Herbivory: A Balanced Diet

Mountain laurel (Kalmia latifolia) is not umbrella-shaped in its natural form! It would normally have evergreen leaves all the way to the ground. Hungry deer have eaten this shrub's leaves as high as they could reach. Luckily for the shrub, it was already taller than a deer when it started being so heavily browsed by an exploding deer population. Young oak seedlings aren't so lucky.
Photographer: Sam Sheline, courtesy of NatureServe
A balanced level of herbivory (animals eating plants) is an important natural process in most if not all natural communities.

Imagine what would happen if every seed that even one tree produced fell to the ground, germinated and grew all the way to maturity! The new trees would crowd each other out and die. So in the grand scheme of things, the fact that animals and insects eat plants and seeds is a big part of what keeps things humming along in nature.

Plants are robust and can survive the loss of some proportion of their leaves and stems, and tree populations produce enough seeds to successfully regenerate (replenish their population) even if many of those seeds or seedlings end up being eaten by animals.

However, a very dense population of native herbivores, or the introduction of non-native animals (including insects), can have devastating effects.

An overabundance of herbivores such as deer can decimate shrubs and drastically reduce the number of tree seedlings and saplings, to the point where the ability of a forest to regenerate itself is compromised. Exploding populations of non-native invasive insects can repeatedly defoliate large areas of forest and weaken mature trees so that they are more susceptible to disease.

See Population Dynamics, and Non-Native Invasive Insects / Animals in Ecology Basics.