California: An exemplary biodiversity hotspot
To understand why coastal Mediterranean regions are both so unusually biodiverse and so unusually threatened relative to other temperate ecosystems, let’s refocus our attention on the California Floristic Province (or “California'' for short, which we’ll use from now on to refer to the whole bioregion, not just the state), because California is an exemplar of all the things that make Mediterannean habitats so unique. Specifically, we’ll look at the “Big 3” - 3 features that California shares in common with other Mediterranean regions that contribute to its status as a biodiversity hotspot.
A map of the California Floristic Province. (NoahElhardt on Wikimedia Commons, CC BY-SA 3.0)
Size
Each biome generally has its own unique suite of species, uniquely adapted to survive in the particular climatic and environmental conditions it offers. As previously hinted, most temperate biomes stretch across vast stretches of continent, meaning most species have broad distributions and local rates of endemism are relatively low. This is categorically NOT the case for California - the California Floristic Province is miniscule, barely exceeding the length of a single state and not even spanning its entire width! This means that rather than being spread out across a large swathe of the continent, California’s endemic species are jam-packed into a tiny area, allowing for the region’s incredibly high rates of local endemism, as well as its vulnerability to anthropogenic disruption - after all, even small-scale habitat loss could be hugely significant for a region as small as California. The same is true for the other Mediterranean ecosystems.
A view of the rugged San Gabriel Mountains north of Los Angeles.
Topography
If you’ve done any driving or hiking in California, you’ll know the landscape is incredibly rugged. Virtually the entire shoreline of the region is paralleled by coastal mountain ranges, and further afield, the Sierra Nevada towers above the landscape, reaching over 15,000 feet at Tumanguya, or Mt. Whitney. The elevational diversity of the region makes up for its limited size in allowing for diversity - at different altitudes, wholly different habitats flourish, which multiplies the region’s rate of endemism several times what would be possible if the entire landscape was flat. Take the Los Angeles Basin - at sea level, the dominant habitat is coastal sage scrub, a form of Mediterranean shrubland. Travel into the foothills of the San Gabriel mountains, and you’ll find yourself in chaparral, another type of shrubland with woodier vegetation. While shrubs grow on the slopes, at the bases of the canyons, groves of oaks grow tall. Higher still, and the oaks are interspersed with pines, and on the highest ridges of the San Gabriels, dry pine forests much like those thousands of miles further north persist. The drier side of the range is dominated by juniper, and once you plunge into the desert, you’ll find yourself surrounded by a sparse shrubland of Joshua trees and creosote bushes. In just a few miles as the crow flies, a traveler can encounter dozens of habitats harboring hundreds of endemic species - all thanks to rugged topography. Interestingly, every other coastal Mediterranean region features a mountain range as well - the Apennines, Pyrenees, and Atlas Mountains in the Mediterranean, the Cape Fold Belt in South Africa, the Darling Scarp and Stirling Range in Australia, and the Andes in Chile.
These two factors, together, allow California to have both biodiversity and endemism levels higher than anywhere else in temperate North America. California has more native plants (about 5,500, with new species still being discovered) than any other state in the country, some 40% of which are endemic. Unfortunately, this incredible biodiversity is becoming harder and harder for everyday Californians to experience thanks to the third feature that makes the region a biodiversity hotspot.
A magnet for people
If you’ve ever been to a beach either in California or somewhere around the Mediterranean basin, say the Amalfi Coast or the French Riviera, you know how nice Mediterranean-type climates are for humans. The utility of a Mediterranean climate to our species goes beyond beach vacations, however. Prior to European colonization, California featured one of the highest population densities of any region in temperate North America, a fact all the more remarkable when considering that California natives did not practice agriculture - there was simply such an abundance of wild plant and animal food available that farming was altogether unnecessary. This legacy of food productivity continues today. California, like other Mediterranean-type regions, is an agricultural hotspot, churning out well over half of the United States’ fruits and nuts as well as a third of the country’s vegetables. California is also rich with natural resources, with the discovery of gold in 1849 drawing in scores of settlers laying their claims to the land’s bounty. Finally, the prosperous society created by both lucrative food production and abundant resources gave rise to one of the world’s most robust service economies, with Silicon Valley and Hollywood drawing in migrants from across America and immigrants from abroad alike.
The urban environment of Los Angeles. (Remi Jouan on Wikimedia Commons, CC BY-SA 3.0)
Of course, farming, resource extraction, and settlement all require land. Over the past hundred and fifty years, nearly 80,000 farms and over 14 million houses have been established across the state, consuming a massive amount of land. As a consequence of this landscape transformation, the highly diverse ecosystems that make the California Floristic Province so unique have taken blow after blow. Seventy percent of the chaparral, California’s one-of-a-kind woody shrubland, has been cleared to make way for million-dollar mansions and cattle ranches. Ninety percent of California’s incredibly productive wetlands have been drained for oil fields and port facilities. And most shockingly of all, ninety-nine percent of California’s grasslands - the most biodiverse grasslands anywhere in temperate North America - have disappeared, with agricultural fields and invasive weedlots taking their place. As a result, scores of unique plants endemic to California grasslands, as well as dozens of birds, amphibians, and fish species endemic to the chaparral and California wetlands, teeter on the verge of extinction today - a fitting encapsulation of the dire habitat loss crisis that puts California, along with every other Mediterranean ecosystem worldwide, on the list of biodiversity hotspots.
Degraded chaparral in Southern California, mostly overtaken by invasive grasses. (Chaparralian on Wikimedia Commons, CC BY-SA 4.0)
