Environmentalists in California are trying to block a federal plan to build a new security fence to prevent illegal immigrants from crossing into the United States from Mexico.

The 14-mile fence would accompany an existing 40-mile fence that has been credited with causing a massive drop in illegal border crossings since its construction in 1993.

Supporters of the fence say that the increasing number of terrorists who are at large means the United States must be even more vigilant at its borders.

But environmentalists argue that the construction of a fence would disrupt the local ecosystem, cause erosion problems and damage the area where the Pacific Ocean meets the Tijuana River (search), now inhabited by rare birds and insects.

The dispute could pit the California Coastal Commission (search) against the will of the White House, which has the authority to overrule the state body, but could find itself in a legal fight.

Click here to view Anita Vogel's report on the battle over the border fence.