Trait-density relationships explain performance in cladoceran zooplankton

Traits have been used extensively to predict and understand performance in response to the abiotic environment, but their role for understanding competitive interactions is less understood, especially in nonplant systems. In this study, we evaluate how traits interact with intraspecific density to modulate performance (per capita birth rate) and whether the traits associated with intraspecific competitive ability are similar across multiple species. We used an experimental system of four cladoceran zooplankton species, experimentally manipulated the density of conspecifics, and measured a range of morphological and life history characteristics (body mass, body length, second antenna length, eye diameter, relative growth rate, age at first reproduction, and birth rate). With causal modeling, we identified significant trait-density relationships for three out of four species, although the specific traits that predicted birth rate varied from species to species. In general, individuals at higher densities displayed smaller morphological traits and shifts towards slower relative growth rates and delayed onset of reproduction. We also asked more generally if there are consistent trait-mediated impacts of density across multiple species. The interspecific model identified significant trait-density relationships for body length, age at first reproduction, and relative growth rate. Unexpectedly, we found little evidence for trait-based competition due to mechanisms such as limiting similarity or hierarchical competition, and rather noted the potential for trait plasticity and constraints on plasticity affecting performance in response to the competitive environment.