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The dispersal of organisms is a natural process,important for the distribution of life on earth. It is also important for the appearance and expression of biodiversity, strengthening the multiple forms and functions of diversity in living organisms.We, too, profit from this process and its dynamics, and are even dependent on it. On a longer time scale, dispersal is one of the drivers of evolution, responsible for life itself.

Dispersal is limited by multiple barriers,among which geographical barriers are the most evident.However, human dispersal has overcome all biogeographical barriers, and humans now inhabit all parts of the world. Our activities are on a global scale, and we have been working intensively for centuries to connect all parts of the world ever closer together. In human history, roads were the first expression of these interconnections, as well as shipping.Today, we can reach any spot on earth by plane within 24 h, and vessels transport cargo around the globe within a few weeks. In addition, new connections between water drainage systems, lakes and oceans have been constructed.

Two main ways of dispersal of species can be distinguished: natural dispersal and anthropogenic spread, either indirectly or directly.Natural spread is usually slow and occurs within evolutionary times, it hardly crosses biogeographic borders, and is mostly undirectional. Anthropogenic dispersal is enabled or facilitated directly by human activities. This includes domestication and the worldwide spread of selected species, releases into the wild of suitable game, and escapes from captivity. Humans use animals for nutrition in multiple ways (farming, game, aquaculture and mariculture) and, as humans settle in the world, other species accompany them.More recent motivations to spread species worldwide include the demand for luxury and exotic products (e.g. fur farms), biological control and the pet trade. The main directions of anthropogenic dispersal until the 19th century were from Europe to the European colonies and many other parts of the world. Later, with the increasing independence of numerous countries, with growing world trade, and also with the actual step of globalization, species have been distributed to and from everywhere in the world.

At least at a global scale, species transfer through human agency is much more frequent, efficient and effective than through natural mechanisms and has no parallel in evolutionary history (Elton 1958; Mack et al. 2000). As propagule pressure is one of the most powerful bottlenecks in invasions (Williamson 1996), human-mediated dispersal is a key process in the range expansion of non-native plant species.

Due to the role of biological invasions as a major threat to biodiversity, recent research has aimed at identifying pathways in invasions which can be regulated to prevent or, at least, curb negative impacts of non-native species (Carlton and Ruiz 2005). Information on the functioning and effectiveness of different pathways is therefore necessary to set priorities in regulation or management (Mack 2003).

More than 1,000 non-indigenous aquatic species, ranging from unicellular algae to vertebrates, have been found in European coastal waters, including navigational inland waterways for ocean-going vessels and adjacent water bodies. Approximately half of all non-indigenous species recorded to date have established self-sustaining populations (Gollasch 2006). These introductions are of high concern, as many cases have caused major economical or ecological problems (Chaps. 13-19).

Species are introduced unintentionally (e.g. with ships) or intentionally (e.g. for aquaculture purposes and re-stocking efforts). In shipping, the prime vectors for species transportation are ballast water and in the hull fouling of vessels. Further, a considerable number of exotic species migrates through man-made canals. Examples are the inner-European waterways connecting the Ponto-Caspian region and the Baltic Sea. Also, the Suez Canal “opened the door” for Red Sea species migrations into the Mediterranean Sea and vice versa

The earliest civilizations flourished on the banks of navigable rivers. Indeed, their first monumental hydrological construction projects were concerned with irrigation and transport: around 2200 b.c., the first navigable canal, the Shatt-el-hai, linking the Tigris and Euphrates rivers in Mesopotamia, was excavated; in the 6th century b.c., a canal was built which joined the Nile with the northern Red Sea and, in the 4th century b.c., the Grand Canal in China connected Peking to Hangzhou, a distance of almost 1,000 km.The technological innovations of the 18th century led to an expansion of the network of navigable inland waterways, followed in the 19th century and the early part of the 20th century by the excavation of two interoceanic canals: the Suez Canal, which opened a direct route from the Mediterranean Sea to the Indo-Pacific Ocean, and the Panama Canal, which afforded passage between the Atlantic and Eastern Pacific oceans.

Canals connecting rivers over watersheds or seas across narrow land bridges “dissolve” natural barriers to the dispersal of aquatic organisms, thereby furnishing these with many opportunities for natural dispersal as well as for shipping-mediated transport. The introduction of alien aquatic species has proven to be one of the most profound and damaging anthropogenic deeds - involving both ecological and economic costs. Globalization and climate change are projected to increase aquatic bioinvasions and reduce environmental resistance to invasion of thermophilic biota. Navigable waterways serving as major invasion corridors offer a unique opportunity to study the impact of these processes.

While research on biological invasions is becoming more predictive (e.g., Mack 1996; Kolar and Lodge 2001; Peterson 2003; Arim et al. 2006; Mitchell et al. 2006), significant challenges lie ahead. Indeed, it is still not clear what leads some introduced species to remain benign while others become aggressive invaders. Here, we review some principal ecological and evolutionary hypotheses employed to explain biological invasions.We present an overview of these hypotheses, and suggest approaches to integrate them into a more comprehensive framework that will allow potential interactions among them to be examined.

Biological invasions are spatially and temporally continuous processes, encompassing transport, establishment and spread phases (Sakai et al. 2001). We focus here on the spread, or demographic expansion, of non-native species that are established, since this stage will ultimately determine an invader’s impact in a novel environment. Demographic expansions of introduced species can encompass changes within individuals, such as increase in size or fecundity, and within populations, such as increase in geographic spread and density. We refer to demographic expansions of introduced species as invasion success.

Any organism must be equipped for life in a given environment, otherwise it will die. The fundamental question is does an organism need to be “equipped”,or what syndrome of traits must it possess to survive and flourish at a given locality. In the current human-mediated biodiversity crisis, where alien species play an important role, we need to know whether some species are inherently better equipped to become invasive when moved to new areas by humans. If so, we can identify such species and consider management options to prevent, or at least reduce the damaging effects of biological invasions.

Despite the importance of chance and timing in the establishment and spread of alien plants (Crawley 1989), invasions are clearly not entirely random events (Crawley et al. 1996). Much of the early work on invasions was directed at collating traits associated with invasiveness (Booth et al. 2003). The question of whether is it possible to determine a set of traits that predispose a species to be invasive has been a central theme since the emergence of invasion ecology as a discrete field of study.

Central in invasion biology is to understand why alien species, whose initial populations are generally small and genetically depleted, can succeed to establish themselves in environments to which they have had no opportunity to adapt (Sax and Brown 2000). This paradox is usually resolved by invoking pre-adaptations of non-indigenous species to novel environments. The idea is that some species are successful invaders because they have attributes that pre-adapt them to survive and reproduce in novel environments (Mayr 1965). However, do we really have evidence that there exist properties of successful invaders?

The goal of this chapter is to evaluate to what extent establishment success of terrestrial vertebrates may be understood by the existence of pre-adaptations of species to novel environments. This implies answering two interrelated questions: (1) do species differ in their invasion potential? And if so, (2) what are the features of the species that identify some as successful invaders? Answering these questions is important not only to fully understand how animals respond to new environmental conditions, but also to help identify and prevent situations where the risk is high that a species becomes established and causes ecological impact when introduced in a novel region.

The alteration of natural ecosystems by humans and anthropogenic dispersal of plant propagules beyond their native ranges have facilitated the dramatic spread and increase in dominance of nonnative plants worldwide since the late 1800s (Hobbs 2000;Mack et al. 2000). The amount of ecosystem alteration is related to predominant land uses,which can be summarized into four categories of increasing impact: (1) conservation - nature reserves, wilderness; (2) utilization - pastoralism, non-plantation silviculture, recreation; (3) replacement - cropping agriculture, plantation silviculture; and (4) removal - urbanization, mining, industrial development (Hobbs and Hopkins 1990; Hobbs 2000). The rate at which propagules are dispersed into new regions is largely related to the frequency and intensity of human activities, which generally covary with the degree of ecosystem alteration among the four land use categories.

Compared to areas where replacement or removal land uses are the norm, the management of plant invasions tends to be more complicated where conservation or utilization land uses prevail. The latter two land uses emphasize the need to maintain the integrity of natural ecosystems, whereas the former two do not require that natural ecosystem properties be maintained, and in some cases involve replacing them with simpler ecosystems (e.g., cropping monocultures). Options for controlling invading plants are more limited when their potential negative effects on native ecosystems may preclude their usage. This chapter is focused on conservation and utilization land uses that occur where native ecosystems are largely present and functioning, otherwise known as wildland areas.

The invasion of natural ecosystems by exotic species is an important component of global environmental change, and poses a major threat to biodiversity. Other drivers of global change - such as alteration of the atmospheric composition and associated climate change, changing patterns of land use that fragment habitats and alter disturbance regimes, and increasing levels of airborne nitrogen deposition - also influence resource dynamics and species composition of ecosystems (Sala et al. 2000). Consequently, they all have the potential to interact with biological invasions and to accelerate this process, for which evidence is accumulating (Dukes and Mooney 1999; Mooney and Hobbs 2000). In addition, biological invasions themselves can alter the biogeochemistry of ecosystems through particular traits of the invading species (Ehrenfeld and Scott 2001). If we wish to understand and eventually predict the ecological impacts of invasive species, it is thus of particular importance to reveal the many complex interactions between all elements of global change, and their effects on ecosystem processes. In this chapter, we focus on alterations of the nitrogen cycle of terrestrial ecosystems by exotic invasions, and how nitrogen deposition may influence the success of invaders.