Patuxent Wildlife Research Center

Effects of Agricultural Pesticides on Translocated Tadpoles of the Pacific Treefrog in Lassen, Yosemite, and Sequoia National Parks

Deborah F. Cowman1, Donald W. Sparling2, Gary M. Fellers3, and Thomas E. Lacher1
1Wildlife and Fisheries Sciences Department, Texas A&M University, College Station, TX
2USGS Patuxent Wildlife Research Center, Laurel, MD
3USGS Western Ecological Research Center, Point Reyes National Seashore, Point Reyes, CA.
Sierra Nevada Mountains overlooking Kaweah Valley, Sequoia National Park


In recent studies chemical analyses show that highest levels of contaminants in surface water and dry particulate samples in the Sierra Nevada Mountains, CA are pesticides applied in the Central Valley, CA during heavy use periods in summer. Don Sparling, Gary Fellers, and Laura McConnell (2001) have recently found significant levels of pesticides (chlorpyrifos, diazinon, and endosulfan) in tissues of adult Pacific treefrogs, Hyla (Pseudacris) regilla, (Silva 1997) collected in the Sierra Nevadas.

This study tests the null hypothesis that pesticides are not having adverse effects on H. regilla tadpoles. H. regilla tadpoles were translocated in cages among sites located in Lassen, Yosemite, and Sequoia National Parks, and held in place to metamorphosis. Suspected impacts to be evaluated include: (1) increased mortality of larvae due to direct toxicity, cholinesterase (ChE) inhibition, or physical malformations; (2) reduced survivorship of tadpoles; (3) increased percentages of malformations at metamorphosis; (4) increased genetic damage in exposed larvae (measured by flow-cytometry); and (5) elevated body burdens of pesticides and ChE inhibition.

Discovery of detrimental effects in Pacific treefrog larvae and newly metamorphosed frogs may provide significant information towards the evaluation of ranid amphibian declines in the Sierra Nevada Mountains.

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Broad scale field sampling compared with historical analyses of museum records shows an ecosystem level decline of amphibians in the Great Central Valley in California. Counties most affected are Sacramento and those of the San Joaquin Valley (Fisher and Shaffer 1996). The collapse of a regional frog fauna, 5 of 7 species, in the Yosemite area of the California Sierra Nevada, has also been documented (Drost and Fellers 1996). Jennings (1996) reported that all 5 native ranid species in the Sierra Nevada are in need of protection. The Cascade frog (Rana cascadae) is in serious decline and the northern leopard frog (R. pipiens) has disappeared from 99% of its range (Jennings and Hayes 1994). The California red-legged frog (R. aurora draytonii) is listed as threatened (Miller 1994). The foothill yellow-legged frog (R. boylii) is in decline (Drost and Fellers 1996) and the mountain yellow-legged frog (R. muscosa) has disappeared from over 75% of study sites where it was formerly found in California (Bradford 1991, Bradford et al. 1994).

H. regilla was chosen as a surrogate species for testing because ranid frog numbers are seriously low. This treefrog species occurs in the same areas as the ranids and is widespread and abundant. However, their exposure to water is much less than that of ranid frogs and this may help to explain why they are still abundant in areas of ranid frog declines. If adverse effects of pesticide exposure is found in H. regilla, ranid frogs may be undergoing similar effects at an increased rate based on length of exposure.

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Pacific TreefrogWe propose to test the null hypothesis:

Agricultural pesticides have no significant adverse effects on frogs in the Sierra Nevada Mountains, California

Possible impacts to be evaluated include: (1) increased mortality of larvae due to direct toxicity, cholinesterase (ChE) inhibition, or physical malformations; (2) reduced survivorship of tadpoles; (3)increased percentages and type of malformations at metamorphosis; (4) genetic damage in larvae and adults; and (5) elevated body burdens of pesticides and ChE inhibition.

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Phase 1

Prior to tadpole translocations, sampling transects will be established from coastal to mountain locations (see map below).

H. regilla tadpoles, adult males, sediment, and water will be collected along transects at 3 sites per location (red dots indicate locations).

Residue analyses will be conducted on all samples; ChE analyses will also be done on tissues.

Broadscale sampling will (1) provide a baseline of information for the study; (2) will enable us to more clearly define trends in pesticides levels and effects from coastal to mountain areas; and (3) aid in our interpretation of translocation data.

key for sampling transect map map of sampling transects Pacific Treefrog Photo by Martin Ouellet

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Phase 2

Tadpoles (Gosner stage 21-25) were translocated among each of the 3 parks (June 1-Sept.7, 2001).

Tadpoles were held in place in a field-tested design cage (Harris and Bogart 1997)made of Nitex® nylon (see diagram below).
field-tested design cage

Tadpoles were fed boiled romaine lettuce, and sampled at 28 days exposure and at metamorphosis (Gosner stage 45-46).

Cages were monitored daily; water temperatures, water depth, dissolved oxygen, nitrites/nitrates, pH, ammonia, hardness, and turbidity were measured.

A floating platform was placed in the cage for metamorphosed frogs when tadpoles reached Gosner stage 42 (appearance of front limbs).

Post exposure, larvae and metamorphosed frogs will be analyzed for ChE inhibition, DNA damage, and body burdens of pesticides. Data collected in the field will be evaluated for rate of physical malformations, abnormal behavior, and mortality due to direct toxicity.

sample siteCriteria for site selection (Fall 2000): 

3 meadow/ponds per park
Approximate elevation of 7000 ft 
Appropriate access Presence of H. regilla
Accommodation of cages Absence of fish

Each pond selected held 9 cages, 3 cages containing tadpoles per each park (27 cages per park)

Dummy cages were put out 2 weeks prior to the start of the study (and during study) for curious bears.

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Map of Lassen Volcanic National Park Translocation Areas
Lassen Volcanic National Park
= Translocation Areas

Map of Yosemite National Park Translocation Areas
Yosemite National Park
= Translocation Areas

Map of California showing locations of National Parks in study

Map of Sequoia National Park Translocation Areas
Sequoia National Park
= Translocation Areas

Preliminary Results Phase 2

Survivorship of tadpoles to metamorphosis was greatest at Lassen Volcanic National Park

Malformations of hind limbs, brachymelia, (first segment of hind limb truncated; see photo below) observed in tadpoles and newly metamorphosed frogs at all three parks.
Photo by Cyndi Gresser

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Bradford, D.F. 1991. Mass mortality and extinction in a high-elevation population of Rana muscosa. Journal of Herpetology 25:174-177.

Bradford, D.F., Graber, D.M. and Tabatabai, F. 1994. Population declines of the native frog, Rana muscosa, in Sequoia and Kings Canyon National Parks, California. Southwestern Naturalist 39:323-327.

Drost, C.A. and Fellers, G.A. 1996. Collapse of a regional frog fauna in the Yosemite area of the California Sierra Nevada, USA. Conservation Biology 10:414-425.

Fisher, R.N. and Shaffer, H.B. 1996. The decline of amphibians in California’s Great Central Valley. Conservation Biology 10:1387-1397.

Harris, M.L. and Bogart, J.P. 1997. A cage for evaluation of in situ water quality using frog eggs and tadpoles. Herpetological Review 28:134-135.

Jennings, M.R. 1996. Status of Amphibians. Sierra Nevada Ecosystem Project: Final Report to Congress, volume II—Assessments and Scientific Basis for Management Options. Report 37. University of California Davis, Davis, CA. USA.

Jennings, M.R. and Hayes, M.P. 1994. Amphibian and reptile species of special concern in California. Rancho Cordova: California Department of Fish and Game, Inland Fisheries Division.

Miller, K.J. 1994. Endangered and threatened wildlife and plants: Proposed endangered status for the California red-legged frog. Federal Register 59, no.22 (2 February): 4888-95.

Silva, H.R. 1997. Two character states new for hylines and the taxonomy of the genus Pseudacris. Journal of Herpetology 31:609-613.

Sparling, D.W., Fellers, G.M. and McConnell, L.L. 2001. Pesticides and Amphibian Population Declines in California, USA. Environmental Toxicology and Chemistry: 20:1591-1595.

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This study is the result of collaboration among investigators and resource managers from USGS, USDA-ARS, Texas A&M University, USFWS, and NPS. Many thanks to all who have contributed advice and logistical help in the planning of this project. Special thanks to field technicians: Erika Cowman, Shenandoah Marr, Deborah Purce, and Pamela Widder, and to Patrick Kleeman, USGS Western Ecological Research Center, Point Reyes National Seashore, Point Reyes, CA, for all his help in the field.

Maps of California were made by Carlos Hinojosa and Amy Hays of the Land Information Systems, Texas A&M University.

Funding is provided for this study by USGS, Amphibian Research and Monitoring Initiative and USFWS, Endangered Species, Region 1.

Texas A&M University Logo land infoDept. of Wildlife and Fisheries Science, Texas A&M

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U.S. Department of the Interior | U.S. Geological Survey
Patuxent Wildlife Research Center, Laurel, MD, USA 20708-4038
Contact: Deborah Cowman, email:
Last Modified: 03/20/02
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