Evaluating survey methods for monitoring a rare vertebrate, the sonoran desert tortoise

Erin R. Zylstra, Robert J Steidl, Don E. Swann

Research output: Contribution to journalArticle

21 Citations (Scopus)

Abstract

Effective conservation requires strategies to monitor populations efficiently, which can be especially difficult for rare or elusive species where field surveys require high effort and considerable cost. Populations of many reptiles, including Sonoran desert tortoises (Gopherus agassizii), are challenging to monitor effectively because they are cryptic, they occur at low densities, and their activity is limited both seasonally and daily. We compared efficiency and statistical power of 2 survey methods appropriate for tortoises and other rare vertebrates, line-transect distance sampling and site occupancy. In 2005 and 2006 combined, we surveyed 120 1-km transects to estimate density and 40 3-ha plots 5 times each to estimate occupancy of Sonoran desert tortoises in 2 mountain ranges in southern Arizona, USA. For both mountain ranges combined, we estimated density to be 0.30 adult tortoises/ha (95 CI 0.170.43) and occupancy to be 0.72 (95 CI 0.560.89). For the sampling designs we evaluated, monitoring efforts based on occupancy were 836 more efficient than those based on density, when contrasting only survey effort, and 1730 more efficient when contrasting total effort (surveying, hiking to and from survey locations, and radiotracking). Occupancy had greater statistical power to detect annual declines in the proportion of area occupied than did distance sampling to detect annual declines in density. For example, we estimated that power to detect a 5 annual decline with 10 years of annual sampling was 0.92 (95 CI 0.750.98) for occupancy and 0.43 (95 CI 0.350.52) for distance sampling. Although all sampling methods have limitations, occupancy estimation offers a promising alternative for monitoring populations of rare vertebrates, including tortoises in the Sonoran Desert.

Original languageEnglish (US)
Pages (from-to)1311-1318
Number of pages8
JournalJournal of Wildlife Management
Volume74
Issue number6
DOIs
StatePublished - Aug 2010

Fingerprint

Gopherus agassizii
Sonoran Desert
survey method
vertebrate
desert
vertebrates
monitoring
tortoises
sampling
methodology
hiking
mountains
line transect
reptile
surveying
field survey
reptiles
tortoise
transect
cost

Keywords

  • density
  • distance sampling
  • Gopherus agassizii
  • monitoring
  • power analysis
  • site occupancy
  • Sonoran desert tortoise

ASJC Scopus subject areas

  • Nature and Landscape Conservation
  • Ecology
  • Ecology, Evolution, Behavior and Systematics

Cite this

Evaluating survey methods for monitoring a rare vertebrate, the sonoran desert tortoise. / Zylstra, Erin R.; Steidl, Robert J; Swann, Don E.

In: Journal of Wildlife Management, Vol. 74, No. 6, 08.2010, p. 1311-1318.

Research output: Contribution to journalArticle

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