Causes of Amphibian Declines

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Causes of Amphibian Declines

• Julie Robertson
• Biol 402
• November 18, 2008

Pacific treefrog
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(No Transcript)
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• One-third to one-half of the worlds 6000 known
  amphibian species could vanish in our lifetime,
  resulting in the single largest mass extinction
  since the disappearance of the dinosaurs.
• -Vancouver Aquarium Marine Science Center

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Overview

• Trends in Amphibian Populations
• Global Status
• Threats to Amphibians
• Complex Causes of Amphibian Declines
• The Effect of Trematode Infection on Amphibian
  Limb Development and Survivorship
• Conclusions
• 2008 Year of the Frog
• Questions

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Trends in Amphibian Populations

• Declining throughout North America and Europe

Houlahan (2000)
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IUCN Red List Assessment for all 5,918 Known
Amphibian Species
EX Extinct EW Extinct in the wild CR
Critically endangered EN Endangered VU
Vulnerable NT Near threatened LC Least
concern DD insufficient information
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Threats to populations
IUCN 2008
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Broad Causes of Declines

• Embryo Mortality with S. ferax
• Complex causes of amphibian population declines.
• (Kiesecker et al 2001)
• Reduced Fitness with Ribeiroia
• The effect of trematode infection on amphibian
  limb development and survivorship.
• (Johnson et al 1999)

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Background

• Knew that S. ferax outbreaks, in connection with
  UV-B radiation, were correlated with high
  amphibian embryo mortality.
• Disease outbreaks on several species occurring
  simultaneously
• Believed that ozone depletion was causing
  increased UV-B radiation, leading to increases in
  mortality.
• Wanted to know if it was Climate change that was
  increasing UV-B radiation, rather than ozone
  depletion.

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Methods

• Oregon Cascade Mountains between 1990 and 1999
• Measured 3 variables
• Number of embryos deposited
• Percent of S. ferax related mortalities
• Depth at which embryos developed
• Conducted field experiment
• Manipulated depth at which embryos were raised
• Embryos were subjected to either full UV-B
  exposure, or completey shielded.
• Collected summer S.O.I. data and winter
  precipitation data

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Southern Oscillation Index

• Air pressure difference between Tahiti and Darwin
  is recorded
• The monthly fluctuations in this air pressure
  difference is used to calculate the Southern
  Oscillation Index (S.O.I)
• Sustained negative values of S.O.I. are
  associated with El Nino events

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Southern Oscillation Index
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Climate Change Affects Depth

• Knew that El Nino events are increasing in
  frequency and intensity due to climate change
• As S.O.I. increases, precipitation in the North
  Cascades increases

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Climate Change Affects Depth

• Showed that depth of water column at oviposition
  sites was a function of S.O.I.

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UV-B Radiation

• Causes embryo mortality

• Attenuates with depth

White Shielded from UV-B Grey Exposed to UV-B
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Saprolegnia ferax

• Infected eggs easily detected
• Infected eggs do not hatch

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Saprolegnia ferax

• Acts synergistically with UV-B radiation
• Increases embryonic mortality
• Complex transfer from infected fish through soil

Black UV Blocking filter
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Saprolegnia ferax

• More than 50 of embryos that developed in
  shallow water became infected
• Less than 19 of embryos that developed in deep
  water became infected
• No UV-B influence on S. ferax related mortality
  in deep waters

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CLIMATE CHANGE
REDUCED POOLS
UVB EXPOSURE
PATHOGEN OUTBREAKS
EMBRYO MORTALITY
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Broad Causes of Declines

• Embryo Mortality
• Complex causes of amphibian population declines.
• (Kiesecker et al 2001)
• Reduced Fitness
• The effect of trematode infection on amphibian
  limb development and survivorship.
• (Johnson et al 1999)

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Reduced Fitness

• Number of reported deformed amphibians increasing
  since early 1990s
• Causes thought to include
• UV-B radiation
• Biocide contamination
• Parisite infection
• retinoids

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Methods

• Surveyed 35 ponds in Santa Clara County,
  California
• Focused on Pacific tree frogs (Hyla Regilla)
• Egg masses were collected from Eel River
• Six treatments
• Control
• Light
• Intermediate
• Heavy
• Alaria mustelae
• Alaria mustelae and Ribeiroia

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Results

• 85 of tadpoles exposed to Ribeiroia developed
  severly abnormal limbs
• Tadpole survival declined with increasing
  parasite density
• High survival rates in control group
• High rates of abnormalities in Heavy treatment
  group
• Alaria infected tadpoles, but had no effect

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Ribeiroi spp.

• Survivorship frequency decreases with increasing
  Ribeiroia density
• Abnormality frequency increases with increases
  Ribeiroia density

• Survivorship and abnormality frequencies
  unaffected by Alaria infection
• Survivorship frequency significantly lower after
  Ribeiroia treatment
• Abnormality frequency significantly higher after
  Ribeiroia treatment

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Infection to Death

• Direct mortality
• Indirect mortality
• Few abnormal adults found in the field

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Recent Increases

• Limb abnormalities shown to be caused by
  Ribeiroia
• Recent increases could be caused by
• Host species
• Accelerated eutrophication
• Removal of predators
• Further research needed

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Conclusions

• Amphibian populations declining drastically
• Complex interactions between abiotic and biotic
  factors, on global and local scales
• Climate change, depleting ozone layer (and other
  factors?) leaving amphibians more vulnerable to
  disease outbreaks

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2008 Year of the Frog Campaign

• http//www.youtube.com/watch?v2hBdHydtF1I
• http//www.amphibianark.org/online-petition.php

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Questions?
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Literature Cited

• Houlahan J.E. (2000). Quantitative evidence for
  global amphibian population declines. Nature 404
  752-755.
• Johnson P.T.J., Lunde K.B., Ritchie E.G. Launer
  A.E. (1999). The effect of trematode infection on
  amphibian limb development and survivorship.
  Science 284 802-804.
• Kiesecker J.M. Blaustein A.R. (1995). Synergism
  between UV-B radiation and a pathogen magnifies
  amphibian embryo mortality in nature. Proceedings
  of the National Academy of Sciences of USA
  92(24) 11049-11052.
• Kiesecker J.M., Blaustein A.R. Belden L.K.
  (2001). Complex causes of amphibian population
  declines. Science 410 681-684.
• Kiesecker J.M., Blaustein A.R. Miller C.L.
  (2001). Transfer of a pathogen from fish to
  amphibians. Conservation Biology 15(4)
  1064-1070.
• Pounds A.J. (2001). Climate and amphibian
  declines. Nature 410 639-640.
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