PART FIVE
Issues in Conservation
THE FOUR PREVIOUS PARTS of this book have covered geological and climatic events shaping the North American freshwater fish fauna, how fish assemblages are formed and maintained, the functional morphology and life-history characteristics of fishes, and ways in which fish populations and species interact. This part integrates many of the previous topics by focusing on several areas of how fishes interact with their environments, how human-caused impacts have altered these environments and the responses of fishes, and how the relatively new field of conservation biology has contributed to the restoration of habitats and native fish populations. I have selected only several key issues because of the vast and rapidly expanding literature on fish conservation. This is exemplified by the excellent and recent book on fish conservation by Gene Helfman, which provides a much more expansive coverage of the topic (Helfman 2007).
To a great extent, conservation biology is a “crisis discipline” that takes a group of traditionally academic disciplines and applies the knowledge to the real-world challenge of maintaining functional biodiversity (Soulé 1985; Meffe and Carroll 1997). That is, not just keeping extant taxa alive, but also maintaining landscapes and ecosystems in which their populations and assemblages interact and evolve. In addition, conservation biology frequently includes the added challenge of immediate need—the necessity of proposing management actions based on currently available knowledge, without the luxury of additional time for studies. Ideally, the goal is to shift conservation biology from a reactive to a proactive discipline, aimed at avoiding or minimizing problems rather than attempting to restore severely damaged populations and ecosystems (Meffe and Carroll 1997).
Conservation of North American native fish diversity is approaching a crisis situation. In the twentieth century, 40 North American fish species became extinct, reflecting an extinction rate 1,000 times higher than the estimated background rate for fishes. Overall, the extinction rate for North American freshwater species is five times greater than for terrestrial species and is equivalent to the estimated extinction rate of tropical rainforest communities (Ricciardi and Rasmussen 1999).
PART 5, FIGURE 1. The distribution of imperilment (extinct, endangered, threatened, or vulnerable) of North American freshwater fishes. Degrees of imperilment are indicated for ecoregions. Redrawn from Jelks et al. (2008) with permission of the American Fisheries Society.
In spite of conservation efforts for North American freshwater and diadromous fishes, the number of species considered to be imperiled increased from 1989 to 2008. At present, 39% of the described fish species or subspecies are imperiled and 21 taxa are considered to be extinct from wild habitats, being maintained only in captive breeding programs. Not surprisingly, the current North American map of imperiled species (see figure in Part 5) is quite similar to the map (Figure 1.4) of North American species diversity—the richest faunas have the most species to lose (Jelks et al. 2008).
As a matter of convenience, Part 5 is divided into two chapters—Chapter 14 emphasizes conservation biology of lotic systems, including fringing floodplains as used primarily by riverine fishes, and Chapter 15 emphasizes conservation biology of lentic systems, including floodplain lakes as used primarily by lentic specialists. This division is somewhat arbitrary for several reasons. First, man-made impoundments, reservoirs, are treated in both chapters. Chapter 14 deals primarily with the impact of dams on disrupting movement patterns of fishes in streams and in affecting downstream water quality, whereas the nature of the impounded water and its effects on the tributary streams is primarily covered in Chapter 15. Second, rather than considering streams, reservoirs, and natural lakes as discrete entities, the degrees of similarity in ecological structure and function are largely related to water residence times and flow characteristics. Consequently a more useful approach to viewing lentic-lotic distinctions is that of a continuum ordered by water flow and hydrologic flushing; lotic habitats are at one extreme with unidirectional flow and high flushing rates, whereas large lentic systems are at the other extreme with turbulent mixing and low flushing rates (Soballe and Kimmel 1987; Essington and Carpenter 2000; Wetzel 2001). Some of the factors affected by water movement include the time available for attached or suspended biota to interact with transported materials, abrasion and resuspension of materials, spatial and temporal variation, turbidity, and nutrient supply (Soballe and Kimmel 1987). Over time, lakes and streams can converge—streams change in character with age and/or longitudinally, becoming more lake-like with lower gradients and greater phytoplankton and zooplankton development, and lakes fill and gradually return to a riverine, wetland, or terrestrial habitat (Wetzel 2001). Finally, and perhaps most important, the separation of lotic and lentic systems runs the risk of de-emphasizing the important conclusion that ecosystems are rarely closed and that virtually all ecosystems receive benefits from or provide resources to other ecosystems—without which such ecosystems will generally suffer (Lamberti et al. 2010).