Direct, indirect and predator-mediated effects of humans on a terrestrial food web: implications for conservation

Humans influence the density and distribution of other species through direct and indirect effects. Effects are indirect when a species influences another species via an intermediate species, whereas direct effects have no mediator. Humans have strong negative direct effects on wolves (Canis lupus) in areas where wolves kill livestock. Such effects could indirectly influence other species in food webs, particularly wolf prey (i.e., herbivores such as elk; Cervus elaphus) and vegetation species eaten by herbivores. In addition, humans simultaneously directly influence several species in food webs, triggering numerous indirect effects. For example, humans have direct effects on forage (e.g., through agriculture) that could indirectly influence herbivores (e.g. by providing food).;I documented strong direct and indirect effects of humans on an entire food web. To fully understand and where necessary mitigate the impact of humans on species in an ecosystem, managers must not limit their considerations to direct effects, but also consider the indirect effects on species at several trophic levels of food webs.;I studied direct and indirect effects of humans on several interacting species in a terrestrial food web. First, I found that despite the minor cost of livestock predation when evaluated in the broader context of the rural economy, it still constitutes an important issue that compensation programs will not easily eliminate. Livestock predation will likely continue to elicit strong direct effects of humans on wolves. Second, I found direct effects of wolves on prey. Elk selected forest cover and low-food-quality habitat in response to wolf presence. Cattle (Bos taurus) selected roads (likely to seek human protection) and low-food-quality habitat, but only after wolves left pastures, indicating poor anti-predator behaviour. Despite presence of the top-down effects described above, path analysis indicated that humans influenced species distribution from the bottom-up with direct effects on forage and positive indirect effects on herbivores (elk and cattle) and wolves. As a result, human presence influences multi-species assemblages. An overall assessment of mammalian species (humans included) relative density in the study area demonstrated that large herbivores were three times more abundant on high-use roads and trails whereas predators were less abundant.