Does Mother Nature really prefer rare species or are log-left-skewed SADs a sampling artefact?




Intensively sampled species abundance distributions (SADs) show left-skew on a log scale. That is, there are too many rare species to fit a lognormal distribution. I propose that this log-left-skew might be a sampling artefact. Monte Carlo simulations show that taking progressively larger samples from a log-unskewed distribution (such as the lognormal) causes log-skew to decrease asymptotically (move towards −∞) until it reaches the level of the underlying distribution (zero in this case). In contrast, accumulating certain types of repeated small samples results in a log-skew that becomes progressively more log-left-skewed to a level well beyond the underlying distribution. These repeated samples correspond to samples from the same site over many years or from many sites in 1 year. Data from empirical datasets show that log-skew generally goes from positive (right-skewed) to negative (left-skewed) as the number of temporally or spatially replicated samples increases. This suggests caution when interpreting log-left-skew as a pattern that needs biological interpretation.