Places · PLC·13
Ravens in the Mojave Desert: Subsidy, Ecology, and Conservation Conflict
[science] The Mojave Desert has seen a human-subsidised raven population explosion — ravens using interstate rest stops, landfills, and water infrastructure at densities far above historical levels. The ecological consequence is documented predation of juvenile desert tortoises, a federally threatened species. The Mojave case is the clearest documented example of raven overabundance causing conservation harm, and it illustrates the complex consequences of human landscape modification for predator-prey relationships.
The Human Subsidy Effect
[science] Ravens are naturally limited in desert environments by food and water scarcity. Human infrastructure removed these limits in the Mojave:12
- Interstate highways (I-15, I-40): provide continuous road kill, food waste at rest stops, and water at maintenance facilities. Ravens have learned to exploit these linear food subsidies, nesting on highway signs and utility poles and foraging along road corridors.
- Military facilities and towns: Fort Irwin, Twentynine Palms, and other facilities provide refuse, wastewater ponds, and food waste that sustain elevated raven populations year-round in areas that would otherwise be near-uninhabitable for ravens in summer.
- Solar and wind energy facilities: the proliferation of renewable energy infrastructure in the Mojave provides additional nest sites and water sources (cooling systems).
Raven densities near infrastructure in the Mojave are estimated at 10–30× higher than in undisturbed desert far from human influence.
Desert Tortoise Predation: The Evidence
[science] Multiple studies have documented raven predation on juvenile desert tortoises:1
- Boarman & Berry (1995) documented ravens as the leading identified predator of juvenile desert tortoises at multiple Mojave sites, accounting for a substantial proportion of observed juvenile mortality.
- Ravens target juveniles (under ~60mm shell length) that cannot retract fully into their shells — once the tortoise carapace exceeds this critical size, ravens largely cannot breach it.
- Characteristic sign of raven tortoise predation: shell fragments at elevated perch sites (telephone poles, highway signs) where ravens carry juveniles and repeatedly drop them to crack the shell.
- Mark-recapture studies in areas with high raven density show lower juvenile tortoise survival than in comparable areas with lower raven density — directly linking the population-level tortoise decline to elevated raven predation.
The Legal Complication
[science] Managing elevated raven populations in the Mojave is legally complex:2
- Common Ravens are protected under the Migratory Bird Treaty Act (MBTA) — lethal control requires a Depredation Permit from the US Fish and Wildlife Service.
- Desert tortoises are listed as Threatened under the Endangered Species Act — agencies have obligations to protect them from preventable human-caused mortality, including from subsidised predators.
- The tension between protecting a native predator (the raven) and protecting a threatened prey species (the desert tortoise) has no clean legal resolution — management must balance both.
Related Pages
Frequently Asked Questions
Are ravens native to the Mojave Desert?
Common Ravens are native to the Mojave Desert and have been year-round residents of the region for millennia. However, their population density in the Mojave has increased dramatically in the 20th and 21st centuries — driven by the expansion of human infrastructure providing food and water in a landscape where both are naturally scarce. Pre-settlement raven densities in the Mojave were significantly lower; the current elevated population is a human-subsidised artifact rather than a natural baseline.
Do ravens eat desert tortoises?
Ravens do not regularly prey on adult desert tortoises (Gopherus agassizii), which are protected by their shell. However, juvenile tortoises and tortoise eggs are vulnerable to raven predation. Studies in the Mojave have documented ravens as a significant source of mortality for juvenile tortoises: ravens can carry off young tortoises and crack the shells of juveniles up to approximately 60mm in length. In areas with elevated raven densities near human infrastructure, tortoise juvenile survival is significantly lower than in areas with lower raven densities. This predation pressure is a documented conservation concern for the Mojave desert tortoise, which is federally listed as threatened.
How is the raven-tortoise problem being managed?
The US Bureau of Land Management (BLM) and other agencies have implemented raven management programmes in the Mojave to reduce predation pressure on desert tortoises. Management strategies have included: targeted lethal control (permitted removal of ravens near tortoise critical habitat); removal of human-provided food and water sources (closing or modifying rest stops, refuse sites) that subsidise elevated raven populations; nest removal at utility infrastructure; and public education to reduce intentional or inadvertent raven feeding near protected tortoise areas. These measures are controversial — ravens are protected under the Migratory Bird Treaty Act and are native wildlife — and their effectiveness in reducing tortoise predation is debated.
Sources
- Boarman, W.I. & Heinrich, B. (1999). Common Raven (Corvus corax). In Poole, A. (ed.), The Birds of North America, No. 476. Cornell Lab of Ornithology. — Mojave raven densities, tortoise predation evidence, management context.
- Marzluff, J.M. & Angell, T. (2005). In the Company of Crows and Ravens. Yale University Press. — human subsidy effect, infrastructure nesting, management controversy, legal framework.
[science] peer-reviewed data