Beyond ecological opportunity: Prey diversity rather than abundance shapes predator niche variation

23 1. Ecological opportunity, i.e. the diversity of available resources, has a pivotal role in 24 shaping niche variation and trophic specialisation of animals. However, ecological 25 opportunity can be described with regard to both diversity and abundance of resources. It 26 is still relatively unexplored to what degree these two components contribute to niche 27 variation. 28 2. To address this, we utilise an extensive dataset on fish diet and benthic invertebrate 29 diversity and density from 73 sampling events in three Norwegian rivers in order to 30 explore realised trophic niches and the response of dietary niche variation along gradients 31 of resource diversity (potential trophic niches), resource density (as a proxy of resource 32 abundance) and fish density (as a proxy of inter-and intraspecific competition) in a 33 freshwater top predator (the brown trout, Salmo trutta L.). 34 3. Linear models indicated that individual and population niche variation increased with 35 increasing ecological opportunity in terms of prey diversity. However, no simple cause-36 and-effect associations between niche indices and prey abundance were found. Our 37 multiple regression analyses indicated that the abundance of certain resources (e.g. 38 Chironomidae) can interact with prey diversity to determine individual and population 39 realised trophic niches. Niche variation (within-individual component and inter-40 individual diet variation) decreased with increasing inter-and intraspecific competition. 41 4. This study extends prevailing trophic ecology theory by identifying diversity, rather 42 than density, of available prey resources as a primary driver of niche variation in fish of 43 temperate riverine systems with no extensive resource limitation. The study also shows 44 that ecological opportunity may mask the direction of the effect (compression or 45 expansion) of competition on niche variation when food resources are diverse.