Probabilistic models for understanding inland amphipods water population abundances in Andean stream (Caspana, 23°S, Antofagasta region, Chile)

Abstract The Andes mountains in northern Chile have numerous small permanent and intermittent streams that are poorly studied; among the most reported species are amphipods of the genus Hyalella , specifically H. fossamanchini and H. kochi . The aim of the present study was to do a first characterization of populations of both amphipod species in a small stream (Caspana, 23°S; 3200 m a.s.l.), that was visited in September 2022. The first results using a Kruskall-Wallis nonparametric test revealed that the abundances of both species and unidentified juveniles were not significantly different in a comparison of two sites, whereas a Mann-Wittney nonparametric U test gave similar results when each species and juveniles were compared between sites. The results of variance/mean ratio revealed the existence of an aggregated pattern, and a negative binomial distribution for all samples. The occurrence of an aggregated pattern and the consequent negative binomial distribution is a very frequent pattern for inland water invertebrates in Chilean rivers.