Playa wetlands are the major habitat for amphibians in the Southern Great Plains, USA. Agricultural cultivation causes playa sedimentation and related hydroperiod reduction in this intensively-farmed region, which in turn affects amphibian community structure. The goals of our study were (1) to investigate the influence of landuse on larval salamander (Ambystoma tigrinum mavortium) occurrence and densities in playas and whether hydroperiod length is an environmental influence on that occurrence, (2) to examine the polymorphic structure (cannibal, intermediate, and typical morphotypes) of larval salamanders and whether the frequency of occurrence of intermediate and typical morphs is associated with the presence of cannibal morphs and variations in hydroperiod length and conspecific density, and (3) to investigate morphological and life history trait differences among morphotypes (i.e., larval periods, growth rates, and body sizes). Cultivation of watersheds negatively affected presence and density of larval salamanders in playas, and this effect was associated with reduced hydroperiods. Although cannibal morphs were rare in playas, their presence negatively influenced presence of intermediate morphs and, thus, in this situation the majority of larvae were typical morphs. When cannibal morphs were absent, intermediate and typical morphs developed in similar proportions among playas with a tendency for intermediate morphs to predominate. Larval density and hydroperiod length were positively associated with the presence of cannibal morphs. Intermediate morphs had distinct growth patterns compared to cannibal and typical morphs and had body size measures (snout-to-vent length and stomach size) in between the two extreme morphs. Cannibal morphs grew faster and reached larger body size at metamorphosis than non-cannibal morphs. When cannibal morphs were absent, intermediate and typical larvae reached larger sizes than when cannibal morphs were present. We conclude that cultivation surrounding playas can negatively influence Ambystoma occurrence in general and especially the cannibal morphotype by reducing wetland hydroperiod. Moreover, a polymorphic community structure in larval amphibians may be important to a properly functioning playa food web.