  We model vertical breathing mode perturbations in the Milky Way's stellar
disc and study their effects on estimates of the local dark matter density,
surface density, and vertical force. Evidence for these perturbations, which
involve compression and expansion of the Galactic disc perpendicular to its
midplane, come from the SEGUE, RAVE, and LAMOST surveys. We show that their
existence may lead to systematic errors of $10\%$ or greater in the vertical
force $K_z(z)$ at $|z|=1.1\,{\rm kpc}$. These errors translate to $\gtrsim
25\%$ errors in estimates of the local dark matter density. Using different
mono-abundant subpopulations as tracers offers a way out: if the inferences
from all tracers in the Gaia era agree, then the dark matter determination will
be robust. Disagreement in the inferences from different tracers will signal
the breakdown of the unperturbed model and perhaps provide the means for
determining the nature of the perturbation.
