Understanding range limits of a vulnerable desert amphibian (Anaxyrus exsul) with a severely restricted habitat

Background/Question/Methods

Populations of the black toad (Anaxyrus exsul) are limited to desert springs within a single valley in the White-Inyo Range of California; all reported native populations occur in an area of about 24 km² in Deep Springs Valley, giving it one of the smallest documented ranges of any amphibian.  The few recent studies that have been carried out on the black toad have focused on population size and population differentiation between different spring habitats. Variation in vegetation, physical and chemical properties of toad habitats -- and their effects on toad populations -- have to date been poorly described.  Thus we have little information about the habitat factors affecting the success of this species, which is considered Vulnerable on the IUCN Red List and is a Fully Protected species in the state of California. In order to understand the factors limiting the black toad’s breeding range, we explored and mapped all of the areas in which black toads have been reported as well as local water sources in which toads have not been found previously.  At each site, we measured physical (temperature, depth) and chemical properties (pH, dissolved oxygen, electrical conductivity) of the potential toad habitat.  We also recorded general features of the aquatic and shoreline vegetation (dominant species, percent cover). 

Results/Conclusions

Preliminary analyses of natural, spring-fed water bodies in Deep Springs Valley have revealed extreme variation in pH (from fairly acidic, pH of 5.6, to highly alkaline, pH of 9.6).  Additionally, some riparian areas near spring sources had water temperatures that were fairly stable over time while others more closely followed ambient air conditions.  The density and identity of vegetation cover also varied widely amongst riparian areas.  Unfenced cattle-grazed areas contained both open water bodies with sedge-dominated borders as well as areas densely filled by cat-tails or rushes, whereas fenced riparian areas were shady and often dominated by woody vegetation (willows).  Here we report on the occurrence of adult breeding toads, egg masses and tadpoles at each of the local riparian habitats, identifying the habitat characteristics that were associated with successful black toad breeding.   Our findings will be important in developing future stewardship plans designed to maintain stable populations of this unique amphibian.  In this study we were able to characterize the entire habitat of a species, and thus our results also have implications for other studies that seek to use habitat characteristics to define species range limits.