Causes of Amphibian Declines

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Causes of Amphibian Declines

• Presented by Farah Hirani

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Some History 1980s

• Researchers begin to notice amphibian decline
• Heyer Rand (1988) frog reductions and
  extinctions in Brazil
• 1989 First World Congress of Herpetology
  illuminates a possible global decline

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Some History 1990s

• More and more reports being published documenting
  declines
• Pounds et al. (1997) and Lips (1998) show
  declines in Costa Rica
• Laurence et al. (1996) in Australia

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Some Important Questions

• How do we distinguish between natural population
  fluctuations and declines with anthropogenic
  causes? (Pechmann et al., 1991)
• Is there an increasing threat of extinction for
  amphibians? (Collins Storfer, 2003)

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Why Are We Concerned?

• Amphibians are valuable indicators of
  environmental stress
• They comprise a significant component of many
  communities, as herbivores, predators and prey
• Understanding their decline could give insight
  into causes for species decline generally

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This Lead Us to Ask

• What ARE the causes of global amphibian
  decline?????

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Several Hypotheses

• Habitat destruction
• Climate change
• Increasing UV-B radiation exposure
• Environmental contamination
• Introduction of non-native species

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Finally, the 2 main papers

• The Effect of Trematode Infection on Amphibian
  Limb Development and Survivorship
• ? Pieter T. J. Johnson, Kevin B. Lunde, Euan G.
  Ritchie, Alan E. Launer
• Complex causes of amphibian population declines
• ? Joseph M. Kiesecker, Andrew R. Blaustein, Lisa
  K. Belden

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The Effect of Trematode Infection on Amphibian
Limb Development and SurvivorshipJohnson et al.
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Specific Research Question

• Is the Ribeiroia sp. infection responsible for
  the limb abnormalities observed in Hyla regilla?

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Trematode Life Cycle
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Trematode Cercariae (larval stage)
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Hyla regilla (Pacific treefrog)
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What sparked their question?

• 4/13 ponds supporting Pacific treefrogs had frogs
  with abnormalities
• They could not detect pesticides, polychlorinated
  biphenyls, or heavy metals
• These 4 ponds were the only ones supporting
  Planorbella tenuis (aquatic snail)
• Frog dissection showed Ribeiroia metacercariae in
  the tissue around pelvic girdle and hindlimbs

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Planorbella tenuis
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Methods

• Collected H. regilla eggs from site with no
  abnormal frogs
• Kept hatched tadpoles in individual 1L containers
  of water and randomly assigned 1 of 6 possible
  treatments

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Methods

• 0 cercariae (control)
• 16 Ribeiroia cercariae (light)
• 32 Ribeiroia cercariae (intermediate)
• 48 Ribeiroia cercariae (heavy)
• 80 Alaria mustelae cercariae
• 80 Alaria cercariae and 32 Ribeiroia cercariae

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Methods

• Tadpoles exposed to parasites in 4 equal doses
  over a 10-day period

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What They Found

• Exposure to Ribeiroia cercariae induced abnormal
  limb development in 85 of surviving frogs
• Tadpole survivorship declined with increased
  parasite load
• Tadpole survivorship lt50 in intermediate and
  heavy treatments

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What they found
Abnormality frequency
Survivorship frequency
Survivorship decreased with Ribeiroia
density. Abnormality increased with Ribeiroia
density.
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What they found
Abnormality frequency
Survivorship frequency
Alaria has no impact on survivorship or
abnormality
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In response to the question

• Is the Ribeiroia sp. infection responsible for
  the limb abnormalities observed in Hyla regilla?

They conclude
YES. Ribeiroia sp. infection IS responsible for
these abnormalities in Hyla regilla.
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• Complex causes of amphibian population declines
• Kiesecker et al.

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Specific Research Question

• Does an increase in the frequency/magnitude of El
  Nino events increase incidence/severity of
  Saprolegnia ferax outbreaks by increasing extent
  to which Bufo boreas embryos are exposed to
  sunlight in shallow water?

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Bufo boreas (Western toad)
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Saprolegnia ferax

• A fungus that causes infections in frog eggs,
  leading to mortality.

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Relationships between different factors

• Water depth influence on frog embryo mortality
  via S. ferax infections
• El Nino/Southern Oscillation (ENSO) cycles
  influence on water depth via precipitation
  variability
• UV-B radiation influence on S. ferax infection
  outbreaks

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What sparked their question?

• S. ferax outbreaks due to increased UV-B
  radiation is a cause for embryo mortality
  (Kiesecker et al., 1995)
• Amphibian embryos often develop under direct sun
  exposure
• Precipitation patterns are linked to ENSO cycles
  in Pacific Northwest

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Methods

• Monitored B. boreas breeding activity from
  1990-1999
• During each breeding event, quantified
• Number of embryos deposited
• of embryonic mortality due to S. ferax
  infections
• Water depth at which embryo developed

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Methods

• Compared relationship between summer Southern
  Oscillation Index (SOI) and winter precipitation

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Southern Oscillation Index (SOI)
Indicates presence of an El Nino or La Nina
effect Negative value El Nino event
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Methods

• Performed a field experiment to determine
  relationship between varying water depth and UV-B
  radiation on S. ferax-associated embryo mortality
• Manipulated depth of embryos (10, 50, 100cm) and
  UV-B radiation exposure (full vs. shielded)
• Monitored UV-B radiation exposure of embryos

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What they found
Less precipitation in the winter following a
summer El Nino event
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What they found
Lower precipitations cause shallower depth during
embryonic development
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What they found
Shallower depth results in lower survival of
embryos
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What they found
UV-B shielded
UV-B exposed
UV-B blocked No difference in survival rates
with different depths UV-B exposed Shallower
depths lower survivorship
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What they found
UV-B flux at different depths
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So in response to the question

• Does an increase in the frequency/magnitude of El
  Nino events increase incidence/severity of
  Saprolegnia ferax outbreaks by increasing extent
  to which Bufo boreas embryos are exposed to
  sunlight in shallow water?
• (which hopefully makes sense now)

They answer
YES!!!
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They conclude

• Previous El-Nino event
• Decreased precipitation next winter
• Shallower embryo development
• Higher UV-B exposure
• Higher susceptibility to S. ferax infection
• Higher mortality of embryos

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How are the papers connected?

• Johnson et al. paper establishes pathogen
  infection as a cause of abnormality/ mortality
• Kiesecker et al. paper begins with this
  relationship already established
• The research done by Kiesecker et al. in this
  paper is the next step for Johnson et al. with
  respect to finding causes for increased Ribeiroia
  infection in treefrogs

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Future Direction

• Determine the role of trematodes in
  abnormalities/declines of other amphibian species
• Determine the causes for increases in trematode
  infections (increased density of other host
  species?)
• Kiesecker et al. has shown the presence of
  complex interactions interacting factors, rather
  than just ONE cause, should be focused on

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• THE END