Population ecology of the spectacled caiman (Caiman crocodilus) in the Apaporis River middle basin

Population ecology studies on spectacled caimans (Caiman crocodilus) in Colombia have been few and far between with many covering short periods and defining population parameters based on relative indices (i.e., individuals/km). This reflects a lack of information on the general effects that environmental variables have on annual cycles of population dynamics, as well as a bias in abundance estimations due to the uncertainty of detection error. Keeping this in mind, we assessed the abundance and demographic structure of the spectacled caiman population inhabiting the Apaporis River middle basin over a year, based on robust hierarchical model that accounts for imperfect detection. We recorded a total of 1156 caiman observations between December 2018 and November 2019, estimating an average predicted value for abundance across all surveys of 29.99 ± 13.17 individuals, slowly increasing as the transect length increases and increasing variation as months passed by. The average detection probability was 0.69 ± 0.25 across all surveys, with no apparent effect as water temperature and relative humidity change across space-time and slowly decreasing as months go through. The population size estimated based on the top-performing model was 1763 ± 786 caimans across ~7.1 km² assessed. We estimate the commonly used relative abundance (encounter rate) index as well as a generalized linear model and discuss how those relate with the values predicted by N-mixture models. We also discuss the relevance and cautions researchers should have when using N-mixture models to better understand spectacled caiman ecology.