Data from: A test of the effects of timing of a pulsed resource subsidy on stream ecosystems

<b>Abstract</b><br/>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 (1) 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, (2) 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 break-down 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.

摘要
空间资源补给可改变跨生态系统边界的群落上行调控与下行调控过程。多数资源补给具有时间波动性，近期理论研究指出，若补给峰值时间与接收生态系统的猎物生产力峰值时间不同步，则可稳定消费者-资源动态。然而，消费者响应的强度本身可能取决于补给时机，这或许是影响群落动态与生态系统过程的关键因素。本研究旨在验证两个假说：（1）接收生态系统中的消费者（切喉鳟，cutthroat trout）对生长季早期出现的资源补给与晚期出现的资源补给是否存在差异化响应；若存在该差异，（2）这种依赖于时机的消费者响应是否会对溪流群落与生态系统功能产生级联效应。为验证上述假说，我们开展了大型野外实验：直接调控进入溪流的陆地无脊椎动物的补给峰值时间（即6-8月峰值与8-10月峰值两组），并保持输入的无脊椎动物总数量恒定。结果显示，切喉鳟的个体生长速率与种群生物量在早期资源脉冲组中显著提升，但在晚期脉冲组中无明显变化。这种依赖于时机的消费者响应进一步产生级联效应：降低了底栖无脊椎动物的丰度与叶片分解速率，并提升了水体营养盐浓度。此外，早期资源脉冲组的切喉鳟在次年春季的成熟率更高，这表明通过消费者的数量响应，补给时机对长期群落动态具有重要影响。本研究结果强调，在解析资源补给对群落与生态系统过程的影响时，需充分考虑依赖于时机的消费者响应。阐明消费者有效利用脉冲式资源补给的内在机制，是深入理解空间耦合生态系统群落动态的重要研究方向。