Variation in richness, evenness, and diversity in diving and dabbling duck guilds in prairie pothole habitats
E. J. Tramer (1969. Ecology, 50: 927–929) proposed that the "regulation" of community diversity by changes in richness or evenness are alternatives found in predictable, nonrigorous environments with "equilibrium" communities and unpredictable, rigorous environments with "opportunistic" communities, respectively. Twenty-six years of diving and dabbling duck census data from relatively benign aspen parkland habitats, and 9 years of the same data from more rigorous mixed prairie habitats, showed little agreement with the hypothesis. Over time, only mixed prairie dabbling ducks showed changes in diversity in agreement with the hypothesis; i.e., diversity changed owing to changes in evenness in the more rigorous environment. Within guilds, changes in diversity over space were all related to changes in evenness, but only dabbling ducks had higher evenness in the less rigorous environment; the reverse was true for diving ducks. Thus, studies that use Tramer's hypothesis as a basis for distinguishing between the mechanisms responsible for community diversity in benign and rigorous environments (i.e., competition versus variation in resource abundance and diversity, respectively) are of limited value.