Below the canopy: global trends in forest vertebrate populations and their drivers

Global forest assessments use forest area as a proxy indicator of biodiversity status, which may mask below-canopy pressures driving forest biodiversity loss and ‘empty forest’ syndrome. The status of forest biodiversity is important not only for species conservation but also because species loss can have consequences for forest health and carbon storage. We aimed to develop a global indicator of forest specialist vertebrate populations to improve assessments of forest biodiversity status. For this purpose we used the Living Planet Index methodology, developing a weighted composite Forest Specialist Index for the period 1970-2014. We then investigated potential drivers of forest vertebrate population change, including tree cover change, to determine whether forest area is a good proxy for forest biodiversity. The effects of satellite-derived tree cover trends and other pressures on the average rate of change of forest vertebrate populations were analysed. We reviewed the literature to gain more context-specific information relating to drivers of forest specialist population change. On average, forest vertebrate populations declined by 53% between 1970 and 2014. We found little evidence of a consistent global effect of tree cover change on forest vertebrate populations but a significant negative effect of exploitation threat on forest specialists. However, time-series cross-correlation analyses showed some forest specialist populations are closely aligned to tree cover change. The literature review identified several drivers of population change that cannot be detected remotely and may cause populations to change independently of tree cover. Forest vertebrate populations have more than halved since the 1970s. In conclusion, we found that forest area is a poor proxy of forest biodiversity status. For forest biodiversity to recover, we must monitor and manage all threats to vertebrates, including those below the canopy.