The current state of freshwater biodiversity, and that of freshwater fishes in particular, can only be described as dismal (Dudgeon et al., 2006; Burkhead, 2012; Chapters 1 and 2). Freshwater extinction rates have been estimated to be around 1000× above background extinction rates (Ricciardi & Rasmussen, 1999; Burkhead, 2012). Using North America as an example, a recent American Fisheries Society assessment indicates that 39% of North American freshwater fish taxa are imperilled: 230 are vulnerable, 190 are threatened, 280 are endangered and 61 are extinct or extirpated (Jelks et al., 2008). Global assessments paint the same general picture (www.iucn.org). What are the factors ultimately responsible for this situation? The objective of this chapter is to examine the role of non-indigenous fishes (hereafter NIF) in the decline and imperilment of native freshwater fishes. We provide a short primer to clarify a few key terms relating to invasion biology Box 8.1).
A biological invasion should not be viewed as an event, but rather a process comprised of several successive stages (Figure 8.1). The non-indigenous species introduction process begins with uptake of individuals, transport to a new area (transport outside of native range), and subsequently release into the wild (introduction to the wild). These released individuals may then establish a self-sustaining population in the new area (establishment), and an established population may increase in abundance and expand its geographic range (spread). Typically it is only when a species becomes abundant and/or widespread that it is perceived to cause ecological or economic harm (impact), such that the species is considered invasive (Lockwood et al., 2013). Human values and perceptions greatly influence how and whether a non-indigenous species is viewed as having undesirable consequences. Alternatively, an introduced species population may remain small and localised in its distribution, with few or no tangible ecological or economic effects. To pass through the invasion stages shown in Figure 8.1, a species must overcome a suite of barriers to advancement. At each stage, there are ecological, environmental and stochastic forces that can either facilitate or hinder transition to subsequent stages. These may include the characteristics of the species (e.g. reproductive rate, physiological tolerance), environmental conditions (e.g. temperature or salinity match) and the nature of the introduction event (e.g. timing of introduction, number of individuals moved).