Forest fragmentation is a major threat that triggers declines in biodiversity at regional and global scales. Importantly, forest fragmentation affects tree species diversity and composition by limiting seed dispersal. Biodiversity losses caused by forest fragmentation may occur with time-lags because of species becoming extinct due to past events (i.e., extinction debt; EB) and the immigration of new individuals (i.e. immigration credit; IC). Species traits correlated with EB (e.g., generation time) and IC (e.g., dispersal ability) are vital determinants of species diversity and species abundances in fragmented landscapes. In this study, we investigated whether past forest cover affects contemporary species diversity and species abundances in fragmented forest landscapes in Borneo. We also investigated the effects of functional traits of species (i.e., generation time and dispersal ability) on EB and IC. We conducted the study in two rural areas of Sarawak, Malaysian Borneo, where forest fragmentation has been ongoing since the 1960s. Eight fragmented forests were selected in each area. In the first area, 16 survey plots of 0.25 ha were established, whereas in the second area, 19 plots were established. All trees with DBH > 10 cm were identified at the species level in each plot.
Results/Conclusions We identified a total of more than 531 tree species in both areas. Alpha diversity in fragmented forests was similar to that observed in primary forests. For investigating historical transitions of forest cover in the area, we analyzed Landsat satellite images from five time periods between 1972 and 2015 and found that forest cover of natural forest decreased over time in both areas. Then, we quantified the effects of past forest cover on contemporary species diversity and species abundances with Generalized Linear Mixed Models. The best model to account for contemporary species diversity was the one including forest cover in 2006, and those were positively correlated. The effects of forest fragmentation on species abundances differed among functional groups, with pioneer species showing negative correlations with past forest cover, and shade-tolerant species showing positive correlations with past forest cover. We conclude that in Borneo forest fragmentation reduces species diversity with time delays, and the effects of fragmentation vary according to functional traits of species. For the prevention of future biodiversity losses in fragmented forest landscapes, conservation strategies should account for the diverse functional groups occurring in biological communities.