The National Invasive Species Council, established by Executive Order 13112 in 1999, defines invasive species as species that are: “…nonnative (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”

A “non-native” (or “alien”, “exotic” or “nonindigenous”) species is one that has been introduced by human action, intentionally or accidentally, into an ecosystem in which it did not previously occur. Introductions occur along a variety of pathways, or vectors, such as through commercial trade of a species or by accidental means. Invasive species can be plants, animals and other organisms (e.g., fungi).

An invasive species grows, reproduces and spreads rapidly, establishes over large areas and persists. In general, species that become invasive succeed due to favorable environmental conditions and an ability to grow and reproduce rapidly where resource availability is high (Daehler 2003). As invasive species spread and dominate ecosystems, they decrease biodiversity by displacing native plants and animals (Texas Invasives.org).

Thus, the definition of invasive used here has two components: 1) nonnative status and 2) the ability or potential to cause harm. It is important to note that not all nonnative species are considered invasive because many do not, or are not likely to, cause economic or environmental harm or harm to human health (Williamson 1996). Many non-native species support human livelihoods or a preferred quality of life. Examples include most crops and a number of exotic ornamentals (IASC 2006). Conversely, in some situations native species can cause economic or environmental harm or harm to human health. Examples include the economic impact of mesquite (Prosopis glandulosa var. glandulosa) spreading through a Texas rangeland, Ashe juniper (Juniperus ashei) altering the hydrology of the Hill Country or a painful rash caused by poison ivy (Toxicodendron radicans) growing along Lady Bird Lake. While these species can cause problems, and do require management, they are not considered invasive because they are native to these particular ecosystems.