# Estimating the impact of disease in species threatened by amphibian chytrid fungus: Comment on Murray et al.

Schmidt, B R (2010). Estimating the impact of disease in species threatened by amphibian chytrid fungus: Comment on Murray et al. Conservation Biology, 24(3):897-899.

## Abstract

Chytridiomycosis is an emerging disease of amphibians
caused by the chytrid fungus Batrachochytrium dendrobatidis
(hereafter Bd) (Fisher et al. 2009). Chytridiomycosis
is thought to be contributing significantly to global
amphibian declines (Berger et al. 1998; Stuart et al. 2004;
Skerratt et al. 2007). In the Neotropics the disease has
spread in a wave-like fashion (Lips et al. 2008), and arrival
at previously pathogen-free sites in this region results in
mass mortalities and extinctions (Lips et al. 2008). Little
is known, however, about the effects of Bd after population
declines have occurred. In some places, Bd persists
where it has invaded (Retallick et al. 2004). Thus, amphibians
may evolve resistance to this novel pathogen
such that the negative effects of Bd are attenuated over
time.
Murray et al. (2009) address this very important question.
They used a multistate mark-recapture model to estimate
survival of Bd-infected and Bd-free frogs (Litoria
pearsoniana) in Queensland, where Bd has been present
for 30 years. The use ofmark-recapturemethods is a clear
improvement over earlier studies in which survival was
estimated but did not account for imperfect detectability
of animals in field studies (Faustino et al. 2004; Schmidt
2004). Murray et al. found that Bd causes a substantial
reduction in survival. This matters because a reduction
(Schmidt et al. 2005) and because it implies that
Bd remains a dangerous pathogen, such that amphibian
populations are unlikely to recover from population declines
caused by Bd for many decades, if ever.
Here, I point out that the apparent monthly survival estimates
ofMurray et al. are likely confoundedwith perma-
Paper submitted November 11, 2009; revised manuscript accepted December 14, 2009.
nent emigration from the study site (Sandercock 2006). If
infected and uninfected frogs have different emigration
probabilities, then one would obtain different apparent
survival estimates from the mark-recapture analysis for
Bd-positive and Bd-negative frogs.

## Abstract

Chytridiomycosis is an emerging disease of amphibians
caused by the chytrid fungus Batrachochytrium dendrobatidis
(hereafter Bd) (Fisher et al. 2009). Chytridiomycosis
is thought to be contributing significantly to global
amphibian declines (Berger et al. 1998; Stuart et al. 2004;
Skerratt et al. 2007). In the Neotropics the disease has
spread in a wave-like fashion (Lips et al. 2008), and arrival
at previously pathogen-free sites in this region results in
mass mortalities and extinctions (Lips et al. 2008). Little
is known, however, about the effects of Bd after population
declines have occurred. In some places, Bd persists
where it has invaded (Retallick et al. 2004). Thus, amphibians
may evolve resistance to this novel pathogen
such that the negative effects of Bd are attenuated over
time.
Murray et al. (2009) address this very important question.
They used a multistate mark-recapture model to estimate
survival of Bd-infected and Bd-free frogs (Litoria
pearsoniana) in Queensland, where Bd has been present
for 30 years. The use ofmark-recapturemethods is a clear
improvement over earlier studies in which survival was
estimated but did not account for imperfect detectability
of animals in field studies (Faustino et al. 2004; Schmidt
2004). Murray et al. found that Bd causes a substantial
reduction in survival. This matters because a reduction
(Schmidt et al. 2005) and because it implies that
Bd remains a dangerous pathogen, such that amphibian
populations are unlikely to recover from population declines
caused by Bd for many decades, if ever.
Here, I point out that the apparent monthly survival estimates
ofMurray et al. are likely confoundedwith perma-
Paper submitted November 11, 2009; revised manuscript accepted December 14, 2009.
nent emigration from the study site (Sandercock 2006). If
infected and uninfected frogs have different emigration
probabilities, then one would obtain different apparent
survival estimates from the mark-recapture analysis for
Bd-positive and Bd-negative frogs.

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