Outline. Understanding the significance and challenges of juvenile migration in amphibians. Aquatic-breeding amphibians

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1 Understanding the significance and challenges of juvenile migration in amphibians Betsie B. Rothermel The Center of Excellence for Field Biology Austin Peay State University Outline Background life history, definitions, migratory events in life cycle Juvenile emigration a critical migratory event? Direct effects on migratory success habitat alteration and importance of connectivity Indirect effects on migratory success carryover effects from one stage to another Aquatic-breeding amphibians Egg and larval development in aquatic habitats (Ambystoma, Bufo, Hyla, Rana) Most of life is spent on land Juvenile stage often lasts 2-3 years Some species require specific terrestrial habitats during nonbreeding season 1

2 Definitions Homing navigation to a familiar home range Returning to breeding pond every year (or following a displacement) Migration round-trip, seasonal movements between habitats (within a population) Annual breeding migrations of adults Dispersal one-way, usually once-in-a-lifetime movements (between populations) Permanent movement away from natal pond Dispersal vs. Migration AMPHIBIAN METAPOPULATION dispersal Dispersal vs. migration True dispersal is a one-way, once-in-a-lifetime event migration AMPHIBIAN LIFE CYCLE 4 Dispersers Juvenile Egg 3 Breeding Adult Nonbreeding Adult 1 5 Paedomorph Metamorph Larva AQUATIC TERRESTRIAL

3 Migration distances (reviewed by Semlitsch & Bodie 2003) Salamanders (Ambystoma) Frogs (Rana) Toads (Bufo) Mean maximum migration distance 248 m 362 m 425 m Maximum distance in any study 625 m (A. jeffersonianum) 1,046 m (R. catesbeiana) 1,600 m (B. bufo) Type(s) of nonbreeding habitat or grassland Burrows Wet meadows (summer) Permanent aquatic (winter) A variety of terrestrial habitats (generalists?) Movement behavior at habitat edges Total Number of Captures Field ** 30 * Ambystoma maculatum Ambystoma texanum Bufo americanus Rothermel & Semlitsch 2002 Juvenile Survival Spotted salamander (A. maculatum ) Mean (SE) proportion known alive Initial Fall 01 Spring 02 Fall 02 Spring 03 Edge Field Rothermel & Semlitsch

4 Migratory success in fields Predicted probability of recapture at forest edge Spotted salamanders American toads Distance to nearest forest (m) Rothermel 2004 Postdoctoral Research: Land-use Effects on Amphibian Populations NSF Collaborative Study University of Maine M. L. Hunter University of Missouri R. D. Semlitsch, PI University of Georgia - SREL J. W. Gibbons, B. B. Rothermel SUNY ESF J. P. Gibbs LEAP Experimental Array 10 acres CWD Removed Control 50 m 100 m 150 m Thinning CWD Retained Spring

5 Photo: J.D. Willson Habitat choice vs. survival Proportion of total captures at site Captures of juvenile toads in drift fences Mean number of surviving toads Survival of juvenile toads in enclosures 0 Bay 37 Bay 5148 Bay 1000 Bay Initial One month Two months Todd & Rothermel 2006 Direct effects of habitat alteration Disturbed/open habitats = barriers to movement due to: Behavioral avoidance Reduced survival and growth Other potential risks: Increased predation risk? Exposure to pesticides? **Connectivity may be constrained more by migratory ability of juveniles than that of adults Critical habitat and buffers Maintaining connectivity between aquatic/terrestrial habitats is critical to population viability Semlitsch & Bodie

6 Indirect (carryover) effects Juveniles Via density-dependent dynamics operating in larval stage and affecting postmetamorphic fitness Adults Via delayed costs of time spent in breeding site and affecting survival during postbreeding migrations Can be a function of either natural or anthropogenic stressors Indirect effects - size at metamorphosis Mean initial mass (g) Survived Did not survive 0.6 Marbled salamanders Spotted salamanders Rothermel & Semlitsch 2006 Mechanistic basis for indirect effects Natural Hydrology (pond drying) Parasites, disease Anthropogenic Chemical stressors Habitat alteration 6

7 More questions Are short-term studies of migration a good proxy for what happens during dispersal? Does most mortality in terrestrial life stages occur during migratory events? How important is performance during the first postmetamorphic migration? Implications of indirect (carryover) effects for migratory success Acknowledgements Funding: ASIH (Gaige Award) U. S. Service, North Central Research Station NSF (DEB Award ) and SREL (Financial Assistance Award DE-FC09-96SR to University of Georgia from the U.S. Department of Energy) Collaborators: Ray Semlitsch (Ph.D. advisor), Whit Gibbons (Postdoc advisor), Tracy Green Rittenhouse, Michelle Boone, Brian Todd Field & Lab Assistance: B. Bennitt, J. Deters, M. Doyle, C. Mank, N. Mills, L. Murray, A. Nold, S. Rothermel, and Z. Slinker 7