West Mojave Plan FEIR/S - Desert Managers Group

Predation Versus Scavenging of Juvenile Tortoises: Ravens are both scavengers (i.e.,feeding on carcasses of animals they did not kill), and predators (i.e., killing and feeding onanimals).The Recovery Plan (USFWS 1994b) cited three types of evidence that ravens prey on,and not just scavenge, juvenile tortoises: (a) Ravens have been observed killing juveniletortoises; (b) Large numbers of juvenile carcasses show signs consistent with raven predation;and, (c) large numbers of juvenile carcasses are found in and at the base of raven nests, as well asnear perches. Boarman and Hamilton (in prep.) concluded ravens prey on tortoises throughoutthe Mojave Desert, but probably not all ravens nesting in tortoise habitat prey on tortoises.Available data suggest that ravens prey on tortoises that are 110 mm (about 4 inches) orless in length [Berry 1985, Boarman and Hamilton (in prep)]. The 136 carcasses Campbell(1983) found at the DTNA were between 36 and 103 mm. Farrell’s (1989) raven-predatedcarcasses in the East Mojave ranged in size from 42 to 110 mm. Intact, adult carcasses are rarelyreported beneath raven nests; finding parts of larger tortoises at raven nests probably signifiesscavenging rather than predation.Effects of Ravens on Regional Tortoise Populations: Although the above anecdotalevidence and focused studies have found that ravens do predate (and scavenge) tortoises, andthat predation may be locally common, the relative impact of ravens on regional tortoisepopulations remains unknown. There are no data available to accurately determine tortoisepopulation levels, so there is no ready means of determining what percent of the population isaffected. Available data indicate that about 9% of the tortoise carcasses found where cause ofdeath could be determined were attributed to raven predation.Available information suggests that ravens are opportunistic predators of small tortoises,and that some individuals or pairs of birds are likely to be responsible, rather than the entireraven population. It is apparent for many predators that they seek out prey items that arerelatively abundant, and that they will switch from one prey species to another if preferred preypopulations diminish. This is suggested by the observations of Woodman and Juarez. In theircase, the first carcasses found beneath the nest were relatively larger than the smaller carcassessubsequently found. A plausible explanation is that the pair of ravens selected relatively larger,more easily found carcasses until they were depleted. Then, gradually, they adjusted their searchimage to find smaller tortoises, which are presumably more difficult to locate.If this scenario proves to be true, raven predation would most likely occur where subadulttortoises are abundant, which coincides with the higher density areas, according to sign countdata. However, the relative impact of ravens may be more significant in depleted tortoisepopulations, where every subadult is relatively more important to the future survival of that localor regional population.Dr. Boarman, probably the foremost expert on raven predation in this population,concluded, “In the sense of hard science, these observations [of raven predation] do not consistof proof that ravens are causing significant harm to tortoise populations, but they do support thehypothesis” (Boarman 1992). In their synopsis, National Ecology Research Center (1990)Chapter 3 3-107