Invasive Plants in California

Introduction

Invasion of natural habitats by nonnative species contributes to biodiversity loss and extinctions. Native species consist of plants or animals that normally thrive in a particular community. Invasive species, by contrast, do not naturally belong in the environment they invade; they are also called nonnative, alien, or foreign species. Introduced species are those that have been deliberately or accidentally released in a new, nonnative environment. Exotic species also arrive deliberately or accidentally, but these species tend to be rare, unique, and highly valued. An orchid known to grow only in Brazil would be an exotic species in Connecticut for instance. By any name, these plants and animals often create biological havoc in the community they invade. This invasion of new habitats can occur by three different means: migration, deliberate introduction, and accidental introduction (Vogt and Armstrong, 696-698).

Invasive species threaten native species by crowding the native populations out of their space and using up their water and food sources. They do this by outcompeting the native species in the natives' own habitat, perhaps aided by any foreign parasites that the invader also brings into the community. In many cases, the native species have no defenses against an unfamiliar pathogen or parasite, and their numbers plunge due to disease.

Invasion's main threat to biodiversity resides in ecosystem imbalance, in which normal food webs and other interrelationships cannot work because one or more of the ecosystem's components have been eliminated. Native species in invaded habitats disappear for the following reasons: forced migrations to new areas, disease, starvation, and unnatural behaviors. Behavioral changes often include conflicts between native animals and the invader, or against other members of the same species. Invaders usually bring aggressive mannerisms with them into their new territory, and, as a consequence, native species can offer little resistance to the takeover.

This entry describes the threats to ecosystems from invasive species and gives examples of invasive species causing trouble in North America. It reviews the series of events that occur after invasion begins and how these steps lead to endangerment and premature extinction. It also reviews programs in which scientists deliberately introduce species into a habitat to return balance to an ecosystem (United States Department of Agriculture, 34 - 100).

Ecosystems Invaded

Nonnative species may be viruses, bacteria, protozoa, algae, fungi, multi cellular plants, or multi cellular animals. Many familiar plants and animals in North America began as non-natives that humans introduced centuries ago. For instance, most of today's cultivated crops and domesticated animals were brought onto the continent by early settlers as food and sources of nonfood products, such as wool and leather. Other nonnative plants served as ornamental additions to gardens, and nonnative animals came as companion animals. These imports arrived in North America from Europe, Asia, and South America centuries ago, but on occasion unwanted imports came, too.

For centuries the holds of ships have carried new animals to new lands: insects, rodents, fish, reptiles, amphibians, and aquatic crustaceans. These travelers disembarked at the same time that settlers and explorers set ...