Amphibian Decline: Global Change By: Brandon Udy December 3, 2009

Causes of Amphibian Decline Disease Habitat modification Killer fungus (Batrachochytriumdendrobatidis)

What Do These Have in Common?

Global Change

Hundreds are on the brink of extinction The Global Amphibian Assessment Lists: 427 species as “critically endangered” 122 species as “possibly extinct”

Reasons Why Global Change Causes Amphibian Decline Climate-change has reduced water depth at oviposition sites which increases embryos’ exposure to UV-B radiation Saprolegniaferax outbreaks due to increased UV-B exposure have been identified as a cause of high amphibian embryo mortality in the Pacific Northwest

Reasons Why Global Change Causes Amphibian Decline Ozone depletion has caused an increase UV-B radiation that harms amphibians’ DNA and their eggs

Reduced Water Depth El Nino: the warm phase of the periodic change in the atmosphere and ocean of the tropical Pacific region Precipitation and water depth/ UV-B exposure are strongly linked to El Nino/ Southern Oscillation cycles Amphibians, particularly those that breed in shallow montane lakes and ponds, may be quite susceptible to climate-induced changes in UV-B exposure

Experiment of Reduced Water Depth The experiment was conducted at a natural oviposition site of Bufoboreas The water depth at which embryos were placed was manipulated to 10, 50 or 100 cm The exposure to ambient UV-B radiation was also manipulated (either exposed to the full complement of ambient UV-B or shielded from UV-B)

Experiment of Reduced Water Depth The percentage of mortality associated with S. ferax infection was monitored in all manipulates

Experiment of Reduced Water Depth: Results climate-induced fluctuations in the water depth influence the exposure of developing embryos to UV-B radiation The percentage of mortality due to S.ferax was dependent on the water depth The hatching success of B. boreasexposed to UV-B radiation was 33% less than that of their counterparts that were shielded from UV-B

Experiment of Reduced Water Depth: Results

Experiment of Reduced Water Depth: Results

Experiment of Reduced Water Depth: Results

Experiment of Reduced Water Depth: Conclusion results suggest a complex series of interactions involving changes in both physical (water depth and UV-B exposure) and biotic (disease outbreaks) factors that alter mortality patterns Changes that alter hydrology may be the precursor to other related mortalities in other population declines (not just B. boreas) Supports that Pacific warming over recent decades is a common denominator for amphibian declines

Experiment of Reduced Water Depth: Conclusion in aquatic ecosystems, climatic changes can increase the exposure of organisms to UV-B Exposure of embryos to UV-B decreases the chances of survival of embryos Increased frequency of El Nino events may increase the incidence of high embryonic mortality experienced by certain amphibians in the Pacific Northwest

Experiment of Reduced Water Depth: Conclusion results suggest that in high mountain lakes, thefluctuation in water levels caused by global change could be of more concern with respect to UV-B exposure than depletion of stratospheric ozone

Chytridiomycosis In the mountains of Costa Rica, seventeen years ago, the Monteverde harlequin frog (Atelopus sp.) vanished along with the golden toad (Bufoperiglenes) The pathogenic chytrid fungus (Batrachochytriumdendrobatidis) is implicated An estimated 67% of the 110 or so species of Atelopus, which are endemic to the American tropics, have met the same fate

Chytridiomycosis Batrachochytriumdendrobatidiscauses chytridiomycosis It is believed that amphibians die from this because of interference with skin functions including maintenance of fluid balance, electrolyte homeostasis, respiration and role as a barrier to infections

How Does This Have Anything To Do With Global Change? At many high habitats, temperatures are shifting towards the growth optimum of Batrachochtrium, thus increasing outbreaks.

The Climate-Chytrid Paradox shifts in temperature or related variables often influence disease dynamics In fact, as temperatures rise, climate fluctuations may cross thresholds for certain pathogens, triggering outbreaks However, Batrachochytrium, according to theory, becomes more pathogenic at lower temperatures.

2 Solutions: Warm or dry conditions may stress amphibians, thus increasing susceptibility to disease “Lower temperatures benefiting the chytrid” may be and oversimplification of the pathogen’s response to climate.  warm years could favor the Batrachochytrium directly.

The Climate-Chytrid Experiment assess large-scale altitudinal patterns of extinction risk show that the timing of the widespread extinctions is strongly tied to large-scale temperature signals explore local climate from a chytrid’s viewpoint to frame a solution to the paradox

1. Assess large-scale altitudinal patterns of extinction risk Bars indicate the number of species known per altitudinal zone, and the grey- shaded portions represent the estimated percentage of species lost from each.

1. Assess large-scale altitudinal patterns of extinction risk The percentage of species lost increases sharply at 200 m, and again at 1,000 m. The percentage of species lost decreases, at 2,400 m, and thus peaks at middle elevations. This suggests that low temperatures as well as high ones may limit the impact of Batrachochytrium

2. Show that the timing of the widespread extinctions is strongly tied to large-scale temperature signals Extinctions showed a positive correlation with warming Around 80% of the species that have disappeared were seen for the last time right after a relatively warm year

3. Explore local climate from a chytrid’s viewpoint to frame a solution to the paradox Shielding them from excessive warmth and fostering moist conditions, cloud cover may promote the chytrid’ssurvival, growth and reproduction increasing cloudiness therefore may increase amphibians’ susceptibility to infection lowlands are often too warm for the chytrids during the day

3. Explore local climate from a chytrid’s viewpoint to frame a solution to the paradox highlands are often too cool for the chytridsat night most Atelopus extinctions have occurred at elevations where the minimum temperature is shifting towards the growth optimum for this pathogen

The Climate-ChytridExperiment: Conclusion Global climate change is already causing extinctions Global climate change is moving towards the optimum temperature for chytrids

References KieseckerJ.M., Blaustein A.R. & Belden L.K. (2001) Complex causes of amphibian population declines. Nature, 410, 681-684 Pounds, J.A. (2006) Widespread amphibian extinctions from epidemic disease driven by global warming.  Nature 439: 161 Stuart, S.N., J.S. Chanson, N.A. Cox, B.E. Young, A.S.L. Rodrigues, D.L. Fischman, and R.W. Waller. 2004. Status and trends of amphibian declines and extinctions worldwide. Science306:1783-1786.