Study finds clues in frog die-off mystery

Scientists suspect UV-B as accomplice in frog die-off mystery…
One of the most disturbing ecological stories of our time has been the mysterious global decline in amphibian populations. What has been so strange is that these declines are occurring in large part across seemingly pristine environments where anthropogenic impacts are not immediately obvious. Various potential causes (habitat loss and modification, alien species, diesease, contamination, etc.) have been implicated, but past research has found that no single cause fully explains what's going on. A complex interaction of factors may be at work.

Striped marsh frog (Limnodynastes peronii) with spawn.A new study published in the journal Global Change Biology examines UV-B exposure from human destruction of stratospheric ozone as a suspect in the amphibian die-off. Past studies have found that in some cases UV-B exposure causes harm, but the results are inconsistent across amphibians. What makes this study unique is that it looks at the potential synergistic effect of UV-B exposure in combination with a biotic stressor - specifically the threat of predation. Synergistic effects occur when two stressors in combination have a substantially greater impact than the sum of either acting indvidually. The researchers attempted to determine whether UV-B exposure and just the threat of predation (versus predation itself) interact synergistically to cause greater mortality.

Creative methodology finds smoking gun...
To find the answer, the researchers exposed tadpoles of striped marsh frog (L. peronii) to treatment of UV+ (i.e. radiation that includes UV-B). They also epxosed tadpoles to predatory chemical cues collected from dragon fly nymphs (Hemianax papuensis). The researchers divided the tadpoles into 4 groups with each receiving a different treatment: 1) No UV+ and No predator cue, 2) Yes UV+ treatement and No predator cue, 3) No UV+ and Yes Predator Cue, and 4) Yes UV+ and Yes Predator Cue. The researchers exposed the tadpoles to treatments for 9 days, during which, they checked survival daily.

The study found that for the tadpoles receivng no exposure, mortality was 0%. For those just receiving the predator chemical cue, mortality was 1.2%, for those receiving just the UV+, mortality was 17.6%, and for those receive both the chemical cue and the UV+, mortality was 36.8%. This result showed a synergistic effect from the predatory stressor in combination with the UV-B (i.e. the UV+ treatment). The authors explain that this synergistic effect may arise because the predatory stressor changes foraging behaviour in the tadpoles which in turn reduces fitness level and corresponding ability to tolerate the UV-B stressor.

In addtition to the increased mortality, the study also found that the exposure to UV-B and predatory chemical cues significantly affected tadpole size and shape. Immediately following the end of the experiment, the researchers photographed the dorsal and lateral views of individuals with a digital camera mounted through a dissecting microscope.Specifically, exposure to UV+ suppressed the development of predator-induced morphological defenses including deeper tails which studies haven shown improve tadpole survival in the face of predators. This shows that synergistic effect in the wild may be even greater than thae measured in the lab.

Image Courtesy of Global Change BiologyConclusions, unfiltered...

"Here, we show that simultaneous exposure to a global abiotic stressor (UV-B radiation), that has recently increased in severity due to anthropogenic activities, and a pervasive natural stressor (predatory chemical cues) has the potential to negatively impact upon amphibians via two different mechanisms: (1) increased mortality; and (2) suppressed predator-induced phenotypic plasticity. Given that these stressors may be acting in habitats not obviously affected by human activities, the interactive effects of these stressors have the potential to explain amphibian declines occurring in seemingly pristine habitats. Future research should therefore be directed at examining the interactive effects of such environmental stressors as well as the underlying mechanisms."

Source:	Global Change Biology
Title:	Risk of predation enhanes the lethal effect of UV-B in amphibians
Authors:	a) Lesley Alton, Robbie Wilson, and Craig Franklin
	a) University of Queensland