Spatio-temporal changes in chimpanzee density and abundance in the Greater Mahale Ecosystem, Tanzania

Species conservation and management require reliable information about animal distribution and population size. Better management actions within a species' range can be achieved by identifying the location and timing of population changes. In the Greater Mahale Ecosystem (GME), western Tanzania, deforestation due to the expansion of human settlements and agriculture, annual burning, and logging are known threats to wildlife. For one of the most charismatic species, the Endangered eastern chimpanzee (Pan troglodytes schweinfurthii), about 75% of the individuals are distributed outside national park boundaries, requiring monitoring and protection efforts over a vast landscape of various protection statuses. These efforts are especially challenging when we lack data on trends in density and population size. To predict spatio-temporal chimpanzee density and abundance across the GME, we employed density surface modelling, fitting a generalised additive model to a ten-year time series data set of nest counts based on line transect surveys. Chimpanzee population declined at an annual rate of 2.41%, including declines of 1.72% in riparian forests (hereafter forests), 2.05% in miombo-woodlands (hereafter woodlands) and 3.45% in non-forests. These population declines were accompanied by ecosystem-wide declines in vegetation types of 1.36% and 0.32% per year for forests and woodlands, respectively; we estimated an annual increase of 1.35% for non-forests. Our model predicted the highest chimpanzee density in forests (0.86 chimpanzees/km², 95% CI 0.60-1.23; as of 2020), followed by woodlands (0.19, 95% CI 0.12-0.30) and non-forests (0.18, 95% CI 0.10-1.33). Although forests represent only 6% of the landscape, they support nearly a quarter of the chimpanzee population (769 chimpanzees, 95% CI 536-1,103). Woodlands dominate the landscape (71%) and thus support more than a half of the chimpanzee population (2,294; 95% CI 1,420-3,707). The remaining quarter of the landscape is represented by non-forests and supports another quarter of the chimpanzee population (750; 95% CI 408-1,381). Given the pressures on the remaining suitable habitat in Tanzania and the need of chimpanzees to access both forest and woodland vegetation to survive, we urge future management actions to increase resources and expand the efforts to protect critical forest and woodland habitat and promote strategies and policies that more effectively prevent irreversible losses. We suggest that regular monitoring programmes implement a systematic random design to effectively inform and allocate conservation actions and facilitate inter-annual comparisons for trend-monitoring, measuring conservation success and guiding adaptive management.