Line transect surveying of arboreal monkeys: problems of group size and spread in a highly fragmented landscape.

Line transect surveying is a relatively rapid and efficient procedure for the assessment of primate populations; although the reliability of the resulting density estimates is affected by a number of factors, including habitat configuration. In this study, primate populations were surveyed at a highly fragmented site in northeastern Brazil, where the resident species include the endangered Coimbra-Filho's titi, Callicebus coimbrai, and the critically endangered yellow-breasted capuchin, Cebus xanthosternos. In addition to standard line transect surveys, data were collected on the composition and distribution of resident groups during playback surveys and ecological monitoring. During surveys, a 5.3-km trail system was walked regularly over a 7-month period, accumulating a total transect length of 476 km and 104 sightings of Callicebus and 41 of Cebus (as well as 19 records of a third primate, Callithrix jacchus). Data were analyzed using the DISTANCE program and the Kelker method, using the perpendicular distances to first animal sighted. Estimates of group density were highly discrepant from known values for both species; although in the case of Cebus, this seemed to reflect grouping patterns at the site. For Callicebus, population density was overestimated considerably when known group counts were used. Overall, there seems to be a need for a much more rigorous standardization of procedures, and the analysis and presentation of results, in order to guarantee the most reliable interpretation possible of the data. These considerations are especially important in the case of endangered species, such as those studied here.