Allochthonous Input Across Habitats, Subsidized Consumers, and Apparent Trophic Cascades: Examples from the Ocean-Land Interface

Abstract

The dynamics and structure of food webs often are influenced by the movement of food and organisms among habitats. Nutrients, detritus, prey, and consumers all cross spatial boundaries in many ways and with a variety of effects (see Polis et al. (this volume)). Such allochthonous input may often be a dominant factor shaping the dynamics of key consumers and their resources. Here, we focus on a common but often neglected process: the movement of food (=detritus and prey) from more productive to less productive habitats. Such flow subsidizes consumers in less productive habitats; outside input allows consumer populations to achieve numbers greater than supported by in situ productivity. Subsidized consumer populations may increase to levels where they depress in situ prey in a manner parallel to apparent competition (Holt, 1984), but here consumers use prey originating in different habitats rather than different prey from the same habitat. In some cases, the depression of in situ prey promotes a trophic cascade (Power, 1990; Carpenter and Kitchell, 1993) if the resources of the in situ prey respond by increasing in abundance.