title: Distribution and site selection of Le Conte's and Crissal thrashers in the Mojave Desert: A multi-model approach
Creators: name: Fletcher, Dawn Marie
Publisher: name: University of Nevada, Las Vegas
Year: 2009

description: Information on the distribution and habitat requirements of a species  are critical components to the development of meaningful conservation  plans. Such knowledge, however, is particularly difficult to obtain for  species that are elusive and occur at low densities, such as the Le  Conte's ( Toxostoma lecontei ) and Crissal (Toxostoma crissale ) thrashers. In association with a regional conservation plan, I  evaluated the distribution and habitat selection of these thrashers  within Clark County, Nevada in the eastern Mojave Desert. I used a  call-broadcast approach to sample 432 stratified random locations,  detecting Le Conte's thrashers at 45 locations and Crissal thrashers at  41 locations. To model suitable habitat and predict thrasher occurrence,  I used site-specific and landscape level information to create models  that represented habitat data at two spatial scales. At each of these  spatial scales, I measured variables corresponding to five environmental  categories; plant assemblages, substrate, landform features, climate,  and human disturbance. For analyses, I used logistic regression and  assessed resulting models using an information theoretic approach.  Inclusions in the best-fit model sets were determined using an Akaike  Information Criterion approach. Model-averaging was used to determine  the best possible parameter estimates for predicting thrasher presence  from the complete sets of best-fit models. Results from the models  indicated that Le Conte's thrashers occur within areas of little  topographic relief such as valley bottoms near dry lake beds (playas).  This pattern was strongly evidenced by the negative relationship between  these thrashers and slope, in that they were never observed on slopes  greater than 5 degrees, and by the disassociation with mountainous  habitat and higher-elevation plant assemblages. The site-specific  (ecological model) supported this broad pattern in identifying strong  positive associations with playas and saltbush assemblages  (specifically,  Atriplex polycarpa and  A. canescens ).  Positive associations were also determined for three other plant  assemblages: wash vegetation, cholla, and Mojave mixed scrub (dominated  by Yucca schidigera ). The landscape model confirmed the  important relationship of saltbush and wash vegetation. Crissal  thrashers presence showed a strong negative relationship with  creosote-bursage, shadscale, and creosote-sparse Joshua tree plant  assemblages and with a principal component describing climatic patterns  associated with decreasing temperatures and increased precipitation at  higher elevations. Two plant assemblage categories, riparian and wash  vegetation, and a principal component describing latitudinal patterns in  climate were positively associated with this thrasher. The landscape  model for the Crissal thrasher identified the same variables and  relationships as the site-specific model. Suitable habitat for both  species were predicted in ArcGIS using model average coefficients  derived from best-fit landscape models. The predictive maps greatly  improved on existing habitat models for these species within Clark  County, and provide tools for conservation planning.