Competition not only occurs between species, but (probably more importantly) also among individuals within species, especially, for species occurring in high densities and when resources are limiting. Fluctuating densities and dietary overlap may account for massive ecological and evolutionary changes in fish communities. We sampled a mixed juvenile (0+ stages, being younger than 1 year of age) fish community at the Lower Rhine over three consecutive years, including native and newly established species (~ 64,000 captured individuals and ~ 4200 dissected fish). Our aim was to develop a new approach in which data on density, individual consumption rates, and diet overlaps (both intra- and interspecific) solely originating from the natural environment, were used and combined to estimate species-specific competitive strengths within and between seasons and species. The juvenile fish community at the food-limited Rhine was dominated by invasive gobies. Species-specific consumption rates decreased depending on the dietary overlap with other juveniles. Two invasive gobies (Neogobius melanostomus and N. fluviatilis) showed the greatest competitive abilities, followed by a third invasive Gobiid species (Ponticola kessleri) and the native Aspius aspius, a species utilizing an exclusive food source. Both other native species (Perca fluviatilis and Sander lucioperca) had the lowest estimates of food-related competitive strength, indicating that these species are forced into a juvenile competitive bottleneck by the invasive gobies, before they finally can develop into successful predators on gobies later in life. Our new analytical approach to measure dietary competitive strength among individuals provides a powerful tool to empirically study eco-evolutionary feedback dynamics in the field.

中文翻译：

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6 种鱼类群落中的竞争在不同年份和季节之间波动：来自该领域的经验证据

竞争不仅发生在物种之间，而且（可能更重要）也发生在物种内的个体之间，特别是对于高密度和资源有限的物种。波动的密度和饮食重叠可能是鱼类群落大规模生态和进化变化的原因。我们连续三年对莱茵河下游的混合幼鱼（0+ 阶段，小于 1 岁）鱼类群落进行了采样，包括本地和新建立的物种（约 64,000 条捕获个体和约 4200 条解剖鱼）。我们的目标是开发一种新方法，其中关于密度、个人消费率和饮食重叠（种内和种间）的数据完全来自自然环境，被使用和组合来估计季节和物种内和之间的物种特异性竞争优势。食物有限的莱茵河的幼鱼群落以入侵虾虎鱼为主。特定物种的消费率下降取决于与其他幼鱼的饮食重叠。两种入侵的虾虎鱼（Neogobius melanostomus 和 N.fluviatilis）表现出最大的竞争能力，其次是第三种入侵的 Gobiid 物种（Ponticola kessleri）和本地的 Aspius aspius，一种利用独家食物来源的物种。其他两个本地物种（Perca fluviatilis 和 Sander lucioperca）对与食物相关的竞争强度的估计最低，表明这些物种被入侵的虾虎鱼逼入了幼年竞争的瓶颈，在它们最终能够在以后的生活中成为成功的捕食虾虎鱼之前。我们测量个体饮食竞争强度的新分析方法为实证研究该领域的生态进化反馈动态提供了一个强大的工具。