A tad too high: Sensitivity to UV-B radiation may limit invasion potential of American bullfrogs in the Pacific Northwest invasion range

Background/Question/Methods

Biological invasion potential can be strongly influenced by abiotic factors such as temperature, water availability and solar radiation. Invasive species that possess phenotypically plastic traits can mediate impacts from these stressors with alterations to life history characteristics. Invasive individuals, however, may be unable to recognize or respond appropriately to novel stressors or novel degrees of stress. Understanding potential constraints on appropriate trait responses induced by abiotic stressors can aid in the management and control of important invaders. Our study explored tolerance and plastic trait response to UV-B radiation in an invasive anuran, the American bullfrog (Lithobates catesbeianus). We quantified tadpole mortality rates, refuge use, and color change responses across two size classes to UV-B exposure rates typically found at low elevation and high elevation breeding sites. Bullfrogs in the Pacific Northwest invasion range have been extremely successful at establishing at low elevations but are relatively rare in high elevation habitats. We predicted that individuals would respond to UV-B exposure with increased refuge use and darker body coloration but that individuals would suffer increased mortality in novel, high UV-B conditions due to ineffective defenses.

Results/Conclusions

We found an increase in tadpole mortality rates across both size classes at both low and high UV-B exposure rates (10-12µW/cm² and 20-24µW/cm², respectively). Only bullfrog tadpoles in the small size class exhibited a significant darkening in body color when exposed to high UV-B treatments. Although this smaller size class did exhibit color plasticity in response to stressful UV-B conditions, individuals did not correlate changes in body coloration with changes in behavior. Regardless of body coloration, larvae did not increase refuge use in low or high UV-B environments to mediate impacts on survivorship. These results suggest ontogenetic differences (estimated by size) in plastic color response to UV-B stress as well as constraints on behavioral use of refuge. These findings are important in understanding differences in bullfrog occupancy of breeding habitats across an elevational gradient, particularly in Oregon’s Cascade Mountain Range where bullfrog distributions are currently restricted at elevations above 1000m.