Two-decade changes in habitat and abundance of Japanese macaques in primary and logged forests in Yakushima: Interim report

Continuous, rather than discrete, data on animal population are ideal for identifying the causal relationships driving population fluctuations. In this study, we reveal the effect of forest regeneration after logging on the habitat, food availability, and abundance of Japanese macaques over two decades, spanning the period from 2000 until 2019, based on a continuous population census conducted every year. Three vegetation types, covering an area of 7.5 km², make up our study sites: primary forests, forests logged from 1984 until 1995 and then naturally regenerated (natural regeneration), and forests logged from 1975 until 1983 and then planted with cedars (plantation). In natural regeneration, both individual and total tree biomass increased, in particular naturally grown cedars. In plantation, the number of trees decreased and the total biomass did not change, mainly due to thinning (partial removal of planted cedars to harvest timber) in 2015. A state space model suggests that the relative fruit production in natural regeneration to that in the primary forest tended to be low after 2012, a condition brought about by cleaning cutting to remove non-cedar broad-leaved trees and shrubs in natural regeneration. An analysis of aerial photos detected an increase in naturally grown cedar patches in natural regeneration between 2009 and 2014. We also observed a population decrease of Japanese macaques in natural regeneration but not in primary and plantation forests. The state space model indicates that this decrease occurred in the first half of the study period, which was earlier than the replacement of cedar patches in natural regeneration. The high time resolution of our data refutes the simple scenario that macaques decreased their number in natural regeneration due to the reduced food availability there. The less frequent use of natural regeneration began as a stochastic event; i.e., the disappearance of multiple individuals from a group ranging there, and then the subsequent changes in the habitat reinforced this tendency.