Disease driven amphibian extinctions have weak effects on alpine lake communities in California’s Sierra Nevada
Mountain yellow-legged frogs (Rana muscosa and R. sierrae) in California’s Sierra Nevada are nearly extinct, largely due to the recent emergence and spread of the lethal amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd hereafter). Emerging diseases such as Bd can strongly affect host population dynamics, which may in turn affect a host’s community. Disease driven declines and extinctions of frogs in the Sierra Nevada created a natural experiment in which we investigated how emerging diseases and non-random single species extinctions can affect ecological communities. We analyzed previously collected snapshot data describing both frog presence-absence and benthic macroinvertebrate communities in 157 lakes throughout the southern Sierra Nevada. In addition, we performed new surveys of invertebrate communities in 22 lakes, repeated annually between 2007 and 2012. We chose lakes in three categories: lakes where Bd naïve frog populations were extant, lakes where Bd epizootics were ongoing and frog populations were declining, and lakes where Bd epizootics occurred prior to 2006 and frog populations were locally extinct. Among frog population categories in both studies, we compared invertebrate abundance, rarefied richness, evenness, and community composition
In the snapshot data, invertebrate richness was higher in lakes where frogs were present. However, in new surveys which accounted for frog population dynamics, invertebrate richness was lower where frogs were extant, but higher where frog populations were declining and extinct. In both cases, richness differed by fewer than two taxa. Community composition differed between lake categories, but we saw neither secondary extinctions nor invasions concurrent with frog extinctions. In the previously collected data, burrowing taxa (a caddisfly, an alderfly, and oligochaetes) differed in abundance between lake categories. However, in the new surveys, taxa that differed between categories were not exclusively burrowers, but were generally grazers. One mayfly increased in abundance where frogs were declining but not where frogs were extinct, while two taxa (midges and one mayfly) only decreased where frogs were declining. Similarities between lakes with and without frogs contrast with lakes with ongoing Bd driven die-offs, and suggest that Sierra Nevada lake communities may be resilient to extinctions of mountain yellow-legged frogs. Our results highlight how both host species’ traits and community properties influence community responses to emerging diseases and species extinctions.