Frequency of singletons 2013 species represented by single individuals 2013 is anomalously high in most large tropical arthropod surveys (average, 32%). We sampled 5965 adult spiders of 352 species (29% singletons) from 1 ha of lowland tropical moist forest in Guyana. Four common hypotheses (small body size, male-biased sex ratio, cryptic habits, clumped distributions) failed to explain singleton frequency. Singletons are larger than other species, not gender-biased, share no particular lifestyle, and are not clumped at 0�2520131 ha scales. Monte Carlo simulation of the best-fit lognormal community shows that the observed data fit a random sample from a community of ~700 species and 120132 million individuals, implying approximately 4% true singleton frequency. Undersampling causes systematic negative bias of species richness, and should be the default null hypothesis for singleton frequencies. Drastically greater sampling intensity in tropical arthropod inventory studies is required to yield realistic species richness estimates. The lognormal distribution deserves greater consideration as a richness estimator when undersampling bias is severe.
