Even so, many significant changes are suggested (see tables 14 through 25). In 

 stands where Bouteloua gracilis was a dominant species, canopy-coverage 

 decreased by an average of 24?4 in five exclosures ,and increased by an average of 

 38% in three others. Overall, canopy-coverage decreased an insignificant 3%. 

 Carex filifolia coverage decreased by an average of 44% in three exclosures 

 where it is a major species. Stipa comata decreased by an average of 34% in 

 three exclosures, but did not change significantly in two others where Agropyron 

 spicatum was a dominant. Except for the variable response of B_^ gracil is , these 

 results are not surprising in view of weather variations. Coverage of Artemisia 

 cana did not change significantly. Andropogon scoparius coverage decreased by 

 an average of 39% in two exclosures. Agropyron spicatum coverage decreased by 

 an average of 31% in two exclosures. Coverage of Distichlis stricta did not 

 decrease significantly. 



With the exception of B_^ gracilis . the behavior of the major range grasses 

 previously mentioned is rather consistent. However, Agropyron smithii coverage 

 decreased by an average of 74% in five exclosures, a far greater decrease than 

 shown by any other major grass. In the Agsm/Bogr exclosure, coverage of A^ 

 smithii decreased by 97%. 



The observed variation in coverage is thought to have been caused by changes 

 in the amount of water available to vegetation relative to the amount needed. 

 It is true that decreasing plant coverage is correlated with increasing litter, 

 but litter accumulation is thought to be a manifestation of climatic conditions 

 and livestock exclosures , and only a secondary cause of the coverage changes. 

 Another wet year coupled with weather measurements in the study area would prove 

 or disprove this hypothesis. 



Species Density 



Species density is the simplest measure of diversity. Figures 18 through 29 

 portray, for each community type, the average number of species per 0.1 m" plot 

 and the number of species encountered in twenty such plots. 



Using canopy-coverage data for all communities combined, the average number 

 of species per plot decreased 19?o between 1978 and 1979 and 25% between 1978 and 

 1980. Species density decreased 5% in all communities between 1979 and 1980, 

 even though it showed an increase in four communities. Canopy-coverage and the 

 derived measure of species density appear to be less sensitive to climatic 

 conditions than productivity. 



In the course of mapping the expanded study area, a number of species not 

 previously encountered were collected and identified. Appendix A is a revised 

 species list for the Circle West study area. Possibly most exciting is the 

 occurrence of Rorippa calycina (Engelm. Rydb. var. calycina ) , a plant recently 

 classified as threatened in Endangered and Threatened Plants of the United 

 States (Ayensu and DeFlllips 1978). However, a report entitled 'An Enumeration 

 of Species of Federal Concern--Listed, Proposed, or Candidate Under the 

 Endangered Species Act of 1973, 25 April 1980," indicates that the status of 

 Rorippa calycina has changed from "threatened" to "species whose status is 

 insufficiently known and need more study." Final determination of the status of 

 this species can be expected soon. 



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