Trophic dynamics and the exchange of materials across ecosystem boundaries are topics that have each been treated in detail, but separately, in freshwater ecosystems. The observation that common food web motifs occur – if not flourish – in spite of a boundary between water and air, which divides realms that impose fundamentally different physiological demands, would suggest that the forest doesn't end at the lake, and vice versa. This is an important observation because it further suggests that conservation and restoration cannot be successful without considering these linkages explicitly. Further, the watershed food web provides a rich framework for a more comprehensive management paradigm. In this chapter, we review some of the few, but salient, examples of cascading trophic interactions across the land–water interface. Many of these examples are “incomplete” cascades that include strong interactions across two, but not more, trophic levels. We also discuss how a dynamic systems framework can be used to conceptualize and organize the diverse array of trophic interactions in which exchange of organic matter and mobile animals across the land–water boundary may instigate strong top-down dynamics.
Aquatic–terrestrial exchange of carbon, organisms, and energy
Energy exchange between land and freshwater ecosystems has a long history in stream and lake ecology. Terrestrial organic matter is the dominant source of carbon (C) to streams in many temperate catchments (Fisher and Likens, 1973) and its removal has consequences for abundances of primary consumers (detritivores) and their predators (Wallace et al., 1999). Strong energetic dependence by rivers on terrestrial forests is the basis for the River Continuum Concept (RCC; Vannote et al., 1980). Similarly, in tropical floodplain rivers, terrestrial plants – especially fruit – provide key nutrition for fish. This observation forms the basis for the Flood Pulse Concept (FPC; Junk et al., 1989). The RCC and the FPC continue to be influential concepts and productive lines of empirical pursuit to this day (Cross et al., 2011; 2013; Jardine et al., 2012a; 2012b).