Detecting trends in body size: empirical and statistical requirements for intraspecific analyses

Attributing biological explanations to observed ecogeographical and ecological patterns require eliminating potential statistical and sampling artifacts as alternative explanations of the observed patterns. Here, we assess the role of sample size, statistical power, and geographic inclusivity on the general validity and statistical significance of relationships between body size and latitude for 3 well-studied species of turtles. We extend those analyses to emphasize the importance of using statistically robust data in determining macroecological patterns. We examined intraspecific trends in body size with latitude in Chelydra serpentina, Chrysemys picta, and Trachemys scripta using Pearson's correlations, diagnostic tests for influential points, and resampling. Existing data were insufficient to ascertain a latitudinal trend in body size for C. serpentina or T. scripta. There was a significant relationship for C. picta, however, resampling analyses show that, on average, 16 of the 23 available independent populations were needed to demonstrate a significant relationship and that at least 20 of 23 populations were required to obtain a statistically powerful correlation between body size and latitude. Furthermore, restricting the latitudes of populations resampled shows that body size trends of C. picta were largely due to leveraging effects of populations at the edge of the species range. Our results suggest that broad inferences regarding ecological trends in body size should be made with caution until underlying (intraspecific) patterns in body size can be statistically and conclusively demonstrated.