Causes of amphibian decline

1
Causes of amphibian decline

• Presented by Heather Farnden

2
What are the causes of amphibian decline?
Natural fluctuations
Anthropogenic causes
Have the studies been able to demonstrate why?
Reasons supporting causes
What can we do about it?
What direction should research take?
3
1970s Variation in larvae survivorship and
developmental rate attributed to local conditions
-Anderson, J.D., Hassinger, D.D., and Dalrymple,
G.H. 1971 Natural mortality of eggs and larvae
of Ambystoma t. tigrinum Ecology
521107-1112 Attempted to correlate observed
mortality with various factors such as
temperature, pH and predation
4
Some studies looked at amphibian response to
chemicals
-Cooke, A.S. 1973 Response of Rana temporaria
tadpoles to chronic doses of pp-DDT Copeia
1973 647-652
-Weis, J.S. 1975 The effect of DDT on tail
regeneration in Rana pipiens and R. catesbeiana
tadpoles Copeia 1975 765-767
5
1980s
-Cooke, A.S.1981 Tadpoles as indicators of
harmful levels of pollution in the
field Environmental Pollution 25A
123-133 -mortality, growth, rate of metamorphosis
and occurrence of deformities
http//student.britannica.com/comptons/art-52902/T
he-eggs-of-a-leopard-frog-develop-into-tadpoles-in
?articleTypeId31
1989 - Decline in amphibian populations is
reported at the First World Congress of
Herpetology held in Canterbury England
6
1990s
Several suggestions put forward as to reason for
declines
Climate change UV-B radiation Infection Habitat
loss Pollution Introduced species
7
Blaustein, A.R., Hokit, D.G., OHara, R.K. and
Holt., R.A. 1994 Pathogenic fungus contributes
to amphibian losses in the Pacific
Northwest Biological Conservation 67 251-254
Blaustein, A.R., Hoffman, P.D., Hokit, D.G.,
Kiesecker, J.M., Walls, S.C., and Hays, J.B.
1994 UV repair and resistance to solar UV-B in
amphibian eggs A link to population
declines? Proc. Nat. Acad. Sci. 91 1791-1795

Kiesecker, J.M. and Blaustein, A.R.
1995 Synergism between UV-B radiation and a
pathogen magnifies amphibian embryo mortality
in nature Proc. Nat. Acad. Sci. 92 11049-11052
8
Carey, C. and Bryant, C.J. 1995 Possible
interrelations among environmental toxicants,
amphibian development, and decline of amphibian
populations Environmental Health Perspectives
103 13-17
Berger, L. et al 1998 Chytridiomycosis causes
amphibian mortality associated with population
declines in the rain forests of Australia and
Central America Proc. Nat. Acad. Sci. 95
9031-9036
Pounds, J.A., Fogden, M.P.L., and Campbell, J.H.
1999 Biological response to climate change on a
tropical mountain Nature 398 611-615
9
The effect of trematode infection on amphibian
limb development and survivorship

• Pieter T.J. Johnson, Kevin B. Lunde, Euan G.
  Ritchie, and Alan E. Launer
• Science. 284 802-804 (1999)

10
Pacific treefrog - Hyla regilla

http//www.werc.usgs.gov/fieldguide/hyre.htm
Photo by Chris Brown
Is Ribeiroia infection responsible for the limb
abnormalities observed in H. regilla?
11
Observations
Survey of ponds found that 4 of 13 ponds with H.
regilla had high incidence of frogs with hindlimb
abnormalities Water tests excluded pesticides,
PCBs, or heavy metals Eggs hatched in lab setting
were normal These 4 ponds also had the snail
Planorbella tenuis (first host of trematode
Ribeiroia sp.)
http//members.aol.com/Martinkcl/Shells/Ptenuis150
.jpg
12
Asexual reproduction of parasite in
snails Cercariae (free swimming) released -
infect frog larvae Encyst around developing limbs
Metacercariae (resting stage) Birds such as
heron consume infected frogs - Parasite eggs
released in feces Eggs hatch - Miracidia infect
the snails
http//limnology.wisc.edu/personnel/pieter/Hidden
20Stuff/Amphibian20Deformities20Research.htm
13
Methods
H. regilla egg masses were collected where
abnormalities unknown Individually placed in 1 L
container with commercial springwater after
hatching Tadpoles were assigned to a specific
treatment and then exposed to the parasites in
four equal doses over 10 days with each dose
equivalent to 1/4 of the total parasite load
14
Randomly assigned to one of the following
treatments 1. Control ( 0 Reibeiroia
cercariae) 2. Light (16 Reibeiroia
cercariae) 3. Intermediate (32 Reibeiroia
cercariae) 4. Heavy (48 Reibeiroia
cercariae) 5. 80 Alaria mustelae cercariae 6.
Mixed 80 Alaria mustelae and 32 Reibeiroia
cercariae
15
Findings
85 of frogs that survived to metamorphosis
developed hind limb abnormalities
Tadpole survivorship fell below 50 with
intermediate and heavy treatments
Control group showed 88 survivorship and 0
abnormal limb development
16
Abnormality frequency Survivorship to
metamorphosis
Increased density of Ribeiroia cercariae leads
to increased abnormal limb development and
decreased tadpole survivorship
17
Solid - survivorship Clear - abnormality frequency
Shows that Alaria has no effect on survivorship
or on limb deformities
18
(No Transcript)
19
Complex causes of amphibian population declines

• Joseph M. Kiesecker, Andrew R. Blaustein Lisa
  K. Belden
• Nature. 410 681-684 (2001)

20
Western toad - Bufo boreas
http//www.werc.usgs.gov/fieldguide/bubo.htm
Photo by Chris Brown
How are variability in precipitation, UV-B
exposure and infection by the pathogenic fungus
Saprolegnia ferax linked to amphibian decline?
21
Predictions

• Mortality associated with S. ferax infections at
  natural ovipostion sites is related to the water
  depth in which embryos develop
• Water depth at an ovipostion site is a function
  of variability in precipitation associated with
  ENSO cycles and
• Outbreaks of S. ferax infections observed in
  shallow water are mediated by exposure to UV-B
  radiation

22
Methods
Measurements of mortality at ovipostion sites
Number of eggs estimated (breeding pairs x 12000
eggs per female) Measurement of water depth to
top of egg mass on four separate dates.
(Standardized and averaged) S. ferax infected
eggs are covered in visible white hyphae and do
not hatch 1m2 grid with 0.1m2 squares - count of
number of dead and healthy eggs per square -
average over grid estimate of mortality in five
randomly assigned locations
23
50 times at each site - 100 eggs removed and dead
and healthy eggs counted - averaged for per cent
mortality at that site (1998 and 1999) Compared
summer SOI and winter precipitation in the north
Cascade Mountains of Oregon
24
Field experiment
Lost Lake, Linn County, Oregon - May 15 to May
29, 1996 Ten eggs from ten clutches were placed
in each enclosure Enclosures were opaque plastic
with 500 ?m mesh floors allowing water flow and
S. ferax transmission
25
Enclosures were covered with a UV-B blocking
filter or acetate (80 UV-B transmitting)
Placed at 10, 50 or 100 cm depth Temperature
recorded 6 times per hour at three enclosures at
each depth UV-B levels measured every 5 cm to
100 cm
26
http//www.bom.gov.au/climate/current/soi-1993-200
0.shtml
The Southern Oscillation Index (SOI) is
calculated from the monthly or seasonal
fluctuations in the air pressure difference
between Tahiti and Darwin.
27
Relationship between summer SOI and winter
precipitation in the north Cascade Mountains
(1989-1999)
Negative SOI is associated with El Niño - below
average precipitation in the Pacific Northwest
during the following winter
28
Relationship between winter precipitation
(1990-1999) and water depth at oviposition sites
during embryonic development
Depth during embryonic development increases as
precipitation increases
29
Relationship between water depth during embryonic
development and S. ferax-associated mortality
Higher percentage of embryos hatch when
development takes place in deeper water
30
Depth increased survival to hatching UV-B
radiation decreases as depth increases
Open - UV-B blocked Closed - UV-B transmitted
31

Conclusions
Both papers have established that infection can
cause population declines in amphibians
Johnson et al suggest that eutrophication may
increase incidence of infection
Kiesecker et al have forged the link between
infection and climate-related factors
32
Directions
Further research into complex causes needs to be
done to make connections between local, regional
and global declines in amphibian populations
Understanding the reasons may help initiate the
solutions to eradicate population declines
33
The end