Fragmentation poses one of the greatest threats to freshwater fish biodiversity (Nilsson et al., 2005; Reidy-Liermann et al., 2012). Whereas damming of large rivers is perhaps the most obvious form of fragmentation (e.g., Nilsson et al., 2005), smaller, semipermeable barriers such as road crossings (Perkin & Gido, 2012) or water withdrawals that dry sections of a river network (Falke et al., 2011) also pose a conservation challenge. In glacial regions, lakes that are naturally connected through waterways are increasingly being isolated by summer evaporation and groundwater loss (Baki et al., 2012). Climate and land-use changes also isolate populations in headwater reaches by increasing temperatures (Rahel et al., 1996) or drying of streams (Falke et al., 2011) in downstream reaches. Finally, barriers can form when the occurrence of a species, such as a large predator, inhibits the movement of prey through a dispersal corridor (Fraser et al., 1995). This severing of connectivity in aquatic habitats affects species persistence through multiple stressors (Chapters 4 and 6) including limiting dispersal necessary to fulfil important life stages, exacerbating negative species interactions, and inhibiting recolonisation following disturbance. Barriers to movement isolate small populations leading to reduced genetic diversity (Chapter 16) and potentially compromise long-term population persistence (e.g. Wofford et al., 2005).
In this chapter, we discuss how fragmentation disrupts dispersal and migration of freshwater fishes and the long-term consequences for population diversity and stability. We begin with a global overview of the problem followed by a review of theoretical and empirical methods for quantifying the effects of fragmentation on population viability. We conclude with a discussion of conservation challenges along with future research and management recommendations. The primary tenet of our review is that persistence of species in fragmented systems is dependent on the nature of barriers to dispersal and ecological traits of species, particularly their ability to complete critical life-history stages within fragmented habitats (Figure 10.1). We often refer to the terms fragmentation, isolation and connectivity. Whereas there are instances where these might be used interchangeably, we consider fragmentation to represent habitats that have been partitioned into smaller habitats and by extension result in smaller populations. The terms connectivity and isolation refer to the ability or lack of ability, respectively, of fishes to disperse into or out of particular habitats.
In every respect, the valley rules the stream.
The challenges facing natural resource managers occur over entire landscapes and involve landscape components at many scales. Many resource managers are shifting their approach from managing resources such as fish, wildlife, and water separately to managing for the integrity of entire ecosystems (Christensen et al., 1996). Indeed, nearly all resource management agencies in the USA have recognized that informed management decisions cannot be made exclusively at the level of habitat units or local sites. It is generally accepted that ecological patterns and processes must be considered over large areas when biodiversity and ecological function must be maintained while the goods and services desired by the public are provided. For example, forest managers must determine the patterns and timing of tree harvesting while maintaining an amount and arrangement of habitats that will sustain many species. Managers of parks and nature reserves must be attentive to actions occurring on surrounding lands outside their jurisdiction. Aquatic resource managers must broaden their perspective to encompass the terrestrial and human landscape to manage stream and lake resources effectively (Hynes, 1975, widely regarded as the father of modern stream ecology and quoted above; Naiman et al., 1995). Landscape ecology also is implicit in the paradigm of ecosystem management (Grumbine,1994; Christensen et al., 1996).
Despite the acknowledged importance of a landscape perspective by both scientists and resource managers, determining how to implement management at broader scales is very much a work in progress.
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