Abstract
Resource subsidies in the form of energy, materials, and organisms can support the productivity of recipient ecosystems. When subsidies increase the abundance of top predators, theory predicts that top‐down interactions will be strengthened. However, the degree to which subsidies intensify predation should be constrained by the strength of interactions between predators and their prey. To test the potential for subsidies to drive strong top‐down control by two stream predators, steelhead (Oncorhynchus mykiss) and Pacific giant salamander (Dicamptodon tenebrosus) we reduced terrestrial prey and manipulated the presence of predators in 32 stream reaches. Prey subsidies supported elevated growth of predatory steelhead in our study system and in the absence of allochthonous prey steelhead experienced a 187% reduction in growth. Despite the high biomass of subsidized predators, there was little support for strong top‐down control of herbivore biomass, or a trophic cascade as measured by changes in AFDM and chlorophyll‐a. This result was consistent across subsidy treatments, suggesting that predatory steelhead are unable to increase exploitation of aquatic prey in the absence of terrestrial prey subsidies. The potential for top‐down control was apparently limited by the fact that most (82%) herbivores in our study system were armored and relatively invulnerable to predation. These results demonstrate the potential importance of behavioral and morphological adaptations that can temper predator prey interactions in highly subsidized ecosystems.
Highlights
Resource subsidies, the movement of nutrients, organic material, and prey from adjacent habitats, support elevated primary and secondary production in many ecosystems, and play an important role in mediating the dynamics of recipient food webs (Polis et al 1997)
For each response variable we considered all possible combinations of our main effects and their interactions, as well as percent canopy cover as a proxy for light availability
Growth of juvenile steelhead from both age classes (1þ, young of the year (YOY)) was best predicted by the availability of terrestrial prey; models of YOY growth which included light availability (% canopy cover) received some support with YOY growth declining across a gradient of increasing canopy closure (À2.31 6 1.96 g)
Summary
The movement of nutrients, organic material, and prey from adjacent habitats, support elevated primary and secondary production in many ecosystems, and play an important role in mediating the dynamics of recipient food webs (Polis et al 1997). Polis 1998, Hilderbrand et al 1999, Sabo and Power 2002, Wipfli and Gregovich 2002, Kawaguchi et al 2003), creating the potential for strengthened top-down control of in situ prey through the process of apparent competition (Holt 1984, Polis et al 1997, Henschel et al 2001, Murakami and Nakano 2002) In such instances, subsidized predators may elicit changes in the biomass and composition of multiple trophic levels within the recipient food web (Polis and Hurd 1996, Henschel et al 2001, Leroux and Loreau 2008)
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