It starts with one, maybe a few, hitchhiking bugs.

They arrive attached to an exotic plant or in the moist bottom of a vase after crossing a border or an ocean or the sky.

Sometimes – maybe even most of the time – the invasive creature dies off soon after landing in Southern California. But a few find the region to their liking, thriving with no natural predators and abundant food. A few even gain such a powerful foothold in their new home that they become threats to crops and forests and industries.

http://www.ocregister.com/2017/06/09/these-invasive-species-are-eating-southern-california-out-of-house-home-crops-and-businesses/

Native trees, especially the oaks of our region, are at risk due to urban encroachment and agricultural activities; making space for them in our urban environment is a good gesture.

http://www.davisenterprise.com/local-news/garden-doctor-valley-oaks-may-not-be-appropriate-for-urban-landscape/