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Estimating site occupancy rates for aquatic plants using spatial sub-sampling designs when detection probabilities are less than one

Nielson, R.M., Gray, B.R., McDonald, L.L., Heglund, P.J., 2011, Estimating site occupancy rates for aquatic plants using spatial sub-sampling designs when detection probabilities are less than one: Aquatic Botany, v. 95, p. 221– 225

Abstract

Submersed aquatic vegetation (SAV) is commonly estimated in lakes and rivers using a modified garden rake. The method is valuable in that it is less expensive than alternative methods and safer than at least one alternative method (snorkeling).  Unfortunately, the prevailing method of estimating the probability of site occupancy from rake data leads to an estimator of occupancy probability that is biased low, with bias expected to increase in magnitude as SAV biomass and SAV coverage within sampling sites decrease. This study evaluated the use of the site occupancy model used in wildlife ecology to estimate occurrence probabilities that have been adjusted for detection errors, and did so for two species common to the Upper Mississippi River, Canadian waterweed and wild celery. Point estimates of site occupancy increased by 36% and 19%, respectively, when adjusted for detection errors and incomplete within-site coverages. The method requires further evaluation for settings where the number of rake detections are small and detection probabilities and/or within-site coverages vary substantially among sites. Questions may be directed to Brian Gray (brgray@usgs.gov) at the Upper Midwest Environmental Sciences Center.

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