Sampling effort, regression method, and the shape and slope of size-abundance relations

1. Despite a substantial body of work there remains much disagreement about the form of the relationship between organism abundance and body size. In an attempt at resolving these disagreements the shape and slope of samples from simulated and real abundance-mass distributions were assessed by ordinary least squares regression (OLS) and the reduced major axis method (RMA). 2. It is suggested that the data gathered by ecologists to assess these relationships are usually truncated in respect of density. Under these conditions RMA gives slope estimates which are consistently closer to the true slopes than OLS regression. 3. The triangular relationships reported by some workers are found over smaller mass and abundance ranges than linear relations. Scatter in slope estimates is much greater and positive slopes more common at small sample sizes and sample ranges. These results support the notion that inadequate and truncated sampling is responsible for much of the disagreement reported in the literature. 4. The results strongly support the notion that density declines with increasing body mass in a broad, linear band with a slope around - 1. However there is some evidence to suggest that this overall relation results from a series of component relations with slopes which differ from the overall slope.