Forest structure and stream salamander diets: Implications for terrestrial-aquatic connectivity

摘要

The concept of the ecological boundary is emerging as a useful tool for investigating and understanding ecological processes in heterogeneous landscapes (Cadenasso et al. 2003). Ecosystem ecologists have provided valuable perspectives on the behavior of ecological boundaries, elucidating the regulation of boundary permeability and its implications for energy and nutrient cycles within contiguous patches of different habitat types (Bormann & Likens 1979, Hedin et al. 1998). These boundary-mediated fluxes and direct effects of ecological boundaries on the distribution and dispersal of organisms (e.g., Haddad 1999) can strongly influence emergent properties of natural systems, including resource dynamics (Jef-feries 2000), secondary productivity (Polis & Hurd 1996), and food web composition and stability (Hux-el & McCann 1998). The ecological boundary between aquatic and terrestrial habitats exerts fundamental control on headwater stream systems, where both ecosystem processes and food webs of terrestrial and stream habitats are widely viewed as coupled (Ross 1963, Wallace et al. 1997). The structure and composition of terrestrial vegetation influence the transfer of dissolved substances to the stream (e.g., Likens & Bormann 1995) and regulate in-stream processing of these substances by affecting light, temperature, and hydrologic regimes (Hill et al. 2001, Valett et al. 2002). Species inhabiting headwater systems rely on the flow of resources from terrestrial to stream habitats (Wallace et al. 1997, Nakano et al. 1999b), stream to terrestrial habitats (Helfield & Naiman 2001, Sabo & Power 2002), and reciprocal flows between these two habitats (Nakano & Murakami 2001). Research is limited on how terrestrial-aquatic connectivity in headwater food webs responds to natural variation in terrestrial vegetation conditions, and the research that has been done focuses on stream fish (Wipfli 1997, Allan et al. 2003). Broader un-derstanding in this area will create valuable opportunities to bridge ecosystem- and community-oriented research in stream systems, and to elucidate more fully the implications of well-established patterns of forest succession (e.g., Yoda et al. 1963, Schwarz et al. 2001). This understanding of how riparian forest vegetation affects terrestrial-aquatic links may also be important for forest management that aims to preserve natural ecosystem processes.