11.07.2015 Views

West Mojave Plan FEIR/S - Desert Managers Group

West Mojave Plan FEIR/S - Desert Managers Group

West Mojave Plan FEIR/S - Desert Managers Group

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

The spatial distribution of 12 of 13 raven-predated carcasses (see Map 3-12) relative tohigher density areas reveals an interesting relationship. Of the 12 carcasses where coordinateinformation was available, 9 (75%) were within or immediately adjacent to tortoiseconcentration areas. It was reported previously that 43% of observed subadult tortoises areassociated with these tortoise concentrations, which occur in only 17% of the surveyed areas.This provides clear direction that raven management should be focused on these higherconcentration areas to maximize its effectiveness.Boarman and Hamilton (in prep.) found 266 shells under an unknown number of ravennests throughout the <strong>Mojave</strong>. Boarman (2002) points out that not all of these tortoises definitelydied from predation, as ravens are also scavengers and may have scavenged carcasses rather thankilled living animals (see discussion below). Between 1991 and 1997, the most shells found at asingle nest in one year were 28 (this occurred in the East <strong>Mojave</strong>). Berry (1990, as amended)reported that among juvenile and small immature carcasses found throughout the <strong>Mojave</strong> <strong>Desert</strong>,72% of the deaths were attributed to raven predation (Boarman 2002). Berry (1985) evaluated403 juvenile tortoise shells found on 27 desert tortoise study plots throughout the <strong>Mojave</strong> <strong>Desert</strong>,and determined that ravens killed about 35% (i.e., 141) of them.Observations of Raven Foraging: In the East <strong>Mojave</strong>, Camp et al., (1992) foundvertebrates comprised half of the total weight of raven food items, with mammals being the mostimportant taxa. Reptiles were unimportant in terms of biomass, but occurred in almost 76% ofall pellets, with iguanid lizards being most common. Invertebrates occurred in 90% of all pellets,and consisted primarily of ants and beetles. Human refuse occurred in almost a quarter of thepellets. (Boarman 2002).Sign count and distance-sampling data suggest that in the <strong>West</strong> <strong>Mojave</strong> tortoises may betwice as active in wetter years than in drier years. These findings may be significant relative toraven management. Presumably, ravens are more likely to predate subadult tortoises that areaboveground. There are no reported accounts of ravens entering burrows to remove tortoises.Given that subadult tortoises are more likely to be aboveground in wetter than drier years, it isplausible that raven predation may affect relatively more tortoises in wetter years.Chapter 3 3-105

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!