1. Ecological opportunity (i.e. the diversity of available resources) has a pivotal role in shaping niche variation and trophic specialisation of animals. However, ecological opportunity can be described with regard to both diversity and abundance of resources. The degree to which these two components contribute to niche variation remains unexplored.
2. To address this, we used an extensive dataset on fish diet and benthic invertebrate diversity and density from 73 sampling events in three Norwegian rivers in order to explore realised trophic niches and the response of dietary niche variation along gradients of resource diversity (potential trophic niches), resource density (as a proxy of resource abundance) and fish density (as a proxy of inter‐ and intra‐specific competition) in a freshwater top predator (the brown trout, Salmo trutta L.).
3. Linear models indicated that individual and population niche variation increased with increasing ecological opportunity in terms of prey diversity. However, no simple cause‐and‐effect associations between niche indices and prey abundance were found. Our multiple regression analyses indicated that the abundance of certain resources (e.g. Chironomidae) can interact with prey diversity to determine individual and population realised trophic niches. Niche variation (within‐individual component and inter‐individual diet variation) decreased with increasing inter‐ and intra‐specific competition.
4. This study extends prevailing trophic ecology theory by identifying diversity, rather than density, of available prey resources as a primary driver of niche variation in fish of temperate riverine systems with no extensive resource limitation. The study also shows that ecological opportunity may mask the direction of the effect (compression or expansion) of competition on niche variation when food resources are diverse.
5. Our study provides novel empirical insight to the driving forces behind niche variation and reveals that diversity, rather than density, of available prey resources may be a primary driver of niche variation in freshwater fish. Our study supports the view that a broader potential trophic niche promotes broader realised trophic niche variation by individuals, which leads to individual niche diversification by opening access to alternatives resources, resulting in a concomitant rise in the realised trophic niche width of the population.

中文翻译：

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超越生态机会：猎物的多样性而不是丰度塑造了捕食者的生态位变化

1. 生态机会（即可用资源的多样性）在塑造生态位变异和动物营养特化方面具有关键作用。但是，可以就资源的多样性和丰富性来描述生态机会。这两种成分对生态位变化的贡献程度尚待探索。
2. 为了解决这个问题，我们使用了来自挪威三条河流中73次采样事件的鱼类饮食和底栖无脊椎动物多样性和密度的广泛数据集，以探索已实现的营养生态位和饮食生态位变化对资源多样性梯度的响应（潜在营养营养位） ，淡水顶级捕食者（褐鳟，Salmo trutta L.）的资源密度（作为资源丰富度的替代）和鱼类密度（作为种间和种内竞争的替代）。
3. 线性模型表明，随着猎物多样性的增加，个体和种群生态位的变化随着生态机会的增加而增加。但是，没有发现利基指数和猎物丰富度之间存在简单的因果关系。我们的多元回归分析表明，某些资源（例如猫科）的丰富性可以与猎物多样性相互作用，以确定个体和种群实现的营养位。生态位变化（个体组成和个体间饮食变化）随着种间和种内竞争的增加而降低。
4. 这项研究通过确定可用猎物资源的多样性而不是密度，将其作为温带河系系统鱼类生态位变化的主要驱动力，而没有广泛的资源限制，从而扩展了普遍的营养生态学理论。研究还表明，当粮食资源多样化时，生态机会可能掩盖竞争对生态位变化的影响（压缩或扩展）的方向。
5. 我们的研究为生态位变化背后的驱动力提供了新颖的经验性见解，并揭示了可用猎物资源的多样性而非密度可能是淡水鱼生态位变化的主要驱动力。我们的研究支持以下观点：更广泛的潜在营养生态位促进了个体更广泛的实现的营养生态位变异，这通过开放获取替代资源的途径而导致了个体生态位多样化，从而导致了人口的实际营养生态位宽度随之上升。