Exotic effects of capital accumulation.

T he accumulation of capital in Europe is strongly and positively associated with the accumulation of alien invasive species. In a study of the drivers behind biological invasions in Europe, Pyšek et al. (1) use European macroecological, economic, and demographic data to explain the variation in alien species richness of bryophytes, fungi, vascular plants, terrestrial insects, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. They find only two significant explanatory variables of the stock of alien species: national wealth and human population density. They interpret these as proxies for the direct drivers behind invasions—propagule pressure along new pathways of introduction, along with the disturbance of both freshwater and terrestrial habitats. The study is a timely and important contribution to our growing understanding of the role of economic activity in the dispersal of species beyond their natural range. It provides fresh evidence for the cumulative effects of anthropogenic stress along two axes. One is the role of trade in opening up new pathways and in developing the propagule pressure along existing pathways. The other is the role of the production of goods and services in native habitat disturbance and hence in the vulnerability of ecological systems to invasion. Just as interestingly, it also provides evidence of a less alarming kind— evidence that, where the threats posed by invasive species are high enough, they have been excluded. The stock of wealth in any country is the cumulative effect of past investments and so is the best measure of a process that, in many countries, has deep historical roots. In Europe, current wealth reflects 2,000 y (and more) of efforts to build productive capacity—through pillage and plunder as often as through trade and investment. The net effect is a legacy of assets that bears the imprint of many sources, reflecting not just the ebb and flow of empires but the progressive integration of the global economic system. The latter process has been far from smooth, but it has been sure. Globalization means that few places have been untouched by world trade, and the European legacy carries the stamp of much of the rest of the world. As Pyšek et al. (1) show, that stamp includes an unrivaled collection of invasive species. Recent analysis of the link between trade and biological invasions includes a number of economic studies on the relationship between the opening of new markets or trade routes and the introduction of new species and between the growth in trade volumes (the frequency of introduction) and the probability that introduced species will establish and spread (2–4). It has been shown that the volume and direction of trade are good empirical predictors of which introduced species are likely to become invasive (5, 6) and which countries are the most likely sources of particular species (7, 8). The second axis of anthropogenic stress, disturbance, is also well understood. Pyšek et al. (1) have elsewhere observed that, although some ecosystems are fundamentally more vulnerable to invasion than others, the vulnerability of all systems increases with fragmentation and disturbance (9). Direct habitat loss through land use change also affects the vulnerability of ecosystems to invasion (10). There is, however, an important dimension of the problem that is not identified by analyzing what species are there. It is the effectiveness of efforts to exclude or eradicate harmful pests and pathogens. The movement of both goods and people is as strongly implicated in the spread of harmful species as it is in the spread of benign species. It is, for example, directly implicated in the emergence both of human diseases, such as H5NI (11), West Nile virus (12), and sudden acute respiratory syndrome (13), and a series of livestock diseases (14, 15). In all cases of this kind, however, the potential harm posed by the pest or pathogen has induced a response aimed at changing the likelihood of its introduction, establishment, and spread. The risk depends on both the likelihood of establishment and the resulting damage. Governments have undertaken measures to detect, intercept, eradicate, or control pests or pathogens, depending on their expectations of the harm they will cause. The risk reflects these measures (16). Historically, Europe has been confronted by a long list of extremely harmful organisms. The bubonic plague introduced in the 14th century, for example, caused Fig. 1. Data on two categories of animal disease reported to the World Organization for Animal Health during the period 1996–2004. List B disease outbreaks (Upper) and list A disease outbreaks (Lower) are reported against the value of average imports of risk materials in the same period. Data are ranked by disease outbreaks. Sources: Disease outbreaks, World Organisation for Animal Health at http://www.oie. int/eng/info/en_infold.htm?e1d5. Trade volumes, Comtrade database at http://comtrade.un.org/db/.