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.
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.