title: A test of the effects of timing of a pulsed resource subsidy on stream ecosystems.
abstract: Spatial resource subsidies can alter bottom-up and top-down forces of community regulation across ecosystem boundaries. Most subsidies are temporally variable, and recent theory has suggested that consumer-resource dynamics can be stabilized if the peak timing of a subsidy is desynchronized with that of prey productivity in the recipient ecosystem. However, magnitude of consumer responses per se could depend on the subsidy timing, which may be a critical component for community dynamics and ecosystem processes. The aim of this study was to test (i) whether a recipient consumer (cutthroat trout) responds differently to a resource subsidy occurring early in its growing season than to a subsidy occurring late in the season and, if this is the case, (ii) whether the timing-dependent consumer response has cascading effects on communities and ecosystem functions in streams. To test those hypotheses, we conducted a large-scale field experiment, in which we directly manipulated the timing of augmentation of the terrestrial invertebrates that enter stream (i.e. peak timing of June-August vs. August-October), keeping constant the total amounts of the invertebrates entered. We found large increases in the individual growth rate and population biomass of the cutthroat trout, in response to the early resource pulse, but not to the late pulse. This timing-dependent consumer response cascaded down to reduce benthic invertebrates and leaf breakdown rate, and increased water nutrient concentrations. Furthermore, the early resource pulse resulted in higher maturity rate of the cutthroat trout in the following spring, demonstrating the importance of the subsidy timing on long-term community dynamics via the consumer's numerical response. Our results emphasize the need to acknowledge timing-dependent consumer responses in understanding the effects of subsidies on communities and ecosystem processes. Elucidating the mechanisms by which consumers effectively exploit pulsed subsidies is an important avenue to better understand community dynamics in spatially coupled ecosystems.
authors.affiliation: The Hakubi Center for Advanced Research, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501, Japan.
authors.forename: Takuya
authors.initials: T
authors.lastname: Sato
authors.affiliation: Department of Biology, University of Victoria, Victoria, BC, V8W 3N5, Canada.
authors.forename: Rana W
authors.initials: RW
authors.lastname: El-Sabaawi
authors.affiliation: Department of Forest & Conservation Sciences, University of British Columbia, Forest Sciences Centre 3041, 2424 Main Mall, Vancouver, BC, V6T 1Z4, Canada.
authors.forename: Kirsten
authors.initials: K
authors.lastname: Campbell
authors.affiliation: Tomakomai Research Station, Field Science Center for Northern Biosphere, Hokkaido University, Takaoka, Tomakomai, Hokkaido, 053-0035, Japan.
authors.forename: Tamihisa
authors.initials: T
authors.lastname: Ohta
authors.affiliation: Department of Forest & Conservation Sciences, University of British Columbia, Forest Sciences Centre 3041, 2424 Main Mall, Vancouver, BC, V6T 1Z4, Canada.
authors.forename: John S
authors.initials: JS
authors.lastname: Richardson
mesh_terms: D000818:Animals; D059001:Aquatic Organisms / Q000502:physiology; D018533:Biomass; D001955:British Columbia; D017753:Ecosystem; D020387:Food Chain; D007448:Invertebrates / Q000502:physiology*; D017683:Oncorhynchus / Q000254:growth & development*; D045483:Rivers; D012621:Seasons
keywords: allochthonous input; ecosystem function; functional response; numerical response; nutrient recycling; riparian ecosystem; temporal subsidy