First integrative trend analysis for a great ape species in Borneo

Truly Santika, Marc Ancrenaz, Kerrie A. Wilson, Stephanie Spehar, Nicola Abram, Graham L. Banes, Gail Campbell-Smith, Lisa Curran, Laura d’Arcy, Roberto A. Delgado, Andi Erman, Benoit Goossens, Herlina Hartanto, Max Houghton, Simon J. Husson, Hjalmar S. Kühl, Isabelle Lackman, Ashley Leiman, Karmele Llano Sanchez, Niel Makinuddin, Andrew J. Marshall, Ari Meididit, Kerrie Mengersen, Musnanda, Nardiyono, Anton Nurcahyo, Kisar Odom, Adventus Panda, Didik Prasetyo, Purnomo, Andjar Rafiastanto, Slamet Raharjo, Dessy Ratnasari, Anne E. Russon, Adi H. Santana, Eddy Santoso, Iman Sapari, Jamartin Sihite, Ahmat Suyoko, Albertus Tjiu, Sri Suci Utami-Atmoko, Carel P. van Schaik, Maria Voigt, Jessie Wells, Serge A. Wich, Erik P. Willems, Erik Meijaard

For many threatened species the rate and drivers of population decline are difficult to assess accurately: species' surveys are typically restricted to small geographic areas, are conducted over short time periods, and employ a wide range of survey protocols. We addressed methodological challenges for assessing change in the abundance of an endangered species. We applied novel methods for integrating field and interview survey data for the critically endangered Bornean orangutan (Pongo pygmaeus), allowing a deeper understanding of the species' persistence through time. Our analysis revealed that Bornean orangutan populations have declined at a rate of 25% over the last 10 years. Survival rates of the species are lowest in areas with intermediate rainfall, where complex interrelations between soil fertility, agricultural productivity, and human settlement patterns influence persistence. These areas also have highest threats from human-wildlife conflict. Survival rates are further positively associated with forest extent, but are lower in areas where surrounding forest has been recently converted to industrial agriculture. Our study highlights the urgency of determining specific management interventions needed in different locations to counter the trend of decline and its associated drivers.