Nutritive Quality and Mineral 

 Content of Potential Desert 

 Tortoise Food Plants 



E. Durant McArthur 

 Stewart C. Sanderson 

 Bruce L. Webb 



INTRODUCTION 



The Mojave desert tortoise (Gopherus agassizii) 

 population north and west of the Colorado River is 

 protected under provisions of the Endangered Species 

 Act (Federal Register 1990). Desert tortoise subpopula- 

 tions in southwestern Utah and adjacent Arizona on 

 the Beaver Dam Slope and north of St. George, UT, 

 are at the northeastern limit of the species' distribu- 

 tion (fig. 1; Patterson 1982; Woodbury and Hardy 1948). 

 The three study areas are City Creek, about 4 kilo- 

 meters north of St. George; Woodbury-Hardy, on the 

 Beaver Dam Slope about 3 kilometers north of the 

 Arizona border; and Littlefield, about 3 kilometers 



Figure 1— Location of plant collection study 

 areas. 



northeast of Littlefield, AZ, also on the Beaver Dam 

 Slope (fig. 1). The health of tortoises in these subpopu- 

 lations apparently differs (Bostick 1990; Glenn and 

 others 1990; Grover and DeFalco in press; Jarchow 

 1987). 



The City Creek subpopulation was deemed to be the 

 healthiest, the Woodbury-Hardy subpopulation was 

 deemed least healthy, and the Littlefield subpopula- 

 tion was deemed intermediate, based on population 

 density and structure, and carapace characteristics. 

 The Woodbury-Hardy subpopulation has a high fire- 

 quency of animals with bone deformations and thin- 

 ning plastrons and carapaces (Jarchow 1987). 



This study was designed to evaluate the hypothesis 

 that nutritive quality and mineral content of potential 

 desert tortoise food plants differed at the three areas, 

 possibly contributing to the apparent health differ- 

 ences. A second hypothesis was that plant mineral 

 content would be correlated with that of associated 

 soil. Mineral nutrition has been shown to be impor- 

 tant in desert tortoise ecophysiology (Nagy and Medica 

 1986). Information about the nutritive quality and 

 mineral status of the plants should be useful for those 

 concerned with the health and management of a wide 

 range of herbivores. 



MATERIALS AND METHODS 



Eighteen plant species common to the three study 

 areas form the core of this study (table 1). These spe- 

 cies were chosen based on three criteria: (1) annual 

 and perennial plants were included; (2) grasses, forbs, 

 shrubs, and succulents (cacti) were included; and 

 (3) there was evidence that they were, for the most 

 part, desert tortoise food plants (Coombs 1974; Hansen 

 and others 1976; Hohman and Ohmart 1980; Woodbury 

 and Hardy 1948). A few additional species were sam- 

 pled early or late in the study (for example, Abronia 

 fragrans, Plantago patagonica, Schismus barbatus, 

 and Sphaeralcea grossulariifolia) (table 1). 



Hansen and others (1976) and Nagy and Medica 

 (1986) have listed desert tortoise food plants fi-om other 

 areas. After our study began, Esque and others (1991) 

 and Esque (1992) documented the primary food plants 



1 



