Why are invasive species bad?

cluster of purple loosestrife on a river bank
Taken from: http://www.sgnis.org/publicat/slide/pl_s4.html

Invasive species have large impacts on the ecosystems they invade. They can affect the chemical makeup of the water, availability of resources, and even alter the food web. Invasive species impact other species more directly through competition for resources. Because invasive species evolved with other competitors and native species evolved in the absence of invasive species they lack the ability to compete with invasive species. This gives invasive species the upper hand when resources become scarce. This leads to an overall decrease in biodiversity as the area invaded becomes less populated by native species and more taken over by invasive species.

Invasive species are also expensive. They are costly to society in many ways through both, direct management, and decreased production. The annual estimated cost in the United States of invasive species control is about 120 billion dollars per year. It is estimated that the global cost of invasive species management is equal to five percent of the global economy.