes sense to use trade policy (tariffs) to control the damage from alien plant and/or animal species. The analysis in this paper can be extended in a number of directions. In what follows, we suggest two possible extensions of this paper's research. First, in our model the rate of alien species introductions depends only on the volume of imports. Therefore, it would be useful to determine the extent to which one can obtain results from a model in which, in addition to the volume of imports, the rate of species introductions depends also on the number of previously successful introductions. Second, it would be useful to analyze the impacts of import quotas to see if our basic result from the previous paragraph also holds in the case of quotas. Studies of alien species management that incorporate these aspects of the problem into the analysis will provide additional insights into a management problem that has considerable economic and ecological implications.