Competition fluctuates across years and seasons in a 6-species-fish community: empirical evidence from the field

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 (similar to 64,000 captured individuals and similar to 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.