A non-native species is an organism that is not historically known to have occurred naturally in an area. In California, non-native species were a rare instance until settlement by the Spaniards and Europeans. The expansion of these settlers into California created more trade prospects for humans but opened the door for the introduction of non-native species. Animals previously unseen in the area began to arrive on the wagons of settlers and in hulls and on the ballasts of ships. Today non-native species are still introduced in many of the same ways. Wagons have become vehicles and wooden vessels have been replaced by large commercial trading ships with containers from all over the world. Increased recreation in our natural areas has also become an avenue for non-native spices introductions. The majority of these introductions are done accidentally, usually by humans, but some are done deliberately. Over the years some wildlife species have been brought in to replace a species that has gone extinct, serve as a biological control, add to ornamental design, or be traded as a pet. Unfortunately, many of these species have been introduced without the proper research being conducted to discover their potential effects on local ecology.