Mammal extinctions and the increasing isolation of humans on the tree of life

Abstract

A sixth great mass extinction is ongoing due to the direct and indirect effects of human pressures. However, not all lineages are affected equally. From an anthropocentric perspective, it is often purported that humans hold a unique place on Earth. Here, we show that our current impacts on the natural world risk realizing that expectation. We simulated species loss on the mammalian phylogenetic tree, informed by species current extinction risks. We explored how Homo sapiens could become isolated in the tree if species currently threatened with extinction disappeared. We analyzed correlates of mammal extinctions risks that may drive this isolation pattern. We show that, within mammals, and more particularly within primates, extinction risks increase with the number of known threat types, and decrease with geographic range size. Extinctions increase with species body mass, trophic level, and the median longitudinal extent of each species range in mammals but not within primates. The risks of extinction are frequently high among H. sapiens close relatives. Pruning threatened primates, including apes (Hominidae, Hylobatidae), from the tree of life will lead to our species being among those with the fewest close relatives. If no action is taken, we will thus not only lose crucial biodiversity for the preservation of Earth ecosystems, but also a key living reference to what makes us human.

Keywords: evolutionary history; extinction risks; phylogenetic originality; primates; threats.

Figure 1

Illustration of phylogenetic patterns in extinction risks and in the number of species that are more original than Homo sapiens in (a) primates and (b) mammals. We used the Springer et al. tree in (a) and the Rolland et al. tree in (b) to provide partial representations of the primate and mammal phylogeny. In (a), tips of the tree are the primate families except for the paraphyletic New World families (Atelidae, Aotidae, Callitrichidae, Cebidae, Pitheciidae) that we grouped into a single clade. In (b), tips of the tree are the monophyletic mammal orders. Given that we displayed a simplified version of the trees, we used the thickness of the terminal branches in trees to better indicate how many species there are in each family (for primates) or order (for mammals). The thickness is equal to log(1 + N)/log(2)*u, where N is the number of species, and u is the basic thickness when N = 1. Next to each phylogenetic tree, a table gives the number of species in each terminal clade (N) including data‐deficient species (IUCN 2016), the number of species (NO) that were more original than H. sapiens according to index ED in our simulations of missing data effects and its standard deviation over all simulations (SD), the same number of species (NO*) if currently threatened species are driven extinct (vulnerable, endangered and critically endangered species). A bar plot gives the percentage of species in each IUCN category (IUCN 2016)

Figure 2

Illustration of the effect species extinctions may have on species originality thanks to a theoretical example. Here we considered a set of 18 species named from letter a to letter r and their theoretical phylogenetic tree. We provide the extinction risk status of each species next to its name (LC: least concern; EN: endangered; CR: critically endangered). Two bar plots in front of the phylogeny indicate the level of originality of each species according to index ED, first (dark gray bars) with all species and second (light gray bars) when currently threatened species (classified CR and EN) are dropped out from the phylogenetic tree