  We study the coupling of a Fisher-Kolmogorov-Petrovsky-Piskunov (FKPP)
equation to a separate, advection-only transport process. We find that the
front dynamics can be described by an FKPP-like equation only at sufficiently
fast diffusion or large coupling strength. For such parameter regimes, we find
a mapping to an effective FKPP equation. We also find that FKPP equation is
fragile with respect to the coupling to an advection-only mechanism, discover
conditions when the front width diverges, and when front speed is insensitive
to the coupling. At zero diffusion in this mean-field description, the downwind
front speed goes to a finite value as the coupling goes to zero.
