Undersampling bias: the null hypothesis for singleton species in tropical arthropod surveys

Undersampling bias: the null hypothesis for singleton
species in tropical arthropod surveys

Jonathan A. Coddington1*, Ingi Agnarsson1,2, Jeremy A. Miller4, MatjazKuntner1,3 and Gustavo Hormiga 5
1
Department of Entomology, National Museum of Natural History, NHB-105, Smithsonian Institution, PO Box 37012,
Washington, DC 20013-7012, USA; 2 Department of Biology, University of Puerto Rico, P.O. Box 23360, San Juan,
PR 00931-3360, Puerto Rico; 3 Institute of Biology, Scientific Research Centre of the Slovenian Academy of Sciences and Arts, Novi trg 2, PO Box 306, SI-1001 Ljubljana, Slovenia; 4 Nationaal Natuurhistorisch Museum Naturalis, Darwinweg 2,2333 CR Leiden, The Netherlands; and5 Department of Biological Sciences, The George Washington University, 2023 G Street NW, Washington D.C., 20052, USA.

Summary
1.      Frequency of singletons – species represented by single individuals – is anomalously high in most
large tropical arthropod surveys (average, 32%).
2.      We sampled 5965 adult spiders of 352 species (29% singletons) from 1 ha of lowland tropical
moist forest in Guyana.
3.      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 0·25–1 ha scales.
4.      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 1–2 million individuals, implying approximately
4% true singleton frequency.
5.      Undersampling causes systematic negative bias of species richness, and should be the default
null hypothesis for singleton frequencies.
6.      Drastically greater sampling intensity in tropical arthropod inventory studies is required to yield
realistic species richness estimates.
7.      The lognormal distribution deserves greater consideration as a richness estimator when undersampling
bias is severe.
Key-words:
Araneae, Guyana, neutral theory, sampling intensity, species richness estimation, spiders
Journal of Animal Ecology
Null models in biology perform the useful function of explaining many data in often infuriatingly simple ways (Gotelli & Graves 1996; Colwell & Lees 2000; Harte et al 2001; Hubbell 2001; Green & Ostling 2003). Often they counterbalance ad hoc explanations of the pattern at hand. In this paper, we propose that the high frequency of ‘singleton’ species (those represented by single individuals) in tropical arthropod inventories or surveys is simply explained as undersampling, and use a large but incomplete survey of spiders in Guyana to make the point. Species richness estimation continues to play an increasingly important role in conservation and biological inventory assessment in multiple contexts (Cardoso et al . 2008; de Thoisy, Brosse, & Dubois 2008; Shen & He 2008; Schoeman, Nel, & Soares, 2008).

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