When does heterogeneity average out in coarse-grained ecological communities?

Aggregating components into a few variables, such as species into total biomass, is common in complex systems, yet differences among components can still shape aggregate dynamics. For dynamics near a stable equilibrium, we quantify this irreducible dynamical heterogeneity as the spectral mismatch between the aggregate's fluctuations and those of the best closed aggregate model, an Ornstein--Uhlenbeck process of the same dimension. In weakly interacting random communities, increasing species richness can reduce or increase this heterogeneity, depending on interaction structure. Our framework clarifies when ecological and evolutionary differences average out and when their dynamical effects persist.