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Distribution patterns of species are a consequence of long evolutionary histories. Biogeographical barriers have resulted in separate developments of biota with specific adaptations to their native ecosystems and associated environmental conditions. Especially during the past centuries, human activity has helped species to surmount these natural barriers, so that present-day patterns of alien species result from natural drivers as well as man’s history of land exploitation and construction of traffic routes. Humans created new pathways of species introductions (Chaps. 2 and 3), and also new habitats. Introduced species were thus able to invade both (semi-)natural and humanmade habitats, which differ considerably in their proportion of alien species.

With the arrival of aliens in a novel environment, interactions between resident species are disrupted, and interactions among resident and invading species have to be newly established. Though unplanned and mostly unwanted, biological invasions are considered to be an important ecological experiment, well suited for ecological studies. Because many aspects are better known in alien species than in native ones (e.g. time of isolation from the original gene pool, and we have replications by introductions into multiple localities), species invasions provide a unique opportunity to test general ecological theories as an alternative approach to focused experimental manipulations which might be more constrained by time, space, research budgets, etc.

Invasive alien plant species pose significant challenges to managing and maintaining indigenous biodiversity in natural ecosystems. Invasive plants can transform ecosystems by establishing viable populations with growth rates high enough to displace elements of the native biota (Rejmánek 1999) or to modify disturbance regimes (Brooks et al. 2004), thereby potentially transforming ecosystem structure and functioning (Dukes and Mooney 2004). Because the numbers of invasive plant species and the extent of invasions are increasing rapidly in many regions, concern has grown about the stability of these novel,emerging ecosystems (Hobbs et al. 2006). The question of how climate change will interact in this global process of ecosystem modification is becoming highly relevant for natural resource management.

Although many studies have addressed the potential threats to ecosystems from invasive alien plants and climate change separately, few studies have considered the interactive and potentially synergistic impacts of these two factors on ecosystems (but see Ziska 2003). Climatic and landscape features set the ultimate limits to the geographic distribution of species and determine the seasonal conditions for establishment, recruitment, growth and survival (Rejmánek and Richardson 1996; Thuiller et al. 2006b). Human-induced climate change is therefore a pervasive element of the multiple forcing functions which maintain, generate and threaten natural biodiversity.

The impacts of invasive species on ecosystem services have attracted worldwide attention. Despite the overwhelming evidence of these impacts and a growing appreciation for ecosystem services, however, researchers and policymakers rarely directly address the connection between invasions and ecosystem services.Various attempts have been made to address the ecosystem processes that are affected by invasive species (e.g., Levine et al. 2003; Dukes and Mooney 2004), but the links between these mechanisms and ecosystem services are largely lacking in the literature. Assessments of the economic impacts of invasive species cover costs beyond those associated with ecosystem services (e.g., control costs), and generally do not differentiate by ecosystem service type. Additionally, while advances have been made in quantifying non-market-based ecosystem services, their loss or alteration by invasive species is often overlooked or underappreciated.

Comparatively little research has been conducted on the ecology of invasive organisms in marine ecosystems when balanced against their terrestrial counterparts (Carlton and Geller 1993). Perhaps rates of invasions in marine systems are simply lower than in other systems, but more likely lack of scrutiny, difficulty with taxonomic resolution, and unusual life-history characters of marine organisms cause the vast majority of invasions to go unreported. Regardless, those few well-studied marine invasions have resulted in tremendous ecological, economic, social, and health problems (e.g., Carlton et al. 1990; Hallegraeff and Bolch 1992; Kideys 1994; Grosholz and Ruiz 1995; Chaps. 4 and 5).Among marine communities that have been extensively studied (e.g., the Chesapeake Bay, San Francisco Bay, and the Black Sea), nonindigenous species are extremely common, and encompass a broad range of taxonomic and trophic groups (Ruiz et al. 1997). Moreover, many marine communities contain remarkably large numbers of ‘cryptogenic’ species (i.e., species that have unknown origins) that are, in fact, likely to have been introduced.

This chapter examines the impact of invasive non-native species on the biodiversity in the river Rhine. The occurrence and spread of non-native species are relatively well documented in the Rhine (e.g. Tittizer et al. 2000; Geitler et al. 2002; Rey et al. 2004). Quantitative studies on changes in abundance of non-native species and on species composition of native communities complement these reports (e.g. Van den Brink et al. 1990, 1996; Haas et al. 2002).We review major changes in the biota of the river Rhine, focusing on mechanisms underlying changes in species abundance following the invasion of non-native species. Our emphasis is on benthic macroinvertebrates but interactions with other animals are also considered. Along the way, we identify important gaps in knowledge and suggest areas for further research.

Human activities, such intentional and unintentional transplantations, and habitat alterations including the establishment of migration corridors, generate increasing opportunities for formerly allopatric taxa to meet and to hybridize. There is indeed increasing evidence that these introduced plant and animal taxa (including crop plants and domesticated animal taxa) frequently hybridize with native relatives and with other introduced taxa, leading to a growing concern that these hybridizations may compromise the genetic integrity of native taxa to the point of causing extinctions (Abbott 1992; Rhymer and Simberloff 1996; Levin et al. 1996; Ellstrand and Schierenbeck 2000; Vilà et al. 2000). A decade ago, Rhymer and Simberloff (1996) stated in their review on this topic that the known cases are probably just the tip of the iceberg.Using the search term ‘hybridization and introgression’, the Web of Science database yields a total of 1,178 research articles, of which 935 (or 80 %) have been published after 1995 (Fig. 16.1). Indeed, the evidence for natural and man-induced hybridization and introgression appears to have increased exponentially these last few years.

The release of genetically modified organisms (GMOs) is a controversial subject. Some perceive it to be the single most important development in biology since the discovery of natural selection. Others are concerned that the movement of genes with no reference to natural species boundaries could pose new ecological risks. One conjectural risk is that transgenes will either cause the host species to become invasive or they will escape from the original host species and cause other species to become invasive. Gene flow between species occurs naturally, although the frequency varies within and across kingdoms. Such gene flow is responsible for creating new combinations of genes, with the potential for introgression or speciation. Hybridisation has been proposed as a stimulus for the evolution of invasiveness in plants (Ellstrand and Schierenbeck 2000), suggesting that new combinations can create genotypes with different, and perhaps surprising ecological behaviours. Do transgenes pose particular risks in this respect? Is it possible to predict the probability that transgenes will cause invasiveness in recipient organisms?

Approximately 50,000 plant, animal, and microbe invasive species are present in the United States, and an estimated 500,000 plant, animal, and microbe invasive species have invaded other nations of the world. Immediately, it should be pointed out that the US and world agriculture depend on introduced food crops and livestock.Approximately 99 % of all crops and livestock in all nations are intentionally introduced plants, animals, and microbes (Pimentel 2002). Worldwide, the value of agriculture (including beneficial non-indigenous species) is estimated to total $ 30 trillion per year. Other exotic species have been introduced for landscape restoration, biological pest control, sport, and food processing, also contributing significant benefits.

Calculating the negative economic impacts associated with the invasion of exotic species is difficult.For a few species, there are sufficient data to estimate some impacts on agriculture, forestry, fisheries, public health, and the natural ecosystem in the US and worldwide. In this article, we estimate the magnitude of the economic benefits, and environmental and economic costs associated with a variety of invasive species that exist in the United States and elsewhere in the world.

Biological invasions have been object of ecological research for years.As one objective, natural scientists investigate the effects of invasive species on ecosystems and their functioning (Levine et al. 2003). However, impacts on ecosystems are also of relevance for society. Changes in ecosystems affect humans insofar as ecosystems provide goods and services, such as fresh water, food and fibres or recreation, which might be altered due to invasive species. Therefore, impacts of biological invasions should be an object of socio-economic interest, which is also demanded by the Convention on Biological Diversity (2002).

This chapter aims at providing elements for the analysis of impacts of invasive species from the socio-economic point of view. Such an analysis is politically relevant, since impacts are the focal point of every decision to establish an appropriate management regime. For an all-encompassing analysis, an integrative framework is needed to structure the information on impacts. For that purpose, the concept of ecosystem services (Chap. 13) is introduced (Sect. 19.2). Alternative decisions on the appropriate management of invasive species face trade-offs between outcomes and impacts. For handling such trade-offs, evaluation is needed.As discussed in Sect. 19.3, perception presents the prerequisite of an explicit evaluation. Finally, different evaluation methods are introduced so as to value the information about impacts during the decision-making process.

The economic aspects of invasive alien species (IAS) are increasingly being recognised as highly significant (Perrings et al. 2000; McNeely 2001). Even though the economics of invasive species is often associated solely with economic consequences of species, economics is equally important for the analysis of reasons for invasions. That IAS impose costs upon society is unchallenged. Pimentel et al. (2000, 2005 and Chap. 18) assess the costs of IAS to the US, and the latter paper estimates these at $ 120 billion per annum. These costs are borne by the whole of society and not only those responsible for the initial introductions.