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Research article - Peer-reviewed, 2020

Assessing the performance of index calibration survey methods to monitor populations of wide-ranging low-density carnivores

Droge, Egil; Creel, Scott; Becker, Matthew S.; Loveridge, Andrew J.; Sousa, Lara L.; Macdonald, David W.

Abstract

Apex carnivores are wide-ranging, low-density, hard to detect, and declining throughout most of their range, making population monitoring both critical and challenging. Rapid and inexpensive index calibration survey (ICS) methods have been developed to monitor large African carnivores. ICS methods assume constant detection probability and a predictable relationship between the index and the actual population of interest. The precision and utility of the resulting estimates from ICS methods have been questioned. We assessed the performance of one ICS method for large carnivores-track counts-with data from two long-term studies of African lion populations. We conducted Monte Carlo simulation of intersections between transects (road segments) and lion movement paths (from GPS collar data) at varying survey intensities. Then, using the track count method we estimated population size and its confidence limits. We found that estimates either overstate precision or are too imprecise to be meaningful. Overstated precision stemmed from discarding the variance from population estimates when developing the method and from treating the conversion from tracks counts to population density as a back-transformation, rather than applying the equation for the variance of a linear function. To effectively assess the status of species, the IUCN has set guidelines, and these should be integrated in survey designs. We propose reporting the half relative confidence interval width (HRCIW) as an easily calculable and interpretable measure of precision. We show that track counts do not adhere to IUCN criteria, and we argue that ICS methods for wide-ranging low-density species are unlikely to meet those criteria. Established, intensive methods lead to precise estimates, but some new approaches, like short, intensive, (spatial) capture-mark-recapture (CMR/SECR) studies, aided by camera trapping and/or genetic identification of individuals, hold promise. A handbook of best practices in monitoring populations of apex carnivores is strongly recommended.

Keywords

carnivores; IUCN criteria; lion; Panthera leo; population monitoring; spoor counts; statistical power; track counts

Published in

Ecology and Evolution
2020, volume: 10, number: 7, pages: 3276-3292
Publisher: WILEY

Authors' information

Droge, Egil
University of Oxford
Montana State University
Swedish University of Agricultural Sciences, Department of Wildlife, Fish and Environmental Studies
Zambian Carnivore Programme
Becker, Matthew S.
Montana State University
Loveridge, Andrew J.
University of Oxford
Sousa, Lara L.
University of Oxford
Macdonald, David W.
University of Oxford

UKÄ Subject classification

Ecology

Publication Identifiers

DOI: https://doi.org/10.1002/ece3.6065

URI (permanent link to this page)

https://res.slu.se/id/publ/109705