Can secondary forests mitigate the negative effect of old-growth forest loss on biodiversity? A landscape-scale assessment of two endangered primates

Context Old-growth forest loss drives the global biodiversity crisis. Nevertheless, this impact could be buffered by the increasing expansion of secondary (regenerating) forests, which can provide supplementary habitat for wildlife. Objectives We tested this hypothesis assessing the effect of old-growth and secondary forest cover on the abundance and immature-to-female ratio (proxy of reproductive success) of two endangered primates: Geoffroy’ spider monkeys and black howler monkeys. Methods We measured the response and predictor variables across 18 whole landscapes (landscape-scale approach) in the Lacandona rainforest, Mexico. As there could be tipping points of forest loss beyond which species extinction is accelerated (extinction thresholds), we separately tested the linear and non-linear effect of forest cover on each response, independently for three spatial scales. Results We found stronger and larger-scale negative responses to forest loss in spider monkeys than in howler monkeys. However, the data were better predicted by linear models, giving no support to the extinction threshold hypothesis. In both species, forest loss had stronger negative impacts on monkey abundance when considering old-growth forest, than when considering secondary forest cover, or total (old-growth + secondary) forest cover. Yet, the immature-to-female ratio was weakly related to forest cover in both species. Conclusion Secondary forests seem to have a weak buffering effect in both species, possibly because they are relatively young (< 30 years old) and do not have large trees. This implies that old-growth forests are irreplaceable for preventing primate extirpation, especially for species with specialized diet and large spatial requirements, such as spider monkeys.