  We study the competing effects of simultaneous Markovian and non-Markovian
decoherence mechanisms acting on a single spin. We show the existence of a
threshold in the relative strength of such mechanisms above which the spin
dynamics becomes fully Markovian, as revealed by the use of several
non-Markovianity measures. We identify a measure-dependent nested structure of
such thresholds, hinting at a causality relationship amongst the various
non-Markovianity witnesses used in our analysis. Our considerations are then
used to argue the unavoidably non-Markovian evolution of a single-electron
quantum dot exposed to both intrinsic and Markovian technical noise, the latter
of arbitrary strength.
