Quantifying intraspecific variation in host resistance and tolerance to a lethal pathogen

Area of Science:

• Ecology
• Conservation Biology
• Pathogen-Host Dynamics

Background:

• Amphibian declines due to the fungal pathogen Batrachochytrium dendrobatidis (Bd) are a significant conservation concern.
• Boreal toads (Anaxyrus boreas boreas) in Colorado have experienced severe declines from Bd, while populations in Wyoming appear less affected.
• Intraspecific variation in host tolerance and resistance can influence population-level disease outcomes.

Purpose of the Study:

• To investigate intraspecific variation in tolerance and resistance to Batrachochytrium dendrobatidis (Bd) in wild boreal toad populations.
• To determine if differences in tolerance and resistance explain regional variations in Bd-induced population declines.
• To assess the utility of a multi-state modeling approach for understanding dynamic infection processes.

Main Methods:

• A common garden infection experiment was conducted using post-metamorphic boreal toads from four populations (two in Colorado, two in Wyoming).
• Toads were exposed to Batrachochytrium dendrobatidis (Bd) and monitored for changes in mass, pathogen burden, and survival over eight weeks.
• A multi-state modeling approach was used to estimate weekly survival and transition probabilities between infected and cleared states.

Main Results:

• Wyoming boreal toads exhibited significantly higher tolerance to Bd infection, demonstrated by increased survival probabilities compared to Colorado toads under identical pathogen loads.
• Wyoming toads also displayed greater resistance, with a higher probability of clearing the infection and a longer time to reach peak pathogen burdens.
• Strong intraspecific differences in both tolerance and resistance were observed between the studied populations.

Conclusions:

• Intraspecific variation in tolerance and resistance to Batrachochytrium dendrobatidis plays a critical role in explaining differential population declines in boreal toads.
• Conservation strategies, including captive breeding and translocation, should consider this intraspecific variation.
• The multi-state modeling framework provides valuable insights into hidden disease dynamics when assessing host tolerance and resistance.