California Department of Fish and Wildlife
Stages of bioogical Invasion

Non-native species invade through a five stage invasion process. Eradication becomes increasingly difficult, and eventually impossible, as the invasion advances.


Invasion Curve image

In early stages of establishment, most invaders either go unnoticed or appear harmless.  However, rapid reproduction and spread over time lead to exponential  increases in both the total area infested and associated control costs.  While prevention is the most effective and cost-efficient strategy for managing invasive species, early detection and rapid response methods are necessary to prevent infestations and control costs from reaching unmanageable levels.

About Invasive Species in California

What are invasive species?

Invasive species are organisms (plants, animals, or microbes) that are not native to an environment, and once introduced, they establish, quickly reproduce and spread, and cause harm to the environment, economy, or human health.

How did they get here?

Relatively few non-native species had been introduced to California prior to its settlement by Spaniards, which began in the 1700’s. With the beginning of European settlement, non-native species were carried to California attached to the hulls of ships, submerged in the ships’ ballast, or carried along in shipments of grain.

Today, there are many different ways in which non-native invasive species are introduced to California. Commercial shipping remains a major source of unintentional introductions, along with smaller commercial fishing boats and recreational watercraft. People traveling between natural areas, farms, or waterways for work or recreation unintentionally spread invasive species on their vehicles, boats, equipment and even clothing.

Both historically and today, non-native invasive species have also been introduced purposely, without an understanding of the potential consequences of those introductions. This occurs most commonly with plants used for erosion control, livestock forage, and aquarium or garden ornamentals. Some of the animals that are currently, or were in the past, brought into California as sources of food, fur, or pets have turned into major pests.

Why are they a problem?

Californians have benefited from the introduction of many plant and animal species necessary for food or other human pursuits; however, a small proportion of introduced species become invasive and wreak havoc on the state’s environment and economy.

Invasive species threaten the diversity or abundance of native species through competition for resources, predation, parasitism, interbreeding with native populations, transmitting diseases, or causing physical or chemical changes to the invaded habitat. Through their impacts on natural ecosystems, agricultural and other developed lands, water delivery and flood protection systems, invasive species may also negatively affect human health and/or the economy. Examples of direct impacts to human activities include clogging navigable waterways and water delivery systems, weakening flood control structures, damaging crops, introducing diseases to animals that are raised or harvested commercially, and diminishing sportfish populations.

A large population of an invasive species can start from a very small number of individuals, and those individuals can be difficult to see, so they may easily go unnoticed. The tiny young of invasive shellfish or insects, a fragment of an aquatic weed, or a single plant ready to release its seeds can be enough to establish a population that could ultimately cost the state millions of dollars to address. The longer infestations are allowed to progress, the more extensive the damage and control costs, and less efficient the control efforts. However, if populations are detected early enough, eradication may still be possible. Though prevention is the best strategy for managing invasive species, “early detection and rapid response” efforts are the most effective and cost-efficient responses to invasive species that become introduced and established.