A wolf (Canis lupus) in the snow in Banff National Park, which was recolonized by wolves in the 1980s.

The Bow Valley in Banff National Park, Canada, was recolonized by wolves (Canis lupus) in the mid-1980s, although wolves did not establish themselves in areas near the growing town of Banff. This situation created ideal conditions to test the trophic cascade hypothesis. The hypothesis predicts in the areas the wolves recolonize, the populations of their herbivore prey (elk) should be less dense and the herbivore’s preferred foods (willow and aspen) should experience less grazing pressure. After wolf recolonization, researchers compared elk (Cervus elaphus) and plant populations in the area without wolves to areas the wolves inhabited. The found that wolves reduced elk density, survival, and recruitment, which cascaded down to positively affect plants that the elk browse on. This photograph originally appeared on the cover of Ecology (86:8) in August of 2005.