Sources of Interpopulational Variation in Growth Respones of Larval Salamanders

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Abstract
In central Kentucky, larvae of small—mouth salamanders (Ambystoma texanum) in ponds grow faster, have shorter larval periods, and are larger at metamorphosis than those in streams. Comparisons of populations in the field with those in common environment in the laboratory indicated that variation in growth in nature is due largely to environmental rather than genetic differences between populations. In laboratory experiments, temperature and food level significantly affected larval growth. However, growth occurred independently of density when larvae were fed identical per—capita food allotments at low levels or were fed as libitum. In the field, larval growth rate and size at metamorphosis were positively correlated with mean environmental temperature. However, the relative size of metamorphs in warm (pond) vs. cool (stream) habitats was the reverse of that predicted from laboratory studies. In a field experiment for determining whether food level affects growth, supplementally fed larvae in enclosures in a stream grew faster (P < .0001) than larvae in the stream proper. Regression coefficients of larval density vs. mean size in isolated pools within the stream were uniformly negative, which suggests that larvae exploitatively competed for food. In streams, food limitation caused little direct mortality through starvation. Instead, food shortages lowered larval survivorship indirectly by lengthening the time to metamorphosis and thereby increasing their susceptibility to stream drying, flooding, and fish predation.