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Ryan GE, Nicholson E, Eames JC, Gray TNE, Loveridge R, Mahood SP, Sum P, McCarthy MA. Simultaneous-count models to estimate abundance from counts of unmarked individuals with imperfect detection. Conserv Biol 2019; 33:697-708. [PMID: 30615823 DOI: 10.1111/cobi.13261] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/23/2017] [Accepted: 09/04/2019] [Indexed: 06/09/2023]
Abstract
We developed a method to estimate population abundance from simultaneous counts of unmarked individuals over multiple sites. We considered that at each sampling occasion, individuals in a population could be detected at 1 of the survey sites or remain undetected and used either multinomial or binomial simultaneous-count models to estimate abundance, the latter being equivalent to an N-mixture model with one site. We tested model performance with simulations over a range of detection probabilities, population sizes, growth rates, number of years, sampling occasions, and sites. We then applied our method to 3 critically endangered vulture species in Cambodia to demonstrate the real-world applicability of the model and to provide the first abundance estimates for these species in Cambodia. Our new approach works best when existing methods are expected to perform poorly (i.e., few sites and large variation in abundance among sites) and if individuals may move among sites between sampling occasions. The approach performed better when there were >8 sampling occasions and net probability of detection was high (>0.5). We believe our approach will be useful in particular for simultaneous surveys at aggregation sites, such as roosts. The method complements existing approaches for estimating abundance of unmarked individuals and is the first method designed specifically for simultaneous counts.
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Affiliation(s)
- Gerard Edward Ryan
- School of BioSciences, University of Melbourne, Parkville, Victoria, 3010, Australia
- WWF-Greater Mekong Programme, House 21, Street 322, Sangkat Beoung Keng Kang 1, Khan Chamkar Morn, Phnom Penh, Cambodia
| | - Emily Nicholson
- Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, 221 Burwood Highway, Burwood, Victoria, 3125, Australia
| | - Jonathan C Eames
- BirdLife International Cambodia Program, House 2, St. 476, Sangkat Toul Tom Pong I, Khan Chamkar Morn, Phnom Penh, Cambodia
| | - Thomas N E Gray
- WWF-Greater Mekong Programme, House 21, Street 322, Sangkat Beoung Keng Kang 1, Khan Chamkar Morn, Phnom Penh, Cambodia
| | - Robin Loveridge
- BirdLife International Cambodia Program, House 2, St. 476, Sangkat Toul Tom Pong I, Khan Chamkar Morn, Phnom Penh, Cambodia
| | - Simon P Mahood
- Wildlife Conservation Society Cambodia Program, 21, St. 21, Sangkat Tonle Bassac, Phnom Penh, Cambodia
| | - Phearun Sum
- BirdLife International Cambodia Program, House 2, St. 476, Sangkat Toul Tom Pong I, Khan Chamkar Morn, Phnom Penh, Cambodia
| | - Michael A McCarthy
- School of BioSciences, University of Melbourne, Parkville, Victoria, 3010, Australia
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