Environmental Instability, Competition, and Density‐Dependent Growth and Survivorship of a Stream‐Dwelling Salamander

Abstract

To investigate whether growth in amphibians is density dependent, annual variation in larval growth rate, length of the larval period, size at metamorphosis, and survivorship to metamorphosis were documented in a population of small—mounted salamanders (Ambystoma texanum) for 6 yr. Peak hatchling densities were similar during all years, and varied from 18.2 to 24.5 hatchlings/m2 of bottom surface. Severe flooding during at the last 2 yr of study eliminated 90% if the larvae within 2 wk after hatching. The few larvae that survived these floods grew faster, had shorter larval periods, and were larger at metamorphosis than larvae in years when flooding was less extensive and larval densities were higher. Mean size at metamorphosis was inversely related to, and mean length of the larval period directly related to, larval density at day 15 (the earliest date at which postflooding censuses were available for both years). In a flood year, a larva of any given size contained about twice the volume of food in its stomach as a larva of the same size in a nonflood year. In a nonflood year food was limiting and average larval mass was inversely related to larval density in stream pools. Average air temperatures during the larval period in flood years and other years did not differ significantly. These data strongly support the hypothesis that growth was density—dependent because of competition for food. Survivorship to metamorphosis did not differ significantly in flood and nonflood years because of the manner in which flooding, stream drying, and competition interacted. In nonflood years competition for food lengthened the average time metamorphosis by °17 d relative to flood years. This, coupled with the fact that streams dried relatively early in the season, resulted in °3% survivorship to metamorphosis. In flood years larvae metamorphosed earlier and suffered only slight mortality from stream drying so that survivorship from day 15 to metamorphosis was an order of magnitude higher than in nonflood years. Increased postflooding survivorship compensated for catastrophic losses during floods; consequently, overall survivorship to metamorphosis was greater than in most nonflood years.