Predators can affect the distribution and abundance of their prey. Under normal circumstances predators are not expected to drive their prey to extinction, because as prey densities decline predators should either switch to more abundant prey or starvation will reduce their numbers. If human-provided resources, such as food or water, prevent declines in predator populations in response to declines in prey densities then predators become a much greater threat to prey. These "subsidized" predators have been implicated in the decline of threatened and endangered species.
One of the species I study, the Common Raven (Corvus corax), is a good example of a subsidized predator in the Mojave Desert. Ravens are strongly associated with human developments in the Mojave, and are known to attack juvenile desert tortoise (Gopherus agassizii), a threatened species. Ravens make use of several types of resources obtained from people: they consume anthropogenic foods at landfills, road-killed carrion on roads, drink irrigation runoff, and nest in artificial platforms and ornamental trees. Ravens that breed in close proximity to human-provided resources experience high reproductive rates and enhanced juvenile survival.
Interestingly, breeding ravens and non-breeding ravens are ecologically quite different. Non-breeding ravens will aggregate in large groups at abundant food sources, such as landfills, but breeding ravens defend territories and maintain distance between nests. Consequently, breeding ravens are often found at greater distances from human developments than are non-breeders, but breeding birds do not reach the population densities that non-breeders can attain. Because of these ecological differences, we wished to study whether predatory activity was also different for breeding and non-breeding birds, and how the spatial pattern of distribution of ravens translated into a spatial pattern of predation risk to tortoises. For this work I used attacks by ravens on styrofoam models of juvenile tortoises to measure risk of predation. Risk of predation for desert tortoises is elevated in places where ravens are more abundant, and at successful raven nests. The large aggregations of ravens found at landfills are generally composed of non-breeding individuals, whereas breeding birds are more evenly dispersed out to greater distances from human developments. This means that both non-breeding and breeding ravens are responsible for elevated predation risk, both near developments and away from them.
More recently my students (Linnae DeCamp and Crystel Doyle) have started to study these processes at the San Diego Wild Animal Park and surrounding reserve lands owned and managed by the Zoological Society of San Diego. The park itself attracts large numbers of crows as well as ravens, which take advantage of animal feed and wastes. We will be studying whether crows and ravens attracted to the Park elevate predation risk for animals in surrounding undeveloped lands (the raven to the left is inspecting one of our artificial nests). Additionally, the Park supports breeding colonies of native wading birds, such as Black-crowned Night Herons. We will be assessing whether these birds respond to the spatial distribution of crows and ravens in their nest site choices within the Park.
We are involved in mapping coastal wetlands in San Diego County, as part of a larger effort to map coastal wetlands throughout coastal southern California, in collaboration with CSU Northridge and the Southern California Coastal Water Research Program. We are mapping wetlands by hand, using high-resolution, seamless imagery from the National Agricultural Imagery Program, and other sources. For more information about the mapping project, see: http://www.socalwetlands.com
As part of this project, one of my graduate students (Chrystal Barry) will be attempting to predict the distributions of several invasive exotic plant species using multivariate statistical methods. If this is successful, it should prove substantially less time consuming than digitizing maps by hand.