Standardizing methods to estimate population density: an example based on habituated and unhabituated spider monkeys

Population estimates are critical for making informed conservation decisions. However, methods for data collection and analysis of population estimates from wildlife surveys vary, often preventing comparisons between sites or years. In this study we compared population density estimates of spider monkeys, Ateles geoffroyi, derived from four commonly used methods to the actual density estimate based on known individual monkeys and home-range size and corroborated these results with surveys done on unhabituated monkeys in the same area. We recorded perpendicular distances of individual monkeys in the Otoch Ma’ax yetel Kooh Protected Area during two surveys: within the home range of an individually-recognized spider monkey group (survey one) and largely outside of the home range (survey two). We sighted 278 and 76 spider monkeys for a total effort of 93.74 and 42.78 km in surveys one and two, respectively. The actual density estimate was 65.4 individuals/km ² (survey one). This value lies closer to the population density estimate obtained using the Kelker method (58.2 individuals/km ² ) than conventional distance sampling (CDS; 92.9–93.8 individuals/km ² ). Density estimates obtained with King and maximum perpendicular distance methods deviated substantially from the actual density. Population density estimates using the Kelker method and CDS differed less in survey two. Population density estimates differed little whether transects were walked slow or fast. We recommend using the Kelker method and CDS to estimate population density with a correction for distance estimation errors. We demonstrate how studies on populations of known size can improve the methods to survey populations of unknown size.