Competition is assumed to shape niche widths, affecting species survival and coexistence. Expectedly, high interspecific competition will reduce population niche widths, whereas high intraspecific competition will do the opposite. Here we test in situ how intra- and interspecific competition affects trophic resource use and the individual and population niche widths of two lacustrine fish species, Arctic charr and brown trout, covering a 40 year study period with highly contrasting competitive impacts prior to and following a large-scale fish culling experiment. Initially, an overcrowded Arctic charr population dominated the study system, with brown trout being nearly absent. The culling experiment reduced the littoral Arctic charr density by 80%, whereupon brown trout gradually increased its density in the system. Thus, over the study period, the Arctic charr population went from high to low intraspecific competition, followed by increasing interspecific competition with brown trout. As hypothesized, the relaxed intraspecific competition following the experimental culling reduced individual diet specialization and compressed population niche width of Arctic charr. During the initial increase of the brown trout population, there was a large dietary overlap between the two species. Over the subsequent intensified interspecific competition from the population build-up of brown trout, their trophic niche overlap chiefly declined due to a dietary shift of Arctic charr towards enhanced zooplankton consumption. Contrary to theoretical expectations, the individual and population niche widths of Arctic charr increased with intensified interspecific competition. In contrast, the diet and niche width of brown trout remained stable over time, confirming its competitive superiority. The large-scale culling experiment and associated long-term research revealed pronounced temporal dynamics in trophic niche and resource use of the inferior competitor, substantiating that intra- and interspecific competition have large and contrasting impacts on individual and population niches.

中文翻译：

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种间和种内竞争对个体和种群生态位宽度的影响：关于两种相互作用的鲑鱼的四年研究

假设竞争会塑造生态位宽度，影响物种的生存和共存。预计，高种间竞争将减少种群生态位宽度，而高种内竞争将相反。在这里，我们原位测试了种内和种间竞争如何影响营养资源利用以及两种湖泊鱼类（北极红点鲑和褐鳟鱼）的个体和种群生态位宽度，涵盖了 40 年的研究期，在大型鱼类扑杀实验。最初，过度拥挤的北极红点鲑种群主导了研究系统，几乎没有褐鳟鱼。扑杀实验将沿海北极红点鲑的密度降低了 80%，因此褐鳟鱼在系统中的密度逐渐增加。因此，在研究期间，北极红点鲑种群从高到低的种内竞争，随后与褐鳟的种间竞争增加。正如假设的那样，实验性剔除后放宽的种内竞争降低了北极红点鲑的个体饮食专业化并压缩了种群生态位宽度。在褐鳟鱼种群的最初增加期间，这两个物种之间存在很大的饮食重叠。在随后因褐鳟鱼种群积累而加剧的种间竞争中，它们的营养生态位重叠主要是由于北极红点鲑的饮食转向增加浮游动物消费。与理论预期相反，北极红点鲑的个体和种群生态位宽度随着种间竞争的加剧而增加。相比之下，随着时间的推移，褐鳟鱼的饮食和生态位宽度保持稳定，证实了其竞争优势。大规模剔除实验和相关的长期研究揭示了劣势竞争者的营养生态位和资源使用的显着时间动态，证实了种内和种间竞争对个体和种群生态位产生了巨大且对比鲜明的影响。