Phylogenetic diversity (PD), or the amount of independent evolution contained within a phylogeny, is an important aspect of biodiversity that captures lineage history as well as hidden functional trait diversity and evolutionary potential. This is why it is increasingly used as a conservation metric to triage species as in the popular Evolutionarily Distinct and Globally Endangered (EDGE) Score. Although conservation may shore up local PD, global PD can only increase with time. For example, we gain roughly 5,400 years of independent evolution (PD) every year because the Earth currently supports about 5,400 species of mammals. However, that amount pales in comparison to the ~2.5 billion years of independent evolution that we have already lost since the Ice Age (6% of global mammal PD). If all Endangered mammals go extinct, we could lose another 7%. How long will it take naturally evolving mammals to replenish the PD debt brought about by the Sixth Mass Extinction?
Using a recent phylogeny and trait dataset for late Quaternary mammals, we estimated future PD gains and losses with birth-death tree simulations conditioned on empirical diversification rates for Cenozoic mammals.
Our simulations show that even if extinction rates drop to background levels, mammals would have to evolve over 20 times faster than their average speciation rate to replenish lost PD within the next 250,000 years. These rapid speciation rates would lead to 5 times as many mammal species on earth and are incredibly unrealistic. A full recovery of PD lost during the Sixth Mass Extinction could take millions of years, especially if Endangered and Vulnerable species go extinct. The exponential nature of speciation means that eventual recovery of PD is inevitable given enough time but functional traits will likely take longer to re-evolve. Even if extinction rates fall to zero, the median body mass of mammals could stay depressed for 10 million years or more.