Coincident mass extirpation of neotropical amphibians with the emergence of the infectious fungal pathogen Batrachochytrium dendrobatidis

Abstract

Amphibians highlight the global biodiversity crisis because ∼40% of all amphibian species are currently in decline. Species have disappeared even in protected habitats (e.g., the enigmatic extinction of the golden toad, Bufo periglenes, from Costa Rica). The emergence of a fungal pathogen, Batrachochytrium dendrobatidis (Bd), has been implicated in a number of declines that have occurred in the last decade, but few studies have been able to test retroactively whether Bd emergence was linked to earlier declines and extinctions. We describe a noninvasive PCR sampling technique that detects Bd in formalin-preserved museum specimens. We detected Bd by PCR in 83-90% (n = 38) of samples that were identified as positive by histology. We examined specimens collected before, during, and after major amphibian decline events at established study sites in southern Mexico, Guatemala, and Costa Rica. A pattern of Bd emergence coincident with decline at these localities is revealed-the absence of Bd over multiple years at all localities followed by the concurrent emergence of Bd in various species at each locality during a period of population decline. The geographical and chronological emergence of Bd at these localities also indicates a southbound spread from southern Mexico in the early 1970s to western Guatemala in the 1980s/1990s and to Monteverde, Costa Rica by 1987. We find evidence of a historical "Bd epidemic wave" that began in Mexico and subsequently spread to Central America. We describe a technique that can be used to screen museum specimens from other amphibian decline sites around the world.

Fig. 1.

Timeline of mean relative salamander abundance (line) and mean Bd prevalence (bars) for all sites and species in Mexico. Asterisk indicates zero Bd prevalence for which sample sizes were large and probability of a false negative was low (<10%). Arrow indicates earliest year of Bd detection in Mexico (1972). The absence of Bd occurs during high abundance years and is followed by Bd emergence and increasing Bd prevalence that coincides with the marked decline of salamanders (15) at all sites in Mexico.

Fig. 2.

Results from laboratory trials monitoring Bd infection for one neotropical frog, (A) Plectrohyla matudai, and two neotropical plethodontid salamanders, (B) Pseudoeurycea leprosa and (C) Bolitoglossa rufescens. Dotted lines with square points represent Bd-infected individuals and solid lines with circle points represent Bd-uninfected individuals. All infected salamanders (five P. leprosa and three B. rufescens) increased infection intensity rapidly over time and suffered mortality. The average infection intensity of animals that died was ( ± 1 SE) = 37,841 ± 7,111 zoospore equivalents × swab⁻¹. Uninfected salamanders (10 P. leprosa and three B. rufescens) remained healthy and Bd negative over time. For P. matudai, all individuals (n = 10) were infected but persisted with low levels of infection.

Fig. 3.

Map of the spatial-temporal spread of Bd southward from Mexico (1970s) to Guatemala (1980s) to Monteverde, Costa Rica (1987), and further through lower Central America (1993–2004). Red circles with stars represent Bd emergence points from our data and plain red circles represent previously published points of Bd emergence in lower Central America from Lips et al. (7).