Predator-prey interactions are among the most familiar and important in ecology, and they are also fraught with common misconceptions for students and the lay public. Many assume that the only important impact of such interactions is through direct population size control of prey by the predator (Stamp et al. 2006, Stamp and Armstrong 2007). The notion of potential indirect impacts of predator on prey has not occurred to most students, nor has the idea of trophic cascades, in which the impact of top predators on herbivores is also felt at the level of primary producers, where it impacts variables such as age structure, spatial distribution, or species composition (Pace et al. 1999, Ripple and Beschta 2004b, 2005, 2007; Ripple et al. 2001). This Issue uses the example of wolf extirpation and restoration in the Greater Yellowstone region to address the nature of trophic interactions, both direct and indirect.
Wolves were extirpated in the Northern Range of Yellowstone by 1926 (Ripple and Beschta 2004a,b), and elk populations were fairly closely monitored in the Park beginning around the same time. On their winter range, elk may rely on woody vegetation as an important source of nutrition when grass and forbs become unavailable. Park managers became increasingly aware of changes in the woody riparian vegetation in valleys where the elk over-wintered, but despite human culling of the elk herd (from mid-1920's to 1968) the abundance of tall willows declined sharply, and there was little or no recruitment of cottonwood seedlings after the 1930's (Beschta 2003, 2005; Ripple and Beschta 2004 a,b). Park and range managers debated whether elk population densities were responsible for loss of native vegetation, or whether drought and climatic shifts were the culprit (Anderson and Blackford 2005; Yellowstone National Park 2007). Restoration of wolves in Yellowstone began in 1995, with significant and sometimes unexpected consequences for their elk prey and the woody browse used by elk in their winter range.
In this issue, students will study figures and generate hypotheses about why woody vegetation recruitment declined in Yellowstone National Park. Then, through a progressive disclosure approach, they will be shown data to help refute or support various hypotheses and finally construct a detailed food web concept map showing various trophic level interactions in the Yellowstone ecosystem, allowing them to see first hand how trophic cascades can structure ecosystems.
Note: Many of these references are available as PDFs in the Resource section.