  The species scale is a field-dependent UV cut-off for any effective field
theory weakly coupled to gravity. In this letter we show that, in the context
of inflationary cosmology, a detection of primordial gravitational waves will
set an upper bound on the decay rate $|\Lambda'_s/\Lambda_s|$ of the species
scale. Specifically, we derive this in terms of the tensor-to-scalar ratio $r$
of power spectra of primordial perturbations. Given the targets of current and
next generation experiments, we show that any successful detection would
signify that this upper limit is of the order of unity, which is consistent
with recent discussions in the literature.
