Causes of amphibian declines The Golden Toad (Bufo periglenes), found in Costa Rica's protected national Monteverde Cloud Forest Reserve, was last seen in 1989. Its extinction is a part of the global decline in the amphibian populations. [Source: Wikipedia] Peter Lin

Outline: History / Background information Concerns Suggestions for Causes Research findings / discussions Conclusions Questions

History / Introduction First World Congress of Herpetology in 1989 – Scientists found that global amphibian populations are fast declining and reaching extinction. Houlahan et al (2000) used data from 936 populations and found that declining trend occurred as early as the late 1950s, and has been continuing for many decades.

Global diversity of amphibians species [Source: Global Amphibian Assessment]

Distribution of Threatened Amphibians Worldwide [Source: Global Amphibian Assessment]

IUCN Red List Assessment for all 5,918 Known Amphibian Species [Source: Global Amphibian Assessment] -1,896 species of extant species are globally threatened. -34 species are extinct (EX), and one extinct in the wild (EW) -2,604 species are near threatened (NT) or least concern (LC) -1,383 species are without sufficient information, or data deficient (DD) -456 species are listed as critically endangered (CR) -EN = Endangered; VU = Vulnerable

Concerns: -Almost 1/3 of all extant amphibian species are globally threatened, much higher than those in birds (12%) and mammals (23%). -Occupy an important position(s) within a food web (herbaceous larvae; carnivore adults). -In the life cycle of amphibians, water/moist habitats are required at one or more stages. -Amphibians depend on their moist outer layer of skin (and lung) for breathing. -Amphibians are considered to be an indicator of environmental changes due to their sensitive skin and dual lifestyle. -Very susceptible to changes in the quality of their ecosystems.

Major habitat preferences of amphibians [Source: Global Amphibian Assessment]

Causes: Human collections (for food) Habitat fragmentation / destruction Climate change Increasing UV-B radiation exposure Environmental pollution and contamination Introduction of non-native invasive species (predation or competition) Toxins, parasites, pathogens, diseases, etc. Others

Major threats to amphibians [Source: Global Amphibian Assessment]

This presentation will examine two of the many potential causes: The effect of trematode infection on amphibian limb development and survivorship (Johnson et al 1999) Complex causes of amphibian population declines (Kiesecker et al 2001)

The Effect of Trematode Infection on Amphibian Limb Development and Survivorship -Cases of deformed amphibians has been increasing since the 1990s, but how this relates to the overall declining global trend is still uncertain. -Objective: Is Ribeiroia sp. (trematode parasite) responsible for limb abnormalities in Hyla regilla (Pacific treefrogs)?

Additional information on the complex lifecycle of the parasitic trematode, Riberoria sp. : http://greenmuseum.org/c/vban/img/trematode_drawing.gif -Primary hosts are usually aquatic birds -Trematode eggs are released from the primary host to aquatic habitat, where they become free swimming miracidum -Miracidum penetrates aquatic snails and produce embryos of redia, the next larval stage -Each radia then produce another swimming larval stage called cercaria -Cercariae exit the snails and swim until they reach a secondary host, an amphibian larvae -Cercariae form metacercariaeon the surface of the secondary host, which penetrate into the tissue to cause inflammation and tissue outgrowths -Secondary host eaten by primary host, metacercariae excyst to form mature trematode worms

Snail Primary Trematode Host - (Planorbella tenuis) Freshly excysted Ribeiroia metacercaria from a deformed frog. (photograph by S. K. Sessions). Pacific treefrog (Hyla regilla) with a complete extra pelvis. (©Steve Holt, stockpix.com). http://limnology.wisc.edu/personnel/pieter/Hidden%20Stuff/Frogs/HYRElong1%20copy.jpg http://info.hartwick.edu/biology/def_frogs/Introduction/Badboy.jpg Snail Primary Trematode Host - (Planorbella tenuis) http://www.stockpix.com/images/2890.jpg http://limnology.wisc.edu/personnel/pieter/Hidden%20Stuff/Frogs/clearstain.jpg

Methods: Collected H. regilla egg masses from sites without abnormal frogs Allow the eggs to hatch, and keep the tadpoles in 1-litre containers of spring water and randomly assigned to one of six treatments 0 cercariae (control) 16 Ribeiroia cercariae (light) 32 R. cercariae (intermediate) 48 R. cercariae (heavy) 80 Alaria mustelae cercariae (a second species of trematode) 80 A. cercariae and 32 R. cercariae Expose the tadpoles to parasites in four equal doses (one dose = ¼ of the total parasite load) over 10 days

Control frog (normal) Permanent extension in right hindlimb Limbs completely missing Limbs partially missing 4 extra hindlimbs

Results: 85% of the surviving frogs to metamorphosis have severe abnormal limb development Tadpole survivorship decreased with increased parasite load, and are less than 50% in intermediate and heavy treatments Alaria cercariae entered the tadpoles and are widely distributed in the subcutaneous tissue, but does not cause limb abnormality nor mortality. Ribeiroia are found near pelvic regions and hindlimbs of metamorphic frogs

Results: -Increasing density of Ribeiroia ~ decreases survivorship Abnormality Survivorship -Increasing density of Ribeiroia ~ decreases survivorship ~ increases abnormalities

Results: Survivorship Abnormality -Alaria seems to have no impact on survivorship nor abnormality of frogs

Conclusion: Ribeiroia sp. are responsible for the abnormalities in Hyla regilla. Eutrophication and removal of snail-predators can increase snail population; thus increase the incidence of parasite infection.

Complex causes of amphibian population declines -Global amphibian population decline due to climatic changes, increased exposure to ultraviolet-B (UV-B), and increased prevalence of disease? -Objective: Does El Niño cycles increase the amount of sunlight exposed to amphibian embryos in shallow water, thus increase the incidence and severity of pathogenic (Saprolegnia ferax) outbreaks?

Bufo boreas (Western toad) http://www.californiaherps.com/frogs/images/bbhalophilusrc306.jpg

The end for embryonic western toads (Pounds 2001). Top - healthy eggs. Below - the eggs infected with Saprolegnia ferax (a fungus that infects the eggs and cause mortality). http://www.nature.com/nature/journal/v410/n6829/images/410639aa.2.jpg

Methods: Monitored the breeding activity of B. boreas from 1990 to 1999. For each breeding event, quantified: 1) the number of embryos deposited; 2) % of S. ferax-caused embryonic mortality; 3) the water depth at which the embryos were developed Compared summer Southern Oscillation Index (SOI) and winter precipitation during the years of embryo mortality

What is SOI? The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. Sustained negative values of the SOI often indicate El Niño episodes.(http://www.bom.gov.au/lam/glossary/soid.htm) http://faculty.washington.edu/kessler/ENSO/soi-shade-ncep-b.gif

Field experiment: Evaluated the impact of variation in water depth and UV-B exposure on B.boreas embryo mortality due to S. ferax infection Manipulated: the depth where embryos are raised (10, 50, or 100 cm); exposure to ambient UV-B radiation (fully exposed or fully shielded). Also monitored the UV-B exposure of embryos

Results: An El Niño event in the summer correlated with a lower mean precipitation in the winter. Lower precipitation correlated with a shallower water depth during embryonic development.

Results: Shallower water depth correlated with a lower survival rate of embryos.

Results: UV-blocking filter UV-transmitting filter -When UV-B is blocked, no difference in survival rates across different depths -When UV-B is transmitted, shallower water depth has lower survival rate -UV-B flux diminishes with increasing water depth

Conclusion: Links between climate change and amphibian declines and extinctions (Pounds 2001). http://www.nature.com/nature/journal/v410/n6829/images/410639ab.2.jpg

Overall Discussion: -The first paper looked at the role of trematodes in amphibian abnormalities. -The increase in abnormal amphibians reported could be due to increase in the density of one of the host species, such as the effect of lake eutrophication on snail populations. -Overall, the impact of parasitic infestation on amphibian host populations seems to be an important cause that demand more focuses. -The second paper looked at the responses of amphibians to climatic fluctuation. -In high mountain lakes, fluctuations in water levels due to climate change, will increase UV-B exposure in aquatic systems and increase embryo mortality via disease infections. -Therefore, diseases caused by either fungus or trematodes seem to be one of the more evident causes to amphibian declines.

Overall Conclusion: The amphibian decline crisis: A watershed for conservation biology? (Beebee and Griffiths, 2005)

References: -Beebee T.J.C. and Griffiths R.A. 2005) The amphibian decline crisis: a watershed for conservation biology? Biol. Conserv. 125: 271-28 -Global Amphibian Assessment (GAA) http://globalamphibians.org/overview.htm -Houlahan, J.E., Findlay, C.S., Schmidt, B.R., Meyer, A.H., and Kuzmin, S.L. 2000. Quantitative evidence for global amphibian population declines. Nature. 404:752-755. -Johnson, P.T.J., Lunde, K.B., Ritchie, E.G., and 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., and Belden, L.K. 2001. Complex causes of amphibian population declines. Nature. 410: 681-684. -Pounds, J.A. 2001. Climate and amphibian declines. Nature. 410: 639-640. -Wikipedia

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