Distance sampling data were analyzed using the gdistsamp
function based on the method proposed by Royle et al. (2004)
and extended by Chandler et al. (2011). This model is similar
to the N-mixture model, although it adds a component to the
likelihood to address variability in detection probability as a
function of range from the survey transect. Here transect-level
abundance is modeled the same as the N-mixture model, but
the detection process is modeled as