Data from: Resource composition mediates the effects of intraspecific variability in nutrient recycling on ecosystem processes

Despite the growing evidence for individual variation in trophic niche within populations, its potential indirect effects on ecosystem processes remains poorly understood. In particular, few studies have investigated how intraspecific trophic variability can modulate the effects of consumers on ecosystems through potential changes in nutrient excretion rates. Here, we first quantified the level of intraspecific trophic variability in 11 wild populations of the omnivorous fish Lepomis gibbosus. Outputs from stomach content and stable isotope analyses revealed that the degree of trophic specialization and trophic positions were highly variable between and within these wild populations. There was intrapopulation variation in trophic position of more than one trophic level, suggesting that individuals consumed a range of plant and animal resources. We then experimentally manipulated intraspecific trophic variability to assess how it can modulate consumer-mediated nutrient effects on relevant processes of ecosystem functioning. Specifically, three food sources varying in nutrient quality (e.g. plant material, macro-invertebrate and fish meat) were used individually or in combination to simulate seven diet treatment. Results indicated that intraspecific variability in growth and nitrogen excretion rates were more related to the composition of the diet rather than the degree of specialization, and increased with the trophic position of the diet consumed. We subsequently used microcosms and showed that critical ecosystem functions, such as primary production and community respiration, were affected by the variability in excretory products, and this effect was biomass-dependent. These results highlight the importance of considering variation within species to better assess the effects of individuals on ecosystems and, more specifically, the effects of consumer-mediated nutrient recycling because the body size and the trophic ecology of individuals are affected by a large spectrum of natural and human-induced environmental changes.

尽管越来越多的证据表明种群内部存在个体间的营养生态位（trophic niche）差异，但这类差异对生态系统过程（ecosystem processes）的潜在间接影响仍未得到充分解析。具体而言，鲜有研究探讨种内营养变异（intraspecific trophic variability）如何通过改变营养盐排泄速率（nutrient excretion rates），调控消费者对生态系统的影响。
本研究首先量化了11个野生杂食性鱼类驼背太阳鱼（Lepomis gibbosus）种群的种内营养变异水平。胃含物分析与稳定同位素分析（stable isotope analyses）结果显示，这些野生种群内部及种群间的营养特化（trophic specialization）程度与营养级位置（trophic positions）均存在显著变异。部分种群内部的营养级位置差异超过一个营养级，表明个体摄食了多种植物与动物资源。
随后我们通过实验操控种内营养变异，以评估其如何调控消费者介导的营养盐效应，进而影响生态系统功能的相关过程。具体而言，我们选取3种营养品质存在差异的食物源（例如植物材料、大型无脊椎动物和鱼肉），分别单独投喂或组合投喂，以此设置7种日粮处理组。
结果表明，个体生长速率与氮排泄速率的种内变异更多与日粮组成相关，而非营养特化程度，且随所摄食日粮的营养级位置升高而增大。
我们继而利用微宇宙（microcosms）实验体系开展研究，发现初级生产力（primary production）、群落呼吸（community respiration）等关键生态系统功能会受个体排泄产物变异的影响，且该效应具有生物量依赖性。
本研究结果凸显了考虑物种内部个体变异的重要性，以更精准地评估个体对生态系统的影响，尤其是消费者介导的营养盐循环（nutrient recycling）效应——这是因为个体的体型大小与营养生态均会受到多种自然及人为环境变化的影响。