Figure 4. Length of the Connected Network

Effectively conserving freshwater biodiversity in a changing climate requires protecting geophysical settings that, over an evolutionary timescale, ultimately drive patterns of diversity (Anderson and Ferree 2010, Palmer et al. 2009, Rieman & Isaak 2010). For stream networks this includes variation in gradient, geology, and temperature, as these factors have long been identified as important in shaping freshwater biodiversity (Higgins et al. 2005). Networks with high variation in these properties capture the variety of available microclimates, habitats, and flow velocity conditions that species can exploit during rearrangement in response to environmental changes. Incorporating information on geophysical diversity allows conservation biologists to better encompass genetic and phenotypic diversity by conserving diverse habitat representations across river basins with appropriate redundancy (Rieman & Isaak 2010). We quantified geophysical diversity for two factors: gradient and temperature.

Stream temperature sets the physiological limits where stream organisms can persist and temperature extremes may directly preclude certain taxa from inhabiting a water body. Seasonal changes in water temperature often cue development or migration, and temperature can influence growth rates and fecundity. Many species that are important in coldwater streams are rare or absent in warmwater streams (Halliwell et al. 1999). Many aquatic species, such as brook trout, have adapted to specific temperature regimes, and are intolerant of even small changes in mean temperatures or lengths of exposure to temperatures above certain limits (Wehrly et al. 2007). Ideally a resilient stream network would span a range of current temperatures offering options for both coldwater and warmwater species and provide connected space for species to stay within their thermal preferences in the future.