West Mojave Plan FEIR/S - Desert Managers Group

Although these data indicate subadults comprised about 15% of the tortoise encounters, itdoes not necessarily indicate that subadults comprise 15% of the population. Subadults likelycomprise more than 15% of the population because they are regularly under-represented inregional surveys, but how much more is unknown. In Boarman and Sazaki’s (1996) studypopulation south of Highway 58, the subadult component was estimated to be 20%. Theseobservations are significant with regards to conservation management scenarios compared inChapter 4. Alternative E, for example, would focus management on eliminating common ravensand disease impacts on tortoise in DWMAs. Effective raven management would provide mostbenefits for tortoises under 110 mm in length, which may comprise 10% or less of the DWMApopulation 7 .Krzysik Analysis of Tortoise Survey Results: Dr. Anthony Krzysik has analyzed theresults of the recent tortoise sign count surveys (see Appendix K). Dr. Krzysik found that (a)desert tortoises are closely associated with their sign (i.e., burrows and scats); there is a highlysignificant correlation of live tortoises with burrows, scats, and Total Corrected Sign (TCS); (b)transects associated with live tortoises are typically also associated with appreciable sign counts;(c) both tortoise densities and tortoise mortality rates are similar in the four DWMAs analyzed;and (d) that the four DWMAs appear to be similar to one another in their tortoise and sign countdensities, and therefore, of similar value as desert tortoise conservation areas.Evidence of Tortoise Population Declines Between 1980 and 2002: Comparing signcount data collected prior to 1984 (“older data”) with those of 1998-2002 (“newer data”) shows adecline in the abundance of tortoise sign per transect. There were 213 older and 3,362 newertransects surveyed throughout the planning area. Comparisons are given in Table 3-14.TCSCATEGORIESTable 3-14Tortoise Total Corrected Sign Found in West Mojave Planning AreaBetween 1975 to 1982 and Between 1998 to 2002PREVALENCE OF TOTAL CORRECTED SIGN1975 to 1982 Transects 1998 to 2002 TransectsTotal No. % Of Total Total No. % Of Total0 38 18 1,405 421 to 3 57 27 1,113 334 to 8 45 21 583 179 to 16 46 22 195 617 to 28 20 9 56 129 to 50 6 3 10 50 1 0 0 0Totals 213 100% 3,362 100%7 The 10% figure assumes that the subadult cohort in the surveyed population is 20% (i.e. 5% more than observed toaccount for lower detectability of this age class). It also assumes that many of the subadults encountered werebetween 120 and 180 mm, and therefore less vulnerable to ravens. There is no way to census (i.e., count everyanimal) subadult populations, and the data generally do not include measured lengths (i.e., more often recorded as“subadult” than measured), so these numbers are hypothetical.Chapter 3 3-86