Raptor Electrocution: A Case Study on Ecological Traps, Sinks, and Additive Mortality

The recovery from human persecution of some upper trophic level wildlife species coupled with ongoing expansion of human-dominated landscapes is leading to increased human-wildlife interactions in urban environments. Raptors in particular are drawn to high resource concentrations of potential nest sites and prey, and are colonizing cities across North America. These birds are encountering novel and sometimes dangerous situations such as exposed overhead electric systems, which can operate as ecological traps, create population sinks, and lead to additive mortality. Ecological traps occur when the cues animals use to evaluate habitat quality are decoupled from the true quality of the environment. Sinks occur in areas where mortality exceeds productivity. Additive mortality can occur when mortality factors that are not density dependent continue to operate proportionally as numbers decrease. Herein, I use the situation of Harris's hawks (Parabuteo unicinctus) colonizing Tucson, AZ, to illustrate these concepts to students in an introductory college ecology course. The case is deliberately loaded with the terminology of ecology, and is intended to be delivered early in the course so that as students explore the case and the vocabulary, their questions probe related material. Student inter- est and questions thus dictate the order of content delivery as students realize and seek to expand the boundaries of their knowledge. Ecology is rarely one-sided and multiple competing hypotheses should be encouraged to facilitate student transitions to higher level modes of thinking. Together, these approaches will increase student interest in detailed cover- age of the typical curriculum of an introductory ecology course.