  We investigate the spectral correlations between different species used to
observe molecular clouds. We use hydrodynamic simulations and a full chemical
network to study the abundances of over 150 species in typical Milky Way
molecular clouds. We perform synthetic observations in order to produce
emission maps of a subset of these tracers. We study the effects of different
lines of sight and spatial resolution on the emission distribution and perform
a robust quantitative comparison of the species to each other. We use the
Spectral Correlation Function (SCF), which quantifies the root mean squared
difference between spectra separated by some length scale, to characterize the
structure of the simulated cloud in position-position-velocity (PPV) space. We
predict the observed SCF for a broad range of observational tracers, and thus,
identify homologous species. In particular, we show that the pairs C and CO,
C$^{+}$ and CN, NH$_3$ and H$_2$CS have very similar SCFs. We measure the SCF
slope variation as a function of beam size for all species and demonstrate that
the beam size has a distinct effect on different species emission. However, for
beams of up to 10", placing the cloud at 1 kpc, the change is not large enough
to move the SCF slopes into different regions of parameter space. The results
from this study provide observational guidance for choosing the best tracer to
probe various cloud length scales.
