Most scientists believe that on a global basis, next to habitat destruction, the second greatest destroyers of biodiversity are invasive species. In some cases, the result is the extinction of an entire existing species.

Species have sometimes invaded new habitats naturally. However, human actions are the main means of invasive species introductions. When people settle far from home, they often bring with them familiar animals and plants. Other species, like rats, make the trip unintentionally. In their new habitat there may have fewer predators or diseases, so their populations grow out of control.

Organisms that they might normally prey on may not have evolved defense mechanisms. Native species may not be able to compete successfully for space or food and so are often pushed to extinction. The spread of nonnative organisms destroys healthy, diverse ecosystems. It replaces them with biologically impoverished, homogeneous landscapes.

It is unfortunate, but increased travel and trade are providing many new opportunities for the spread of nonnative species. In addition to which, one important feature that makes
a community susceptible to invasion by nonnative species is the level of human-induced disturbance. For example, nonnative birds such as European starlings and house sparrows do well in ecologically disrupted areas such as cities, suburbs, and farms.