  The counting of the number of light modes in a gravitational theory is
captured by the notion of the `species scale', which serves as an effective UV
cutoff below the Planck scale. We propose to define a moduli-dependent species
scale in the context of 4d, ${\cal N}=2$ theories, using the one loop
topological free energy $F_1$, which we relate to a gravitational version of
the $a$-function. This leads to $\Lambda_{\rm sp}\sim 1/\sqrt{F_1}$ from which
we recover the expected scaling of the species scale in various corners of the
moduli space. Moreover by minimizing $F_1$ we define the center of the moduli
space (the `desert point') as a point where the species scale is maximal. At
this point the number of light degrees of freedom is minimized.
