Effects of Larval Density in Ambystoma Opacum: An Experiment Large‐Scale Field Enclosures

This experiment was designed to measure the effects of larval density on larval traits in the salamander Ambystoma opacum, and to ascertain whether previous studies conducted at smaller spatial scales or higher densities produced artifactual results. Density effects on larval growth, body size at metamorphosis, length of larval period, and survival to metamorphosis were studied in A. opacum in large—scale (41 m² and 23 m²) field enclosures in two temporary ponds. Each enclosure contained indigenous populations of prey (zooplankton and insects) and predators, as well as the range of microhabitats present in these natural ponds. Initial larval densities were chosen to represent high and low levels of naturally occurring mean densities. Larvae at high density grew more slowly, exhibited longer larval periods, were smaller at metamorphosis, and had reduced survival compared with those raised at the low density. At one site where experiments were conducted during three years, larval quality and quantity were enhanced in the year of longest hydroperiod. When ponds dried early, larvae metamorphosed sooner, were smaller, and exhibited lower survival. Slower growth at high densities probably increased mortality through an increased probability of desiccation in the early—drying years, and increased susceptibility to predation in all years. Larval survival in the enclosures was not elevated compared to natural ponds. as may occur in predator—free small enclosures and artificial ponds. These results suggest that, in natural ponds, the importance of intraspecific competition is dependent upon hydroperiod, and the intensity of competition influences predation risk. Thus, both density—dependent and density—independent factors affect body size and recruitment of larval A. opacum into the adult population. The use of large—scale field enclosures has advantages and disadvantages: it allows the examination of density—dependent process under natural conditions and provides high statistical power because of low variability in larval traits; however, experimental designs must be simple and underlying mechanisms are difficult to identify.