Predicting Invasion Success: Freshwater Fishes in California as a Model

S lakes, and estuaries are among the most highly invaded ecosystems in the world; in many of them, alien organisms are now a significant part of the biota (Cohen 2002). The best-documented invaders in these systems are fishes. Because many fish species are widely introduced, there is concern among scientists and managers over the homogenization of freshwater fish faunas worldwide (McKinney and Lockwood 1999, Marchetti et al. 2001, Rahel 2002). Invasions by alien species also have high ecological and economic costs. Efforts to prevent the spread of unwanted fish and other organisms, however, are still small compared with the size of the problem (Meyerson and Reaser 2003), in part because policymakers and the public often fail to appreciate the seriousness of the invasions. Many harmful alien fishes, for example, are also favorite food and game fishes. Thus, to make predictions useful for the management and control of alien species, scientists need to understand the nature of invaders, the invasion process, and the impact of invasions, and to communicate that knowledge effectively. Because “any species can invade and any environment can be invaded” (Moyle and Light 1996), a search for generalities that would be useful for predicting the success of potential invaders may be futile. Kolar and Lodge (2001) argue, however, that quantitative approaches (including meta-analysis of diverse sources of information) can produce useful predictions. In particular, they argue for the need to analyze data on failed invasions. We think there is also a need to focus analyses at regional scales, because studies at continental or global scales tend to yield results with limited predictive ability (Gido and Brown 1999, Lockwood 1999, Ruesink 2005). In this article we address these issues of approach and scale in relation to fish invasions by synthesizing recent studies in California, mainly our own. We focus on these studies because the scope and completeness of our data sets make our studies widely applicable. After introducing California as a laboratory for the study of fish invasions, we focus on the potential for predicting success and impacts of freshwater fish invasions based on (a) symbiotic characteristics (the relationship of the fish to humans), (b) biological characteristics of the invaders, (c) characteristics of the invasion site, and (d) the invasion process. We then briefly address the question, Do alien fishes alter the ecosystems they invade?