Title page for ETD etd-03162004-101746

Type of Document Dissertation
Author Williams, Alison Kay
Author's Email Address alwilli2@vt.edu
URN etd-03162004-101746
Title The influence of probability of detection when modeling species occurrence using GIS and survey data
Degree PhD
Department Fisheries and Wildlife Sciences
Advisory Committee
Advisor Name Title
Berkson, James M. Committee Chair
Angermeier, Paul L. Committee Member
Brewster, Carlyle C. Committee Member
Kelly, Marcella J. Committee Member
Stauffer, Dean F. Committee Member
Zobel, Christopher W. Committee Member
  • probability of detection
  • habitat suitability model
  • salamander
  • Bayesian logistic regression
  • multivariate statistics
  • ecological niche factor analysis
Date of Defense 2003-12-17
Availability unrestricted
I compared the performance of habitat models created from data of differing reliability. Because the reliability is dependent on the probability of detecting the species, I experimented to estimate detectability for a salamander species. Based on these estimates, I investigated the sensitivity of habitat models to varying detectability.

Models were created using a database of amphibian and reptile observations at Fort A.P. Hill, Virginia, USA. Performance was compared among modeling methods, taxa, life histories, and sample sizes. Model performance was poor for all methods and species, except for the carpenter frog (Rana virgatipes). Discriminant function analysis and ecological niche factor analysis (ENFA) predicted presence better than logistic regression and Bayesian logistic regression models. Database collections of observations have limited value as input for modeling because of the lack of absence data. Without knowledge of detectability, it is unknown whether non-detection represents absence.

To estimate detectability, I experimented with red-backed salamanders (Plethodon cinereus) using daytime, cover-object searches and nighttime, visual surveys. Salamanders were maintained in enclosures (n = 124) assigned to four treatments, daytime–low density, daytime–high density, nighttime–low density, and nighttime–high density. Multiple observations of each enclosure were made. Detectability was higher using daytime, cover-object searches (64%) than nighttime, visual surveys (20%). Detection was also higher in high-density (49%) versus low-density enclosures (35%).

Because of variation in detectability, I tested model sensitivity to the probability of detection. A simulated distribution was created using functions relating habitat suitability to environmental variables from a landscape. Surveys were replicated by randomly selecting locations (n = 50, 100, 200, or 500) and determining whether the species was observed, based on the probability of detection (p = 40%, 60%, 80%, or 100%). Bayesian logistic regression and ENFA models were created for each sample. When detection was 80 – 100%, Bayesian predictions were more correlated with the known suitability and identified presence more accurately than ENFA.

Probability of detection was variable among sampling methods and effort. Models created from presence/absence data were sensitive to the probability of detection in the input data. This stresses the importance of quantifying detectability and using presence-only modeling methods when detectability is low. If planning for sampling as an input for suitability modeling, it is important to choose sampling methods to ensure that detection is 80% or higher.

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