A Test of the Effects of Timing of a Pulsed Resource Subsidy on Stream Ecosystems

Spatial resource subsidies that can influence community dynamics are temporally variable. However, we know little about the effects of their temporal pattern on community dynamics and ecosystem processes. We tested the effects of seasonality (i.e., peak timing of June-August vs. August-October) of the resource subsidies, terrestrial invertebrate inputs, on stream ecosystems using large-scale field experiments. We found large increases in the individual growth rates and population biomass of a recipient consumer, 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 break-down rate, and increased water nutrient concentrations. Furthermore, the early resource pulse could influence consumer’s numerical response, suggesting the importance of the subsidy timing on long-term community dynamics. Our results emphasize the need to acknowledge timing-dependent consumer responses to subsidies, which can have cascading effects on community dynamics and ecosystem processes.