The crowding tax in parasites: linking dispersal behaviour to population growth in entomopathogenic nematodes

In mesocosms, Steinernema costaricense and S. kraussei disperse into aggregated host cohorts that sharply reduce per-capita fitness, costing 82% and 34% of peak fitness, respectively.

• Why it matters: Parasites cannot redistribute once inside hosts, so overshooting a fitness optimum may impose a fixed “crowding tax” on population growth, but linking dispersal, within-host distribution, and fitness has been hard.
• What they did: Using lab dose-response experiments, mesocosms with equidistant hosts, and field patterns paired with an R0 model, the study quantified how group size affects infection, emergence, and offspring production.
• The result: Fitness curves folded into the R0 model showed both species aggregate beyond the population-growth optimum, achieving only 72% (S. costaricense) and 40% (S. kraussei) of achievable R0.