Functional traits are defined as the characteristics of an individual that affect its relative fitness in a community via niche constraints and trade-offs. Yet, empirical tests for this assumption in natural plankton communities are scarce. One of the most popular functional traits is body size because metabolism and predator-prey dynamics scale with size and because body size is time- and economically affordable to be measured in the field. Here, I will briefly present two field studies testing the role of functional traits on functioning and community structure.
In the first study we explore how the size diversity of prey (nano-microplankton) and predators (mesozooplankton) influence trophic transfer efficiency (using biomass ratio as a proxy) in natural marine ecosystems. We unveil an interactive effect: prey size diversity hinders trophic transfer efficiency and predator size diversity enhances it, yet predator size diversity becomes influential when prey size diversity is high. This empirical finding provides insights for ecosystem predictive models.
In the second study, we develop a new method to test if species-level categorical traits affect community structuring differently among traits and between regions. The method consists of comparing the frequencies of trait categories in the species-rank abundance distributions of local communities versus their frequencies in the regional average species pool. We unveil different patterns for six traits in subarctic versus subtropical copepod communities. In subarctic communities, medium-sized and cold-water species are selected to dominate, indicating traits affect relative fitness as predicted by classic niche theory. In subtropical communities, most species are small and warm-water, but small and warm-water are not selected to dominate, suggesting that greater species diversity towards the tropics may be due to lesser trait-based fitness difference, which allows more species to coexist.