Main article: Ecology of California

Ecologically, California is one of the richest and most diverse parts of the world, and includes some of the most endangered ecological communities. California's diverse geography, geology, soils and climate have generated a tremendous diversity of plant and animal life. The State of California is part of the Nearctic ecozone, and spans a number of terrestrial ecoregions, and is perhaps the most ecologically diverse state in the United States.

California has a rather high percentage of endemic species. California endemics include relict species that have died out elsewhere, including the redwoods and the Catalina Ironwood (Lyonothamnus floribundus). Many other endemics originated through differentiation or adaptive radiation, whereby multiple species develop from a common ancestor to take advantage of diverse ecological conditions. California's great abundance of species of California lilac (Ceanothus) is an example of adaptive radiation. Many California endemics have become endangered, as urbanization, logging, overgrazing, and the introduction of exotic species have encroached on their habitat. Furthermore, California is home to the largest trees in the world, the Giant Sequoias.

California's native grasses were perennials, which stayed green year-round in most of the state's subclimates^[2]. After European contact, these were generally replaced by invasive species of European annual grasses; and, in modern times, California's hills turn a characteristic golden brown in summer and fall. California's nickname The Golden State is in reference to the California Gold Rush, and not to the golden brown summer hillsides as is sometimes stated.^[1]