It is the first study of its kind. It is specifically dedicated to the development of global animal populations in forests. The authors cite the loss of habitat caused by humans as the main cause of the decline. Deforestation and forest degradation are responsible for 60% of the collapse of animal populations. The development in the tropics is particularly dramatic.

According to the study, amphibians and reptiles are the species that suffer most from the loss of their habitat. But the populations of mammals such as monkeys and forest elephants have also declined. However, there are also examples where species have been able to recover. This is the case with gorillas in Central and East Africa, where the number is expected to rise again - not least thanks to conservation measures. The capuchin monkeys in Costa Rica are also doing better. The number of birds had also increased.

The WWF investigated 268 vertebrate species and 455 populations. Based on the methodology of the Living Planet Report, the "Forest Specialist Index" was developed in cooperation with the UN World Monitoring Centre for Nature Conservation (UNEP-WCMC) and the Zoological Society of London (ZSL). This is the first indicator to provide a representative account of the health of global forest ecosystems. The data refer to the period between 1970 and 2014, the last year from which corresponding data are available.