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Migratory success of juveniles: a potential constraint on connectivity for pond‐breeding amphibians

Rothermel, Betsie B.
Ecological applications 2004 v.14 no.5 pp. 1535-1546
Ambystoma, Bufo americanus, breeding sites, edge effects, fences, forests, habitat fragmentation, habitats, juveniles, landscapes, migratory behavior, models, mortality, pastures, pitfall traps, probability, regression analysis, salamanders and newts, toads
The persistence of pond‐breeding amphibians in highly fragmented landscapes may be constrained by the need for connectivity between aquatic breeding sites and suitable terrestrial habitat, an example of landscape complementation. Although migratory ability determines the spatial scale at which landscape complementation operates, the factors influencing migratory success of amphibians, especially of juveniles, are poorly understood. This study is the first to investigate whether juvenile amphibians possess any innate, long‐distance orientation mechanisms that might improve their chances of locating suitable terrestrial habitat. I conducted experimental releases of spotted salamanders (Ambystoma maculatum) and American toads (Bufo americanus) from 18 artificial pools in replicate pastures at distances of 5–50 m from the nearest forest edges. Using circular drift fences with pitfall traps, I captured, individually marked, and released metamorphosed salamanders (n = 323) and toads (n = 203) leaving each pool. Salamanders exhibited nonrandom orientation at nine pools, but at only one was the mean movement direction consistent with the direction to the nearest forest edge. Emigrating salamanders probably responded to microtopographic or other distinct features of each pool, rather than to distant cues. Migratory success was determined by recaptures of marked juveniles at drift fences along the forest edges. I used logistic regression to model probability of recapture and evaluated alternative models using an information‐theoretic approach. Migratory success of both species was primarily a function of distance to nearest forest. Of salamanders and toads released from 50‐m pools, <15% reached the forest, suggesting that few juvenile amphibians would be able to migrate greater distances across pastures. Breeding sites lacking connectivity to suitable terrestrial habitat may be population sinks due to high mortality of juveniles during emigration. Additional research is needed to determine appropriate threshold distances between breeding sites and terrestrial habitat, as well as the potential effectiveness of movement corridors for migrating amphibians.