The ghosts of mammals past: biological and geographical patterns of global mammalian extinction across the Holocene

Abstract

Although the recent historical period is usually treated as a temporal base-line for understanding patterns of mammal extinction, mammalian biodiversity loss has also taken place throughout the Late Quaternary. We explore the spatial, taxonomic and phylogenetic patterns of 241 mammal species extinctions known to have occurred during the Holocene up to the present day. To assess whether our understanding of mammalian threat processes has been affected by excluding these taxa, we incorporate extinct species data into analyses of the impact of body mass on extinction risk. We find that Holocene extinctions have been phylogenetically and spatially concentrated in specific taxa and geographical regions, which are often not congruent with those disproportionately at risk today. Large-bodied mammals have also been more extinction-prone in most geographical regions across the Holocene. Our data support the extinction filter hypothesis, whereby regional faunas from which susceptible species have already become extinct now appear less threatened; they may also suggest that different processes are responsible for driving past and present extinctions. We also find overall incompleteness and inter-regional biases in extinction data from the recent fossil record. Although direct use of fossil data in future projections of extinction risk is therefore not straightforward, insights into extinction processes from the Holocene record are still useful in understanding mammalian threat.

Figure 1.

Global patterns of Holocene species extinctions and currently threatened species. (a) Number and (b) proportion of extinct species; (c) number and (d) proportion of threatened species. Countries without extinct or threatened species in our dataset are not plotted in (a) and (c). Plots in (b) and (d) use the same colour scale; proportions above 0.5 were set to 0.5. The size of the plotted circles reflects the total number of species (extant + extinct) in (a) and (b), and the number of extant species in (c) and (d), on a logarithmic scale (see size key).

Figure 2.

The phylogenetic pattern of Holocene extinctions on the mammal supertree. Extinct species are marked by red triangles at the tips; major groups are alternately coloured dark and light grey to visually set them apart. The tree contains 5000 extant and 210 extinct species.

Figure 3.

The relationship between extinction risk and body mass within countries, (a) including and (b) excluding extinct species. Circle colour shows the estimated slope of the relationship from a phylogenetic GLS regression, and circle size the significance for this slope (small circle, p > 0.05; big circle, p < 0.05). Models were fitted for countries with more than 10 species with all data and in the phylogeny. Slope values more than 1 were set to 1.

Figure 4.

The median body mass of extinct species and its departure from expectation under random extinction within countries. Circles are only plotted for countries with at least one extinct species. Circle shading shows the median log₁₀-transformed body mass of extinct species; circle size indicates the results of a one-tailed significance test of this observed median extinct body mass against that expected under random extinction of the same number of species within countries (small circle, p > 0.05; big circle, p < 0.05).

Figure 5.

Available radiometric last-occurrence dates of extinct mammal species in relation to timing of first human arrival and European arrival for (a) Madagascar, (b) Cuba, and (c) Hispaniola. Black lines represent 1σ radiometric confidence limits for extinct mammal species last-occurrence dates; shaded areas represent 1σ radiometric confidence limits for first regional prehistoric evidence of human presence; shaded line represents known calendar date of first regional European arrival.