Empirical evidence for effects of invasive American Bullfrogs on occurrence of native amphibians and emerging pathogens

Abstract/Summary

Invasive species and emerging infectious diseases are two of the greatest
threats to biodiversity. American Bullfrogs (Rana [Lithobates] catesbeiana),
which have been introduced to many parts of the world, are often linked with
declines in native amphibians via predation and the spread of emerging pathogens
such as amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd])
and ranaviruses. Although many studies have investigated the potential role of
bullfrogs in the decline of native amphibians, analyses that account for shared
habitat affinities and imperfect detection have found limited support for
clear effects. Similarly, the role of bullfrogs in shaping the patch-level distribution
of pathogens is unclear. We used eDNA methods to sample 233 sites in the southwestern USA and Sonora, Mexico (2016–2018) to estimate how
the presence of bullfrogs affects the occurrence of four native amphibians,
Bd, and ranaviruses. Based on two-species, dominant-subordinate occupancy
models fitted in a Bayesian context, federally threatened Chiricahua Leopard
Frogs (Rana chiricahuensis) and Western Tiger Salamanders (Ambystoma
mavortium) were eight times (32% vs. 4%) and two times (36% vs. 18%), respectively,
less likely to occur at sites where bullfrogs occurred. Evidence for the
negative effects of bullfrogs on Lowland Leopard Frogs (Rana yavapaiensis)
and Northern Leopard Frogs (Rana pipiens) was less clear, possibly because of
smaller numbers of sites where these native species still occurred and because
bullfrogs often occur at lower densities in streams, the primary habitat for
Lowland Leopard Frogs. At the community level, Bd was most likely to occur
where bullfrogs co-occurred with native amphibians, which could increase the
risk to native species. Ranaviruses were estimated to occur at 33% of bullfrogonly
sites, 10% of sites where bullfrogs and native amphibians co-occurred,
and only 3% of sites where only native amphibians occurred. Of the 85 sites
where we did not detect any of the five target amphibian species, we also did
not detect Bd or ranaviruses; this suggests other hosts do not drive the distribution
of these pathogens in our study area. Our results provide landscape-scale
evidence that bullfrogs reduce the occurrence of native amphibians and
increase the occurrence of pathogens, information that can clarify risks and
aid the prioritization of conservation actions.

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