The anomalous combination of temperature and precipitation is behind sudden deaths of frogs, toads and salamanders, according to a study led by the National Museum of Natural Sciences (MNCN-CSIC) together with the Joint Research Institute in Biodiversity and the Spanish Herpetological Association, published in Current Biology.
Far from attributing it solely to the heat, the study reformulates the previous theory and proposes the Theory of Thermal-Hydrological Mismatch : it is the interaction between temperature anomalies and changes in the rainfall regime that triggers pathogens and increases the mortality of amphibians, the most threatened group of vertebrates on the planet.
Key data from the study in Spain
The team analyzed nearly 6.000 specimens sampled throughout Spain , covering cold and humid habitats as well as warm and dry ones, to link infection peaks with unusual weather events.
Amphibians adapted to cold and humid climates suffered more infections from the fungus Batrachochytrium dendrobatidis (Bd) during abnormally warm and dry periods , while species typical of warm and dry environments were more infected in unusually cold and humid phases.
In contrast, the viruses that circulate naturally in our populations thrived under conditions opposite to those of the fungus; this asymmetry explains their limited temporal overlap despite sharing space.
Lead author Barbora Thumsová emphasizes that this is the first evidence that precipitation anomalies, combined with thermal ones , govern the process and that the same logic is confirmed for lethal viruses that affect amphibians.
Fungi and viruses: opposing dynamics and consequences
Chytidomycosis, caused by the Bd fungus —of probable Asian origin—, damages the skin and can cause heart failure ; it is one of the main drivers of the global decline of frogs, toads and newts as it is already distributed throughout the planet.
Ranavirosis, on the other hand, can trigger mass mortalities by proliferating and destroying skin tissue and internal organs; both pathogens respond to different climatic signals, which requires surveillance that takes into account both temperature and water.
The study warns about emblematic species such as the Pyrenean frog or the Betic midwife toad, which have survived for millions of years and are now exposed due to the loss of safe havens in an increasingly unpredictable environment.
Beyond their intrinsic value, amphibians act as indicators of the state of ecosystems , control insect pests and sustain food chains; their decline is a warning sign that points to deep environmental dysfunctions.
The team is already collaborating with managers to translate evidence into action: continuous monitoring of populations and pathogens, restoration of wetlands and refuge corridors, and models that integrate thermal and rainfall variations to anticipate outbreaks.
The evidence converges on a simple idea: when the rhythm of heat and rain becomes unbalanced, pathogens find their window of opportunity and amphibians lose room to maneuver; understanding this imbalance is key to gaining time and preventing further die-offs.
