Managing a Subsidized Predator Population: Reducing Common Raven Predation on Desert Tortoises

Abstract

Human communities often are an inadvertent source of food, water, and other resources to native species of wildlife. Because these resources are more stable and predictable than those in a natural environment, animals that subsist on them are able to increase in numbers and expand their range, much to the detriment of their competitors and species they prey upon. In the Mojave Desert, common ravens (Corvus corax) have benefited from human-provided resources to increase in population size precipitously in recent years. This trend has caused concern because ravens prey on juvenile desert tortoises (Gopherus agassizi), a federally threatened species. In this paper, I discuss management strategies to reduce raven predation on desert tortoises. The recommendations fall into three categories: (1) managing raven populations by reducing access to anthropogenic resources; (2) removing offending ravens or other birds in specially targeted tortoise management zones; and (3) continuing research on raven ecology, raven behavior, and methods of reducing raven predation on tortoises. I also recommend approaching the problem within an adaptive management framework: management efforts should first be employed as scientific experiments--with replicates and controls--to yield an unbiased assessment of their effectiveness. Furthermore, these strategies should be implemented in concert with actions that reduce other causes of desert tortoise mortality to aid the long-term recovery of their populations. Overall, the approaches outlined in this paper are widely applicable to the management of subsidized predators, particularly where they present a threat to a declining species of prey.

Subsidized predators, landscapes of fear and disarticulated carnivore communities

“…Such scenarios become problematic for conservation practitioners when the enhanced predator population influences species of conservation concern. Examples of this are widespread: subsidized ravens influence tortoise ecology (Boarman, 2003); subsidized predatory beetles influence insect herbivore populations (Rand & Louda, 2006); subsidized mountain lions influence bighorn sheep demographics (Rominger et al, 2004); and in the example presented by Shapira, Sultan & Shanas (2008), subsidized red foxes Vulpes vulpes influence gerbil ecology.…”

Turtle shell and mammal skull resistance to fracture due to predator bites and ground impact

“…Specifically, their measurements of breaking force show that F = 1.7t 2.0 (Figure 1, with r = 0.98, t in cm, and F in kgf). Using (1) to infer the value of σ f yields that the tensile strength of eggshells is σ f ≈ 53.6 MPa, a value that is within the range given for mollusk and eggshells [Biggs et al 1976]. Next, [Ar et al 1979;Pritchard 1993] use scaling arguments involving (1) to make a biologically meaningful statement about constraints on eggshells.…”

“…However, a few animals use their teeth to nibble at the edges of the shell, or poke into soft or thin parts of the shell or carapace. This includes ravens, raccoons, and coyotes [Gibbons 1987;Boarman 2003]. Other very large predators dispatch turtles by biting through their shells with great force.…”

“…While the study of safety factors of leg bones during locomotion has a long history [McMahon and Bonner 1985;Currey 1988;Biewener 1990;Biewener 2003;Vogel 2003], less work has been done on shells and armor. Over millions of years, shell-based armor has evolved in a variety of organisms, and has manifested in diverse forms such as eggshells, mollusk shells, skulls, nuts, fingernails, and gourds.…”

Wintering ecology of sympatric subspecies of Sandhill Crane: Correlations between body size, site fidelity, and movement patterns

“…Because ravens are known to fly up to 65 km (40 mi) in a day and range over several hundred kilometers throughout the year, any concentrated food resource, such as a landfill, could influence raven populations over a broad area (Boarman 2003). Crows also are known to forage or fly to roost sites up to 18 km (11 mi) from daytime territories or activity centers (Liebezeit and George 2002).…”