Severe Population Decline of Southern Pygmy Marmosets Cebuella niveiventris in a Protected Forest in Ecuador

Monitoring wild populations of the threatened Ecuadorian primates is the first step towards improving their conservation status. Since 2012, we have monitored a population of the southern pygmy marmoset (VU) in the Tiputini Biodiversity Station (TBS), in northeastern Ecuador. This area is not affected by deforestation, and human impact is low. In 2012 and 2022, from May through August, we surveyed 37 km of transects in gallery forests along the Río Tiputini and eight forest creeks early in the morning and in the afternoon. In addition, from 2013 through 2021, we annually surveyed 10 km of transects along forest creeks in an area of 2 km^2. During each survey, we used auditory and visual cues to detect the marmosets. When we found a group we recorded its location, group size and composition. We estimated the ecological density of the population using data from the 2012 and 2022 surveys, by dividing the total number of pygmy marmosets recorded in each survey by the total surveyed length. The ecological density decreased by 86% from one marmoset/km in 2012 to 0.14 marmoset/km of gallery forest in 2022. We do not know what caused this severe decline, but it could be a combination of a disease outbreak and changes in food availability related to climate change. Epidemiological studies and continuous monitoring of this and other populations of pygmy marmosets throughout the distribution range are urgently needed to develop informed conservation strategies to reverse the declining population trend.