The observed decline of Western Toads (Anaxyrus boreas) over several decades at a novel winter breeding site

The Western Toad (Anaxyrus boreas) population of the Atlin Warm Springs in northwestern British Columbia has persisted since at least 1924. An extraordinary feature of the population has been winter breeding in late February to early March, while nearby cold-water populations breed in late-May. Metamorphosis of tadpoles, enhanced by the warm water, occurs as early as late-March. In 2008, Amphibian Chytrid Fungus (Batrachochytrium dendrobatidis) was documented in toadlets at the warm springs. Until 2005, as many as eight egg clutches and 25 breeding adults had been observed at the warm springs, after which the population declined. In 2017, novel spring breeding occurred in a cooler pond in the spring complex. Future observations will help determine whether the population is recovering and whether breeding phenology and habitat use have changed.

Shifts in temperature influence howBatrachochytrium dendrobatidisinfects amphibian larvae

Many climate change models predict increases in mean temperature, and increases in frequency and magnitude of temperature fluctuations. These potential shifts may impact ectotherms in several ways, including how they are affected by disease. Shifts in temperature may especially affect amphibians, a group with populations that have been challenged by several pathogens. Because amphibian hosts invest more in immunity at warmer than cooler temperatures and parasites may acclimate to temperature shifts faster than hosts (creating lags in optimal host immunity), researchers have hypothesized that a temperature shift from cold-to-warm might result in increased amphibian sensitivity to pathogens, whereas a shift from warm-to-cold might result in decreased sensitivity. Support for components of this climate-variability based hypothesis have been provided by prior studies of the fungusBatrachochytrium dendrobatidis(Bd) that causes the disease chytridiomycosis in amphibians. We experimentally tested whether temperature shifts before Bd exposure alter susceptibility to Bd in the larval stage of two amphibian species – western toads (Anaxyrus boreas) and northern red legged frogs (Rana aurora). Both host species harbored elevated Bd infection intensities under constant cold (15° C) temperature in comparison to constant warm (20° C) temperature. Additionally, both species experienced an increase in Bd infection abundance when shifted to 20° C from 15° C, compared to a constant 20° C but they experienced a decrease in Bd when shifted to 15° C from 20° C, compared to a constant 15° C. These results are in contrast to prior studies of adult amphibians that found increased susceptibility to Bd infection after a temperature shift in either direction, highlighting the potential for species and stage differences in the temperature-dependence of chytridiomycosis.

Movement behaviour of adult western toads in a fragmented, forest landscape

Habitat loss and fragmentation are among the best documented explanations for the dramatic declines amphibians are experiencing globally. The western toad ( Anaxyrus boreas (Baird and Girard, 1852)) is an IUCN red-listed species whose distribution range has been significantly affected by habitat modification. We used radiotelemetry to follow daily, postbreeding movement patterns of 23 adult male toads in a fragmented landscape near Vancouver, British Columbia, Canada, composed of forest patches and small, recent clearcuts (~5 ha). Results showed that toads in forests oriented towards edges of clearcuts from at least as far as 150 m. Greater than 60% of toads released in forest patches chose to enter the clearcuts from adjacent forests, indicating high boundary permeability. Net displacement distance was not significantly reduced in these clearcuts; however, movement rates were significantly lower on clearcuts than in forest. This indicates that there is no structural impediment to movements; however, there still could be fitness consequences or an interaction with summer weather. Toads also used roads for their movement more frequently than at random relative to their area. These results suggest that the western toad’s movements may not be negatively affected by small-scale forest harvesting at our latitude during the spring.