 In contrast, a study published in 2009 demonstrated that multiple species of trees with highly varying autecologies are in fact heavily impacted by the loss of an apex predator.Another study, published in 2011, demonstrated that the loss of large terrestrial predators also significantly degrades the integrity of river and stream systems, impacting their morphology, hydrology, and associated biological communities.The critics' model is challenged by studies accumulating since the reintroduction of gray wolves ("Canis lupus") to Yellowstone National Park.