Density-dependent body-size distributions reveal populations state relative to carrying capacity

Man Qi , Jacques A. Deere, Isabel M. Smallegange, Rob Salguero-Gomez

Phenotypic traits may provide accessible information about population dynamics, but how population-level trait distributions change along density-dependent trajectories remains poorly understood in populations with complex life cycles. Here, we quantified body-size distributions in three experimental populations of the bulb mite (Rhizoglyphus robini) as they increased from low density towards carrying capacity under constant environmental conditions and a fixed food supply. Population-level body-size distributions changed predictably but nonlinearly with relative density. Mean body size initially declined and subsequently increased towards carrying capacity, whereas variation and skewness showed broadly opposing patterns. Counterfactual decomposition showed that these changes reflected both shifts in population structure and changes in body size within developmental stages, with their relative contributions to mean body size varying along the density continuum. Stage-specific patterns also differed across development: later stages generally had smaller mean body sizes at higher densities, whereas early active juveniles showed the opposite pattern. Together, these results show that population-level trait distributions integrate concurrent changes in demographic structure and traits within developmental stages. They provide proof of principle that body-size distributions can inform density-dependent population state, while showing that their interpretation requires accounting for population structure, stage-specific trait change and nonlinear trait–density relationships.