Current extinction rates of reptiles and amphibians

Abstract

There is broad concern that a mass extinction of amphibians and reptiles is now underway. Here I apply an extremely conservative Bayesian method to estimate the number of recent amphibian and squamate extinctions in nine important tropical and subtropical regions. The data stem from a combination of museum collection databases and published site surveys. The method computes an extinction probability for each species by considering its sighting frequency and last sighting date. It infers hardly any extinction when collection dates are randomized and it provides underestimates when artificial extinction events are imposed. The method also appears to be insensitive to trends in sampling; therefore, the counts it provides are absolute minimums. Extinctions or severe population crashes have accumulated steadily since the 1970s and 1980s, and at least 3.1% of frog species have already disappeared. Based on these data and this conservative method, the best estimate of the global grand total is roughly 200 extinctions. Consistent with previous results, frog losses are heavy in Latin America, which has been greatly affected by the pathogenic chytrid fungus Batrachochytrium dendrobatidis. Extinction rates are now four orders-of-magnitude higher than background, and at least another 6.9% of all frog species may be lost within the next century, even if there is no acceleration in the growth of environmental threats.

Keywords: Bayesian methods; amphibians; extinction rates; reptiles.

Fig. S1.

Geographic regions included in the analysis.

Fig. 1.

Cumulative numbers of inferred anuran extinctions in (A) Mesoamerica and Brazil and (B) Madagascar and the Sahul region (where extinctions are strongly concentrated in New Guinea). Figures are sums of individual extinction probabilities (see Table 1 for frogs and Table 2 for squamates) and are extremely conservative (Materials and Methods and Figs. 4 and 5). Global grand totals are 66.1 frog extinctions and 54.0 squamate extinctions.

Fig. S2.

Size of the dataset representing New Guinea. A square-root scale is used on the y axis to make smaller values more visible.

Fig. 2.

Relationship between the current extinction probability and the last year of collection for frogs from the Mesoamerica (A), Brazil (B), Madagascar (C), and the Sahul region (D).

Fig. 3.

Size of anuran datasets representing regions with high (A) and low (B) extinction estimates. (A) The Sahul region (thin line) and Brazil (thick line). (B) The Sunda region (thin line) and southeastern United States (thick line).

Fig. 4.

Simulated extinction proportions recovered by Bayesian analysis in the presence and absence of extinction. Data are for (A) Mesoamerica, (B) Brazil, (C) Madagascar, and (D) the Sahul region. Upper lines (“real”) show actual values. Lower lines (“random”) show data produced by randomizing collection dates, which obliterates any signal of true extinction.

Fig. S3.

Number of species sampled per year in the actual data for frogs from (A) Brazil, (B) Mesoamerica, (C) Madagascar, and (D) the Sahul region (thin black lines) and datasets in which the years of collection have been randomized (thick gray lines; see also Fig. 4). Small offsets and the lack of consistent trends demonstrate that collectors are not focusing more strongly on particular groups of species.

Fig. S4.

(A and B) Effect of truncating the data records at either 1970 or 1990.

Fig. 5.

Simulated extinction proportions and posterior probability sums recovered after imposing artificial extinction events. Proportions were generated by randomly adding events to real occurrence data for frogs from the southeastern United States (closed circles) and for squamates from southern Europe (open squares). Sums are based on Bayesian calculations.