Understanding bias in geographic range size estimates

Authors

  • Seema N. Sheth,

    Corresponding author
    1. Center for Conservation and Sustainable Development, Missouri Botanical Garden, PO Box 299, St Louis, MO 63166-0299 USA
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  • Lúcia G. Lohmann,

    1. Center for Conservation and Sustainable Development, Missouri Botanical Garden, PO Box 299, St Louis, MO 63166-0299 USA
    2. Universidade de São Paulo, Instituto de Biociências, Departamento de Botânica, Rua do Matão, 277, 05508-090, São Paulo, SP, Brazil
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  • Trisha Distler,

    1. Center for Conservation and Sustainable Development, Missouri Botanical Garden, PO Box 299, St Louis, MO 63166-0299 USA
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    • Present address: Audubon California, 4225 Hollis Street, Emeryville, CA 94608 USA.

  • Iván Jiménez

    1. Center for Conservation and Sustainable Development, Missouri Botanical Garden, PO Box 299, St Louis, MO 63166-0299 USA
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Seema Nayan Sheth, Department of Biology, Colorado State University, Fort Collins, CO 80523-1878, USA. E-mail: seema.sheth@colostate.edu

ABSTRACT

Aim  Estimates of geographic range size derived from natural history museum specimens are probably biased for many species. We aim to determine how bias in these estimates relates to range size.

Location  We conducted computer simulations based on herbarium specimen records from localities ranging from the southern United States to northern Argentina.

Methods  We used theory on the sampling distribution of the mean and variance to develop working hypotheses about how range size, defined as area of occupancy (AOO), was related to the inter-specific distribution of: (1) mean collection effort per area across the range of a species (MC); (2) variance in collection effort per area across the range of a species (VC); and (3) proportional bias in AOO estimates (PBias: the difference between the expected value of the estimate of AOO and true AOO, divided by true AOO). We tested predictions from these hypotheses using computer simulations based on a dataset of more than 29,000 herbarium specimen records documenting occurrences of 377 plant species in the tribe Bignonieae (Bignoniaceae).

Results  The working hypotheses predicted that the mean of the inter-specific distribution of MC, VC and PBias were independent of AOO, but that the respective variance and skewness decreased with increasing AOO. Computer simulations supported all but one prediction: the variance of the inter-specific distribution of VC did not decrease with increasing AOO.

Main conclusions  Our results suggest that, despite an invariant mean, the dispersion and symmetry of the inter-specific distribution of PBias decreases as AOO increases. As AOO increased, range size was less severely underestimated for a large proportion of simulated species. However, as AOO increased, range size estimates having extremely low bias were less common.

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