In many areas of the United States, an excess of white-tail deer has caused ecological problems. Deer are seen as a "keystone species" for forests. This means that they can completely change an ecosystem through their behavior.
In the Eastern and Midwestern US, the excess deer populations have had bad effects on the vegetation in forests. Deer tend to prefer certain species of plants as food and they overgraze those species.
When deer overgraze those species, they leave ecological niches open for other species of vegetation. Often the species that take over are detrimental kinds of vegetation, including invasive exotic plant species (plants that aren't native to an ecosystem and tend to disrupt ecosystems they enter).
Follow the links for specific details about problems associated with excessive deer populations.