Present address: Audubon California, 4225 Hollis Street, Emeryville, CA 94608 USA.
Understanding bias in geographic range size estimates
Article first published online: 11 OCT 2011
© 2011 Blackwell Publishing Ltd
Global Ecology and Biogeography
Volume 21, Issue 7, pages 732–742, July 2012
How to Cite
Sheth, S. N., Lohmann, L. G., Distler, T. and Jiménez, I. (2012), Understanding bias in geographic range size estimates. Global Ecology and Biogeography, 21: 732–742. doi: 10.1111/j.1466-8238.2011.00716.x
- Issue published online: 13 JUN 2012
- Article first published online: 11 OCT 2011
- Area of occupancy;
- collection effort;
- geographic range size;
- herbarium data;
- sampling intensity
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.