Invasion by non-native plants, animals, and microorganisms is one of the most significant natural threats a healthy ecosystem can face. Invasive, exotic plant species often thrive in areas throughout the hot and humid south. If left unchecked, they can expand over vast areas and eventually cover, eliminate, or replace native grasses, trees, and shrubs.
What makes a non-native species so potentially disruptive? The reason stems from the fact that all species evolve in concert with all the other species that inhabit and interact within a particular area. Due to the eternal competition for a finite resource base (i.e. food, whether it comes in the form of sunlight, water, and nutrients for plants; or carbohydrates and protein for animals), natural selection ensures that each successful species possesses a suite of characteristics that enable it to grow and reproduce under a given set of environmental conditions. As long as the environmental conditions do not change too drastically, each species is able to occupy its own individual ecological niche, but is kept in check by competition and predation from other species. In this way no one species is able to dominate, but rather exists as part of a complex food web that is more or less "balanced".
However, the balance that an ecosystem obtains over thousands or millions of years can be seriously disrupted when two factors come into play: physical disturbance and biological introduction. Physical disturbance can be either a natural event, like a flood or avalanche, or human-caused, such as habitat destruction. A biological introduction occurs when one species is taken out of its native range, transported across vast distances, and redeposited again by human hands. The problem arises when a species, which has been transported from another part of the country, continent, or world, comes to rest in an area that's been disturbed and made vulnerable. This species may have evolved under similiar environmental conditions (like heat and rainfall amounts) in its original habitat, but in its new location it may face neither competition nor predation from the native species it is now located among and that it evolved in isolation from. If this is the case the introduced, non-native species may grow or reproduce unchecked in the disturbed habitat, and out-compete and dominate the native species. In doing so it reduces or eliminates habitat components that native species rely on for food and shelter, and seriously decreases an area's inherent biodiversity.
Vicksburg NMP is an area that suffered massive disturbance during the Civil War. In subsequent years many non-native plant species, introduced either accidentally or on purpose, became established and currently make up over 25% of the park's total floral species. Some of these species are invasive and have infested many acres of park land. The most pernicious of the invaders are: paper mulberry, Chinese parasol tree, English ivy, Chinese privet, Japanese honeysuckle, Chinaberry tree, nandina, trifoliate orange, kudzu, and johnsongrass. In fact, five of what are considered the Top Ten Worst Weeds in Mississippi can be found in the park. Invasive non-native species control efforts are a primary focus of the park's natural resource management program. Such efforts are directed toward preserving and restoring the park's ecological integrity and biodiversity, and maintaining a flora that is more typical of the Civil War period.