I read this interesting article about a project to re-introduce wolves to Yellowstone National Park and the impact the project has had on the parks eco system. The wolves were introduced in 1995 after having been absent from the environment for 70 years. Since their return there has been a regeneration of the parks population of aspen trees which were previously declining. Whilst the wolves do not have a direct impact on the growth of the aspen tree they do affect it indirectly.
The decline in the trees was attributed to the growth of the parks elf population which having no natural predators was getting out of control. The elk were able to wander around the park freely and eat all the young aspen trees. The re-introduction of the wolves has meant that the elk are no longer able to wander so safely for fear of being eaten. The knock on effect is that there are less elk and the young aspen trees are able to grow to maturity. I find it interesting how restoring the natural balance of predator and prey affects the whole eco system of the park.