Global Patterns of the Fungal Pathogen Batrachochytrium dendrobatidis Support Conservation Urgency

Abstract

The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) is a skin pathogen that can cause the emerging infectious disease chytridiomycosis in susceptible species. It has been considered one of the most severe threats to amphibian biodiversity. We aimed to provide an updated compilation of global Bd occurrences by host taxon and geography, and with the larger global Bd dataset we reanalyzed Bd associations with environmental metrics at the world and regional scales. We also compared our Bd data compilation with a recent independent assessment to provide a more comprehensive count of species and countries with Bd occurrences. Bd has been detected in 1,375 of 2,525 (55%) species sampled, more than doubling known species infections since 2013. Bd occurrence is known from 93 of 134 (69%) countries at this writing; this compares to known occurrences in 56 of 82 (68%) countries in 2013. Climate-niche space is highly associated with Bd detection, with different climate metrics emerging as key predictors of Bd occurrence at regional scales; this warrants further assessment relative to climate-change projections. The accretion of Bd occurrence reports points to the common aims of worldwide investigators to understand the conservation concerns for amphibian biodiversity in the face of potential disease threat. Renewed calls for better mitigation of amphibian disease threats resonate across continents with amphibians, especially outside Asia. As Bd appears to be able to infect about half of amphibian taxa and sites, there is considerable room for biosecurity actions to forestall its spread using both bottom-up community-run efforts and top-down national-to-international policies. Conservation safeguards for sensitive species and biodiversity refugia are continuing priorities.

Keywords: Bd; amphibian chytrid; climate associations; emerging infectious disease; fungal pathogen.

Figure 1

World maps of Batrachochytrium dendrobatidis (Bd) occurrences at unique sites from data compiled for the Global Bd Mapping project through: (A) June 2014 (42); and (B) December 2019. Sites shown with Bd detections also may have sampling results with no detection; records with only country-level coordinates are not mapped.

Figure 2

Regional maps of Batrachochytrium dendrobatidis (Bd) occurrences at unique sites from data compiled for the Global Bd Mapping project: (A) North America; (B) South America; (C) Europe; (D) Africa; (E) Eastern Asia; (F) Australia; and (G) New Zealand. Sites shown with Bd detections also may have sampling results with no detection; records with only country-level coordinates are not mapped.

Figure 3

United States 5^th-field hydrologic unit code watershed maps of Batrachochytrium dendrobatidis (Bd) occurrences from data compiled for the Global Bd Mapping project through: (A) June 2014 (42); and (B) December 2019. Watersheds shown with Bd detections also may have sampling results with no detection.

Figure 4

Relative habitat suitability of Batrachochytrium dendrobatidis (Bd) occurrence (probability ratio output) for each environmental attribute [(A) mean temperature; (B) annual precipitation; (C) maximum elevation; (D) temperature range] from the global presence-only best-fit species distribution model. Each environmental attribute marginal-response plot is calculated while holding all other covariates at the mean.

Figure 5

Absolute probability of Batrachochytrium dendrobatidis (Bd) occurrence for the environmental attributes (A) mean temperature and (B) temperature range from the global presence-absence best-fit species distribution model.

Figure 6

Global maps of predicted Batrachochytrium dendrobatidis (Bd): (A) habitat suitability derived from the best-fit presence-only species distribution model (log₂-transformed [log base 2] probability ratio output, area under curve [AUC] = 0.86); and (B) probability of Bd occurrence from the best-fit presence-absence species distribution model (AUC = 0.62). Both maps were derived using Bd data compiled through December 2019.

Figure 7

Regional predictions of Batrachochytrium dendrobatidis (Bd) habitat suitability from our best-fit presence-only species distribution models (log₂-transformed [log base 2] probability ration output, area under curve values in Table 5) using Bd data compiled through December 2019 for: (A) North America; (B) South America; (C) Europe; (D) Africa; (E) eastern Asia; and (F) Australia.

Figure 8

Regional predictions of Batrachochytrium dendrobatidis (Bd) absolute probability of Bd occurrence from our best-fit presence-absence species distribution models (area under curve values in Table 6) using Bd data compiled through December 2019 for: (A) North America; (B) South America; (C) Europe; (D) Africa; (E) eastern Asia; and (F) Australia.